Imaging medium
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of our application Ser. No. 08/232,725, filed Apr. 25, 1994 (now U.S. Pat. No. 5,441,850). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an imaging medium and to a process for producing an image. 
     Images can be generated by exposing a photosensitive medium to light in an imagewise fashion. Some conventional non-silver halide photosensitive compositions contain molecules which are inherently photosensitive, so that absorption of electromagnetic radiation brings about decomposition of, at most, as many molecules as photons absorbed. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the absorption of each photon generates a catalyst for a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each photon absorbed. For example, systems are known in which the primary photochemical reaction produces an acid, and this acid is employed catalytically to eliminate acid-labile groups in a secondary, radiation-independent reaction. Such systems may be used as photoresists: see, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; and Ito et al., &#34;Chemical Amplification in the Design of Dry Developing Resist Materials&#34;, Polym. Sci. Eng., 23(18), 1012 (1983). 
     Among the known acid-generating materials for use in this type of process employing secondary, non-radiation dependent reactions are certain diazonium, phosphonium, sulfonium and iodonium salts. These salts, hereinafter referred to as superacid precursors, decompose to produce superacids, i.e., acids with a pKa less than about 0, upon exposure to electromagnetic radiation. Other materials decompose to produce superacids in a similar manner. However, in the absence of a spectral sensitizer, the known superacid precursors decompose to produce superacid only upon exposure to wavelengths which the precursors absorb, which are typically in the short ultraviolet region (below about 280 nm). The use of such wavelengths is often inconvenient, not least because special optical systems must be used. 
     It is known that various dyes can sensitize the decomposition of superacid precursors upon absorption by the dye of radiation which is not significantly absorbed by the superacid precursor; see, for example, European Patent Application Publication No. 120,601. Unfortunately, however, due to the very low pKa of the superacid, many such dyes are protonated by the superacid, so that no unbuffered superacid is produced (i.e., the sensitizing dye buffers any superacid produced). Since no unbuffered superacid is released into the medium, decomposition of superacid precursors sensitized by these dyes cannot be used to trigger any secondary reaction which requires the presence of unbuffered superacid. 
     (The term &#34;unbuffered superacid&#34; is used herein to refer to superacid which is not buffered by the sensitizing dye, and which thus provides an acidic species stronger than that provided by the protonated sensitizing dye. Because of the extreme acidity of superacids and their consequent tendency to protonate even species which are not normally regarded as basic, it is possible, and indeed likely, that &#34;unbuffered superacid&#34; will in fact be present as a species buffered by some component of the imaging medium less basic than the sensitizing dye. However, such buffering by other species may be ignored for the present purposes, so long as superacid is present as an acidic species stronger than that provided by superacid buffered by the sensitizing dye.) 
     Crivello and Lain, &#34;Dye-Sensitized Photoinitiated Cationic Polymerization&#34;, J. Polymer Sci., 16, 2441 (1978) and Ohe and Ichimura, &#34;Positive-Working Photoresists Sensitive to Visible Light III, Poly(tetrahydropyranyl methacrylates) Activated by Dye-Sensitized Decomposition of Diphenyliodonium Salt&#34;, J. Imag. Sci., Technol., 37(3), 250 (1993) describe small sub-groups of sensitizing dyes which are sufficiently non-basic that the buffered superacids produced can effect certain acid-catalyzed reactions. However, the need to restrict the choice both of sensitizers and of acid-catalyzed reactions may make it difficult to design an efficient imaging system at a specific desired wavelength. 
     A variety of non-basic, polycyclic aromatic compounds sensitize decomposition of superacid precursors to produce unbuffered superacid upon exposure to longer wavelengths than the superacid precursors absorb themselves. Such materials are discussed in, for example, DeVoe et al., &#34;Electron Transfer Sensitized Photolysis of &#39;Onium salts&#34;, Can. J. Chem., 66, 319 (1988); Saeva, U.S. Pat. No. 5,055,376; and Wallraft et al., &#34;A Chemically Amplified Photoresist for Visible Laser Imaging&#34;, J. Imag. Sci. Technol., 36(5), 468-476 (1992). 
     U.S. application Ser. No. 08/141,852, filed Oct. 22, 1993 (now U.S. Pat. No. 5,453,345) and its parent, U.S. Pat. No. 5,286,612 (both assigned to the same assignee as the present invention) describe a process by which a wider variety of dyes than those discussed above may be used together with a superacid precursor to generate free (unbuffered) superacid in a medium. In this process, acid is generated by exposing a mixture of a superacid precursor and a dye to actinic radiation of a first wavelength which does not, in the absence of the dye, cause decomposition of the superacid precursor to form the corresponding superacid, thereby causing absorption of the actinic radiation and decomposition of part of the superacid precursor, with formation of a protonated product derived from the dye; then irradiating the mixture with actinic radiation of a second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of free superacid. Generation of superacid by exposure to the second wavelength may be sensitized by one of the non-basic, polycyclic aromatic sensitizers mentioned above. (For convenience, the type of process disclosed in this patent will hereinafter be called the &#39;612 process.) 
     However, the use of any of the methods described above for the sensitized decomposition of superacid precursors to generate unbuffered superacid poses a severe problem if the exposing wavelength for the imaging system falls within a wavelength range in which the resultant image is to be viewed, and the sensitizing dye is not removed. This problem will hereinafter be referred to as the &#34;sensitizing/viewing problem,&#34; and is particularly apparent when one attempts to use one of the sensitizers described above to make an image, intended to be viewed by the eye, by means of an exposure to visible wavelengths. Absorption of visible light by the sensitizer leads to a large minimum optical density (D min ) in the final image, lowering its contrast and making its appearance unacceptable (especially in regions intended to be whim, that is diffusely reflective, and non-absorbing in the visible region). Even if the visible absorption of the sensitizer is bleached in the course of the sensitization reaction, this visible absorption will still remain in the originally non-exposed areas of the image. The sensitizing/viewing problem is not confined to visible wavelengths but applies to any system in which the exposing radiation absorbed by the sensitizer falls within a wavelength range in which the resultant image is to be viewed or used (in the case of a photomask or photographic negative). For example, in the graphic arts industry, it is conventional to expose contact and &#34;dupe&#34; films in the near ultra-violet to produce photomasks which display imagewise changes in absorption which are themselves in the near ultra-violet range. Such processes, if sensitized by one of the methods described above, may suffer from a sensitizing/viewing problem. 
     The sensitizing/viewing problem is especially severe if it is desired to construct a full-color imaging system, which requires exposure to at least three different wavelengths to produce images in three primary colors. A conventional approach to this problem would be to resort to &#34;false sensitization&#34;, i.e., to expose the imaging medium at three wavelengths which are not visible to the eye. In practice, however, exclusion of visible wavelengths for exposure leads to great difficulty. 
     Efficient sensitization of superacid precursors at non-visible wavelengths is limited to the near ultra-violet region (between about 330 nm and 420 nm) and the near infra-red region (between about 700 nm and 1200 nm). The non-basic, polycyclic aromatic compounds mentioned above may be used to sensitize decomposition of superacid precursors to make unbuffered superacid in the near ultra-violet region. However, it is difficult to find three near ultra-violet wavelengths which can be generated by a convenient source and which are sufficiently separated from each other to be absorbed by three sensitizers without cross-talk among three color-forming layers. (The practical near ultra-violet wavelength range for this type of imaging process extends only from about 330 nm to about 420 nm, since other components of the imaging medium, such as leuco image dyes if used, often absorb competitively with the sensitizer below about 330 nm, and wavelengths above about 420 nm will appear yellow to the human eye). It is difficult to find a lamp which emits three spaced wavelengths in the 330-420 nm range; a conventional mercury lamp emits only two usable wavelengths in this range. Moreover, a modulating device such as a liquid crystal cell (and polarizers, if required) may be damaged by lengthy exposure to near ultra-violet radiation. Although phosphors emitting in the near ultra-violet are known, cathode ray tubes using such phosphors must be custom-made, and the emission spectra of such phosphors is often broad, leading to possible cross-talk. Another available exposure source, the near ultra-violet laser, is expensive. 
     Although it may at first appear that sensitization at near infra-red wavelengths would solve the problems discussed above, infra-red sensitizers suffer from other problems. Some near infra-red dyes, and all non-basic polycyclic aromatic compounds which absorb in the near infra-red, have visible absorptions which contribute to the D min  of the image. Equally importantly, the photon energy in the near infra-red is less than about 40 kcal/mole, and in practice this low photon energy appears to limit the quantum efficiency at which the sensitizing dye causes the decomposition of the superacid precursor. This limited quantum efficiency limits the overall sensitivity of the process. 
     The problems discussed above are alleviated if at least one of the exposing wavelengths is in the visible region. If at least one exposing wavelength is in the visible or near infra-red region, a maximum of only two exposing wavelengths must be found in the near ultra-violet. Furthermore, the higher photon energy available in the visible region, as compared with the near infra-red, is likely to lead to higher quantum efficiency, and therefore higher sensitivity, than can be expected from near infra-red exposure. There are also other reasons why exposure in the visible region is preferred. Conventional cameras are designed to use visible light, and thus any medium intended to replace conventional silver halide film in cameras and produce a direct positive print or transparency must use visible light to produce a visible image. Likewise, any medium intended to produce a print from a photographic negative must be capable of exposure by visible light. Conventional photographic printers, also, are designed to use visible light. Much effort has been expended in developing sources (for example, light-emitting diodes, cathode ray tubes, lumps and lasers) and control devices (for example, liquid crystal light valves) optimized for visible radiation. Accordingly, using visible radiation rather than (say) infra-red or ultra-violet radiation in an imaging process often enables the cost of the imaging apparatus to be reduced, and may also make it possible to use a standard, commercially available light source or control device rather than a custom-made source or device. 
     U.S. Pat. No. 5,334,489 and U.S. application Ser. No. 08/141,860 (now U.S. Pat. No. 5,395,736) (both of which are assigned to the same assignee as the present application), and the corresponding International Application No. PCT/US93/10224, Publication No. WO 94/10607), all describe processes for the photochemical generation of acid and for imaging using conventional ultra-violet sensitizers; these processes will hereinafter collectively be called the &#34;&#39;489 process.&#34; The aforementioned U.S. Pat. No. 5,286,612 and copending application Ser. No. 08/141,852 (both of which are assigned to the same assignee as the present application), and the corresponding International Application No. PCT/US93/10215 (Publication No. WO 94/10606), all describe an imaging process using an imaging medium comprising an acid-generating layer or phase and a color-change layer or phase. In this patent and these applications, the acid-generating layer or phase comprises a mixture of a superacid precursor, a sensitizer and a secondary acid generator. The secondary acid generator is capable of acid-catalyzed thermal decomposition by unbuffered superacid to form a secondary acid. The color-change layer comprises an image dye which undergoes a change in its absorption of radiation upon contact with the secondary acid. After imagewise exposure and generation of unbuffered superacid in the exposed areas, the medium is heated; in exposed areas, the unbuffered superacid causes acid-catalyzed decomposition of the secondary acid generator, thereby causing the formation of a molar amount of secondary acid much larger than the molar amount of unbuffered superacid present before the heating. In the non-exposed areas, however, since no unbuffered superacid is present, no significant generation of secondary acid takes place during the heating. Thereafter, the medium is further heated (in practice the two heating steps can be combined) to cause the components present in the two layers to mix, so that, in exposed areas, the secondary acid brings about the absorption change in the image dye, thereby forming an image. Thus, the imaging medium is a single sheet which develops its image without any need for treatment with a developing composition and without requiring any waste material to be peeled from the medium to produce the final image. 
     In these processes, the sensitizer or sensitizers and the undecomposed superacid precursor remain present in at least the non-exposed areas of the medium after imaging (these non-exposed areas are the D min  areas, i.e., the regions of minimum optical density, in the normal case where the image dye is colorless before imaging and develops color in the exposed areas during imaging). Accordingly, in practice it is necessary that the image be viewed in a wavelength range which does not include the wavelength used for the imagewise exposure, since otherwise the image will suffer from a sensitizing/viewing problem, with the contrast between the exposed and non-exposed areas reduced by the absorption, in the non-exposed areas, of the sensitizer or sensitizers used. For example, as discussed above, the preferred &#39;612 process requires two exposures, one at a wavelength in the near infra-red region (700-1200) nm and the second at a wavelength in the near ultra-violet region, typically around 365 nm, to produce an image in the visible region (about 400-700 nm). 
     There is a need for a modified form of these imaging media and processes which removes the restriction that the wavelength used for the imagewise exposure be outside the wavelength range in which the final image is viewed. 
     SUMMARY OF THE INVENTION 
     This invention provides a process for producing an image. This process uses an imaging medium comprising an acid-generating layer or phase comprising a mixture of a superacid precursor, a sensitizing dye and a secondary acid generator, and a color-change layer or phase comprising an image dye. The sensitizing dye has a first form and a second form, the first form having substantially greater absorption in a first wavelength range than the second form. The superacid precursor is capable of being decomposed (if necessary, with the addition of a non-basic sensitizer, for example a polycyclic aromatic sensitizer) to produce superacid by actinic radiation in a second wavelength range different from the first wavelength range, but is not, in the absence of the sensitizing dye, capable of being decomposed to produce superacid by actinic radiation in the first wavelength range. The secondary acid generator is capable of acid-catalyzed thermal decomposition by unbuffered superacid to form a secondary acid. The image dye undergoes a change in its absorption of radiation upon contact with the secondary acid. In the present process, while at least part of the sensitizing dye is in its first form, the medium is imagewise exposed to actinic radiation in the first wavelength range. In the exposed areas of the acid-generating layer or phase, this imagewise exposure causes the sensitizing dye to decompose at least part of the superacid precursor, with formation of unbuffered superacid. Thereafter, the medium is heated to cause, in the exposed areas of the acid-generating layer or phase, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid. Next, the components of the acid-generating and color-change layers or phases are mixed, thereby causing, in the exposed areas of the medium, the secondary acid to bring about the change in absorption of the image dye and thereby form the image. Finally, in at least the non-exposed areas of the medium, the sensitizing dye is converted to its second form. 
     This invention also provides an imaging medium as defined in the preceding paragraph. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1D show the acid concentrations in the exposed and non-exposed regions of the acid-generating layer during the various steps of a first preferred process of the present invention; 
     FIGS. 2A-2E show the acid concentrations in the exposed and non-exposed regions of the acid-generating layer during the various steps of a second preferred process of the present invention; and 
     FIG. 3 is a schematic cross section through an imaging medium of the present invention as it is being passed between a pair of hot rollers during the imaging process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the present invention differs from the &#39;489 and &#39;612 processes in that the present process uses a sensitizing dye having a first form and a second form, with the first form having substantially greater absorption in a specific wavelength range (herein called the &#34;first wavelength range&#34;) than the second form. The medium is imagewise exposed to radiation in the first wavelength range while the sensitizing dye is in its first form, so that, in the exposed areas, the sensitizing dye causes decomposition of at least part of the superacid precursor, with formation of unbuffered superacid. As explained in more detail below, in some cases, in addition to the imagewise exposure, further steps may be necessary to produce the imagewise distribution of unbuffered superacid required for later steps of the present process. 
     The unbuffered superacid in the exposed areas (in effect, a &#34;latent image&#34; in superacid) produced in the first part of the present process is then used to cause superacid-catalyzed thermal decomposition of the secondary acid generator, and for this purpose the imaging medium is heated. Next, the components of the acid-generating and color-change layers or phases are mixed, thereby causing, in the exposed areas of the medium, the secondary acid to bring about the change in absorption of the image dye and to form the image. (In saying that the components of the acid-generating and color-change layers or phases are mixed, we do not exclude the possibility that these two layers or phases may be merged to form a single layer or phase, but such complete merger is not necessary, since it is only necessary for the secondary acid and the image dye to come into contact, so that the secondary acid can cause the change in absorption of the image dye.) Finally, in at least the non-exposed areas of the imaging medium, the sensitizing dye is converted to its second form. This conversion of the sensitizing dye to its second form essentially removes from the non-exposed areas of the image the absorption of the sensitizing dye in the first wavelength range, and thus allows the imagewise exposure of the medium to be effected using radiation in the same wavelength range as that in which the image is viewed; at least the non-exposed areas of the final image do not absorb at the wavelength at which imaging is effected. 
     Conversion of the sensitizing dye from its first to its second form may be effected by any technique capable of convening a dye from a first form having a substantial absorption in the desired wavelength range to a second form having substantially less absorption in this wavelength range. However, this conversion should be such as not to adversely affect the image, either by significantly decreasing the maximum optical density (D max ) of the exposed regions or by introducing unwanted color into the regions where the sensitizing dye finally remains in its second form. In addition, of course, the sensitizing dye should be chosen so that it is compatible with the other components of the imaging medium and does not, for example, crystallize out on prolonged storage or undergo slow thermal reactions with other components of the imaging medium during such storage. 
     The conversion of the sensitizing dye from its first form to its second form may be either a reversible or an irreversible chemical reaction. (In theory, no chemical reaction is thermodynamically completely irreversible; however, as any chemist is aware, there are many reactions where the equilibrium lies so far to one side that no detectable trace of the other supposed components of the equilibrium mixture are present, and the term &#34;irreversible&#34; is used herein to mean such reactions which are for practical purposes irreversible.) For example, conversion of the first form of the sensitizing dye to the second form may be effected by contacting the sensitizing dye with a base; in a preferred variant of the present process of this type (hereinafter called the &#34;deprotonation process&#34;), the first form of the sensitizing dye is a protonated form and the second form is a deprotonated form, and the two forms are reversibly interconverted by contact with base or acid. Alternatively, conversion of the first form of the sensitizing dye to the second form may be effected by reacting the sensitizing dye with a nucleophile; this variant of the present process is hereinafter called the &#34;nucleophile process&#34;. In such a process, the chemical change effected by the nucleophile may be irreversible. Other techniques for converting a first form of the sensitizing dye to its second form may also be used, for example heating the imaging medium to cause thermal decomposition of the sensitizing dye, or bringing about decomposition of the sensitizing dye by exposing the imaging medium to actinic radiation of a wavelength which does not affect the other components of the medium. 
     The imaging media of the invention described thus far still contain, after imaging, a substantial amount of unchanged superacid precursor, and are thus susceptible to post-imaging color changes caused by unwanted generation of superacid by ambient radiation striking the superacid precursor, with consequent generation of acid. However, this susceptibility of the imaged media to unwanted color generation can be eliminated by including in the media a &#34;fixing&#34; reagent capable of destroying the superacid precursor without formation of acid therefrom, thus fixing the image. Such fixing reagents, and processes for their use, are described and claimed in copending application Ser. No. 08/232,757, and its continuation-in-part application Ser. No. 08/430,421, of even date herewith and assigned to the same assignee as the present application. The preferred embodiments of the invention described below with reference to Tables 1 and 2 and the accompanying drawings contain such a fixing reagent. 
     In the present process, the imagewise exposure is effected with at least part of the sensitizing dye in its first form. The imaging medium may be prepared with at least part of the sensitizing dye already its first form; for example in a deprotonation process, an appropriate amount of acid may be included in a coating solution from which the acid-generating layer is deposited, or this coating solution may be contacted with a separate phase containing an acid. Alternatively, the imaging medium may be prepared with the sensitizing dye in its second or some other precursor form, and converted to its first form within the acid-generating layer before the imagewise exposure is effected; for example, as illustrated below with reference to FIGS. 1A-1D and 2A-2E, in a deprotonation process, the imaging medium may be prepared with the sensitizing dye in its deprotonated (second) form and this deprotonated form converted to the protonated (first) form prior to the imagewise exposure by generating or introducing acid into the acid-generating layer. Such acid generation is conveniently effected by exposing the whole of the acid-generating layer to radiation of the second wavelength, with consequent formation of superacid from the superacid precursor, and protonation of at least part of the sensitizing dye. A combination of these two methods of providing acid may of course be used, i.e., one could coat a limited amount of sensitizing dye in its first form and generate additional sensitizing dye in its first form immediately before use. 
     To illustrate the complex chemical reactions which may take place during the present process, two preferred deprotonation processes of the present invention will now be described, with reference to Tables 1 and 2 below and FIGS. 1A-1D and 2A-2E of the accompanying drawings. 
     Table 1 and FIGS. 1A-1D of the accompanying drawings show the changes in acid concentration in exposed and non-exposed areas of the acid-generating layer used at various stages during the first preferred deprotonation process. The last section of Table 1, headed &#34;AFTER FIXING,&#34; shows the composition of the combined acid-generating and color-change layers after the components thereof have become intermixed. 
     
                                           TABLE 1                                 
__________________________________________________________________________
EXPOSED AREA        NON-EXPOSED AREA                                      
Component       Moles                                                     
                    Component     Moles                                   
__________________________________________________________________________
PRIOR TO USE                                                              
[S-DYE]         1   [S-DYE]       1                                       
Secondary acid generator                                                  
                10  Secondary acid generator                              
                                  10                                      
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                5   Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                      
                                  5                                       
AFTER INITIAL ULTRA-VIOLET EXPOSURE                                       
[S-DYE-H].sup.+ SbF.sub.6.sup.-                                           
                0.75                                                      
                    [S-DYE-H].sup.+ SbF.sub.6.sup.-                       
                                  0.75                                    
[S-DYE]         0.25                                                      
                    [S-DYE]       0.25                                    
Secondary acid generator                                                  
                10  Secondary acid generator                              
                                  10                                      
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                4.25                                                      
                    Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                      
                                  4.25                                    
AFTER IMAGEWISE VISIBLE EXPOSURE                                          
[S-DYE-H].sup.+ SbF.sub.6.sup.-                                           
                0.25                                                      
                    [S-DYE-H].sup.+ SbF.sub.6.sup.-                       
                                  0.75                                    
Ph-[S-DYE-H].sup.+ SbF.sub.6.sup.-                                        
                0.75                                                      
                    [S-DYE]       0.25                                    
HSbF.sub.6      0.5 Secondary acid generator                              
                                  10                                      
Secondary acid generator                                                  
                10  Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                      
                                  4.25                                    
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                3.5                                                       
AFTER HEATING                                                             
[S-DYE-H].sup.+ SbF.sub.6.sup.-                                           
                0.25                                                      
                    [S-DYE-H].sup.+ SbF.sub.6.sup.-                       
                                  0.75                                    
Ph-[S-DYE-H].sup.+ SbF6-                                                  
                0.75                                                      
                    [S-DYE]       0.25                                    
HSbF.sub.6      0.5 Secondary acid generator                              
                                  10                                      
Secondary acid  10  Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                      
                                  4.25                                    
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                3.5                                                       
AFTER FIXING                                                              
[S-DYE]         0.25                                                      
                    [S-DYE]       1                                       
Ph-[S-DYE]      0.75                                                      
                    HOAc          0.75                                    
HOAc            6   KOAc          1                                       
Image dye/secondary acid salt                                             
                5.5 Secondary acid generator                              
                                  10                                      
Unprotonated image dye                                                    
                1   Cu reagent    1                                       
Ph-image dye/secondary acid                                               
                3.5 PhOAc         4.25                                    
salt                Unprotonated image dye                                
                                  10                                      
Cu reagent      1   KSbF.sub.6    5                                       
KSbF.sub.6      5                                                         
K/secondary acid salt                                                     
                1                                                         
__________________________________________________________________________
 
    
     As shown in Table 1, the imaging medium initially contains the sensitizing dye in its unprotonated form. Both the exposed and non-exposed areas comprise a quantity (shown in Table 1 as 1 mole for simplicity; all references to moles concerning Tables 1 and 2 (see below) refer to moles per unit area of the imaging medium, and are only by way of illustration, since the proportions of the various components may vary widely) of a sensitizing dye, a larger molar quantity of a superacid precursor (5 moles of Ph 2  I +  SbF 6   -   are shown in Table 1; a suitable quantity of a non-basic polycyclic aromatic sensitizer, such as pyrene, is also included in the medium but is not shown in Table 1) and a still larger molar quantity (10 moles are shown in Table 1) of a secondary acid generator. 
     The imaging medium is first blanket irradiated with radiation in the second wavelength range, typically near ultra-violet radiation, the amount of radiation applied being sufficient to cause the decomposition of less than one mole (0.75 mole is used for illustration in Table 1 and FIG. 1A) of the superacid precursor, thus producing a corresponding amount of superacid. This superacid immediately protonates the sensitizing dye, producing a salt of the dye shown as &#34;[S-DYE-H] +  SbF 6   -  &#34; in Table 1, and leaving no unbuffered superacid present in the acid-generating layer. Thus, after this initial ultra-violet exposure, as shown in Table 1, all areas of the acid-generating layer contain 0.75 mole of the sensitizing dye salt, 0.25 mole of unprotonated sensitizing dye, 4.25 moles of superacid precursor and 10 moles of secondary acid generator. This situation is illustrated in FIG. 1A, which shows the acid level as 0.75 times a threshold level (indicated by T in FIGS. 1A-1D) at which all the sensitizing dye becomes protonated. 
     (The secondary reactions that follow the fragmentation of the superacid precursor are not entirely understood at present. However, it is likely that a phenyl radical is generated, which subsequently becomes attached to the radical cation derived from the non-basic polycyclic aromatic sensitizer, following which elimination of a proton occurs. This phenyl radical is ignored in Table 1. Even if some of the phenyl radicals generated do become attached to sensitizing dye molecules, this will not significantly affect the overall course of the process shown in Table 1 and FIG. 1, since a phenylated form of the sensitizing dye would be expected to undergo the same type of protonation and deprotonation reactions, with similar absorption shifts, as the non-phenylated dye.) 
     After the initial ultra-violet exposure, the imaging medium is imagewise exposed to radiation in the first wavelength range; visible radiation is shown for illustration in Table 1. As shown in Table 1 and FIG. 1 B, in the area BC of the acid-generating layer which is exposed to the visible radiation, this visible radiation causes the protonated sensitizing dye to bring about the decomposition of a further 0.75 mole of superacid precursor, with generation of a further 0.75 mole of superacid, so that the total amount of acid present exceeds the threshold T. The additional superacid generated by the visible exposure protonates the remaining 0.25 mole of previously unprotonated sensitizing dye, leaving 0.5 mole of unbuffered superacid in the exposed area BC, as shown in FIG. 1B. (For purposes of illustration, FIG. 1B shows the acid generated in the ultraviolet and visible exposures separately, although of course no difference exists chemically.) In the non-exposed areas AB and CD no change in the acid level occurs, the acid concentration remains below the threshold T, and no unbuffered superacid is present after the visible exposure. 
     Thus, at the end of the imagewise irradiation, unbuffered superacid is present in the exposed areas, whereas in the non-exposed areas no unbuffered superacid is present, all the superacid generated being buffered by the sensitizing dye. In effect, the acid-generating layer now contains a &#34;latent image&#34; in superacid, although this image is not visible to the human eye. 
     It is expected that the decomposition of the superacid precursor by the sensitizing dye during the imagewise visible exposure will be accompanied by phenylation of the photooxidized sensitizing dye by the phenyl radical derived from the superacid precursor, followed by elimination of a proton. Accordingly, at the end of the imagewise exposure, the exposed areas will contain 0.75 mole of a phenylated product derived from the protonated sensitizing dye, this product being denoted Ph-[S-DYE-H] +  SbF 6   -   in Table 1. The remaining 0.25 mole of sensitizing dye will remain in the [S-DYE-H] +  SbF 6   -   form. Also present in the exposed areas will be 0.5 mole of unbuffered superacid, the 3.5 remaining moles of superacid precursor, and the 10 moles of secondary acid generator, which remain unchanged at this point. (The composition of the non-exposed areas of course remains unchanged by the imagewise visible exposure.) 
     The imaging medium is next heated. In the exposed area BC, the unbuffered superacid present catalyzes the decomposition of the secondary acid generator, thus producing a large quantity of the secondary acid (10 moles are shown by way of example in Table 1; FIG. 1C is not strictly to scale). However, in the non-exposed areas AB and CD, no unbuffered superacid is present, and the sensitizing dye/superacid salt does not catalyze the decomposition of the secondary acid generator, so that essentially no decomposition of the secondary acid generator occurs and essentially no secondary acid is generated. 
     In the final step of the process, as discussed in more detail below, the components of the acid-generating and color change layers become intermixed. Table 1 assumes that the color-change layer contains 10 moles of an indicator image dye, 1 mole of copper compound, 1 mole of a reducing agent (the products produced by oxidation of this reducing agent are ignored in Table 1 for simplicity) and 6 moles of a reactive material, shown as potassium acetate in Table 1 (where acetate is abbreviated &#34;OAc&#34;). Table 1 further assumes that the image dye is more basic than the sensitizing dye; the contrary case is discussed below following Table 2. In the non-exposed areas, the copper compound, the reducing agent and the reactive material decompose all remaining superacid precursor, with generation of the corresponding amount of phenyl acetate (Table 1 assumes), phenyl iodide (omitted from Table 1) and potassium hexafluoroantimonate. In the exposed areas, the potassium acetate is protonated by the superacid and by some of the secondary acid. The copper reagent catalyzes decomposition of the remaining superacid precursor with the formation of phenyl cations, which react with the most nucleophilic species remaining, here assumed to be the image dye. (In practice, the decomposition of the superacid precursor is probably somewhat more complicated, and other products may be produced; however, the exact type of decomposition products produced does not affect the essential nature of the present process.) 
     In the exposed areas, the unbuffered superacid and 4.5 moles of the secondary acid are immediately neutralized by the potassium acetate, which also deprotonates the protonated forms of both the original sensitizing dye and the phenylated form of this dye to produce the corresponding unprotonated dyes, thereby removing the absorption in the first wavelength range due to the sensitizing dye. The decomposition of the superacid precursor is catalyzed by the copper compound, leading to the formation of 3.5 moles of phenylated image dye. 5.5 Moles of the secondary acid reversibly protonate and form a salt with the image dye. Both the phenylated and the protonated image dyes are colored. 1 Mole of the image dye remains in its unprotonated, leuco form. FIG. 1D shows graphically the 5.5 moles of secondary acid (3.5 moles of aryl cation are also formed) remaining in the exposed areas. 
     In the non-exposed areas, the potassium acetate deprotonates the sensitizing dye, returning it to its unprotonated form, and thus reducing the D min  of the image in this range (assuming, as is usual, that the absorption change in the image dye is an increase in absorption, i.e., increase in color, in the relevant wavelength range so that the non-exposed areas are the D min  areas of the image). The decomposition of the superacid precursor and the deprotonation of the sensitizing dye consumes 5 moles of potassium acetate; 1 mole of potassium acetate remains in the non-exposed areas. This excess of base is represented in FIG. 1D as -1 moles of remaining acid. None of the image dye is protonated, all remaining in its unprotonated, leuco form. The provision of the excess potassium acetate serves to ensure that, if a small amount of uncatalyzed thermal decomposition of the secondary acid generator does occur in non-exposed areas AB and CD during the heating step, the small amount of secondary acid resulting will be neutralized by base before the secondary acid can effect changes in the image dye, as described in more detail below. The excess potassium acetate also ensures that, if minor decomposition of the secondary acid generator does occur after the imaging process has been completed, the minor amounts of acid generated will be neutralized by the potassium acetate and thus will not affect image dye in the non-exposed areas of the final image. 
     It will be seen from Table 1 that the &#34;neutralization&#34; of the superacid and secondary acid, and deprotonation of the protonated sensitizing dye by the potassium acetate, produce acetic acid. Although acetic acid is normally regarded as an acid, it is insufficiently acidic in the polymeric binders in which the present process is normally carded out to protonate the sensitizing dye or the image dye, and is thus not regarded as an acid for present purposes. 
     From the foregoing description, it will be seen that, in the exposed areas, the superacid catalyzes the breakdown of the secondary acid generator, so that the final quantity of secondary acid present is substantially larger than the quantity of unbuffered superacid produced directly by the imagewise radiation acting on the superacid precursor, although of course the secondary acid is typically a weaker acid than the superacid itself. This &#34;chemical amplification&#34; of the superacid by the secondary acid generator increases the number of moles of acid generated per einstein of radiation absorbed, and thus increases the contrast of the image produced by the present process as compared with simple generation of superacid by a superacid precursor. In practice, it has been found that, under proper conditions, at least 20, and in some cases 100 or more, moles of secondary acid can be liberated for each mole of unbuffered superacid present in the exposed areas following the imagewise irradiation. 
     The first preferred deprotonation process described above is convenient for use with many combinations of indicator sensitizing dye and secondary acid generator. However, this first preferred process requires that the superacid generated during the imagewise exposure effect acid-catalyzed thermal decomposition of the secondary acid generator in the presence of the protonated sensitizing dye, and such acid-catalyzed decomposition requires that the superacid protonate the secondary acid generator. If the sensitizing dye is capable of further protonation, and the singly protonated sensitizing dye is significantly more basic than the secondary acid generator, any superacid generated above the threshold T during the imagewise exposure will tend to doubly protonate the sensitizing dye rather than protonate the secondary acid generator. If the doubly protonated sensitizing dye absorbs the wavelength used for the imagewise exposure and also causes decomposition of the superacid precursor, such double protonation of the sensitizing dye is not a serious problem; the imagewise exposure can be continued until the concentration of acid in the exposed areas exceeds 2T, at which point all the sensitizing dye is doubly protonated, and unbuffered superacid is again available to protonate the secondary acid generator. After this continued exposure, the acid level diagram is similar to that shown in FIG. 1B, except that the acid concentration in the exposed area BC exceeds 2T rather than T. The subsequent heating and base addition steps can be carried out exactly as described above with reference to FIGS. 1C and 1D. 
     If, however, the sensitizing dye can be doubly protonated, the singly protonated sensitizing dye is significantly more basic than the secondary acid generator, and the doubly protonated form of the sensitizing dye either does not absorb the wavelength used for the imagewise exposure, or does absorb this wavelength but does not cause decomposition of the superacid precursor following such absorption, continuing the imagewise exposure will result in the acid concentration increasing to, at most, 2T, at which point all the sensitizing dye is doubly protonated, and further acid generation ceases. The present process can be carried out with this type of sensitizing dye, but requires an additional step, as will now be described with reference to Table 2 and FIGS. 2A-2E. In view of the number of possible species in the second preferred process illustrated in Table 2 and FIGS. 2A 2E and the uncertainties as to, for example, the exact species which is phenylated by the phenyl radicals liberated upon decomposition of the superacid precursor, or which counterion or mixture of counterions is associated with a given protonated sensitizing dye species, for simplicity the various possible protonated species derived from the sensitizing dye are grouped together without regard to their degree of phenylation. Thus, for example the term &#34;doubly protonated S-DYE species&#34; used in Table 2 refers to all doubly protonated species derived from the sensitizing dye, whether those species are substituted with 0, 1 or 2 phenyl groups derived from the superacid precursor. 
     
                                           TABLE 2                                 
__________________________________________________________________________
EXPOSED AREA         NON-EXPOSED AREA                                     
Component        Moles                                                    
                     Component        Moles                               
__________________________________________________________________________
PRIOR TO USE                                                              
[S-DYE]          1   [S-DYE]          1                                   
Secondary acid generator                                                  
                 10  Secondary acid generator                             
                                      10                                  
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                 5   Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                     
                                      5                                   
AFTER INITIAL ULTRA-VIOLET EXPOSURE                                       
Singly protonated S-DYE species                                           
                 0.75                                                     
                     Singly protonated S-DYE species                      
                                      0.75                                
SbF.sub.6.sup.-  0.75                                                     
                     SbF.sub.6.sup.-  0.75                                
[S-DYE]          0.25                                                     
                     [S-DYE]          0.25                                
Secondary acid generator                                                  
                 10  Secondary acid generator                             
                                      10                                  
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                 4.25                                                     
                     Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                     
                                      4.25                                
AFTER IMAGEWISE VISIBLE EXPOSURE                                          
Doubly protonated S-DYE species                                           
                 0.75                                                     
                     Singly protonated S-DYE species                      
                                      0.75                                
Singly protonated S-DYE species                                           
                 0.25                                                     
                     SbF.sub.6.sup.-  0.75                                
SbF.sub.6.sup.-  1.75                                                     
                     [S-DYE]          0.25                                
Secondary acid generator                                                  
                 10  Secondary acid generator                             
                                      10                                  
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                 3.25                                                     
                     Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                     
                                      4.25                                
AFTER SECOND ULTRA-VIOLET EXPOSURE                                        
Doubly protonated S-DYE species                                           
                 1   Doubly protonated S-DYE species                      
                                      0.75                                
SbF.sub.6.sup.-  2   Singly protonated S-DYE species                      
                                      0.25                                
Secondary acid generator                                                  
                 10  SbF.sub.6.sup.-  1.75                                
HSbF.sub.6       0.75                                                     
                     Secondary acid generator                             
                                      10                                  
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                 2.25                                                     
                     Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                     
                                      3.25                                
AFTER HEATING                                                             
Doubly protonated S-DYE species                                           
                 1   Doubly protonated S-DYE species                      
                                      0.75                                
SbF.sub.6.sup.-  2   Singly protonated S-DYE species                      
                                      0.25                                
HSbF.sub.6       0.75                                                     
                     SbF.sub.6.sup.-  1.75                                
Secondary acid   10  Secondary acid generator                             
                                      10                                  
Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                          
                 2.25                                                     
                     Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                     
                                      3.25                                
AFTER FIXING                                                              
Unprotonated S-DYE species                                                
                 1   Unprotonated S-DYE species                           
                                      1                                   
HOAc             6   HOAc             0.75                                
Image dye/secondary acid salt                                             
                 6.75                                                     
                     Unprotonated Image dye                               
                                      10                                  
Unprotonated image dye                                                    
                 1   KOAc             1                                   
Ph-image dye/secondary acid salt                                          
                 2.25                                                     
                     Secondary acid generator                             
                                      10                                  
Cu reagent       1   Cu reagent       1                                   
KSBF.sub.6       5   KSBF.sub.6       5                                   
K/secondary acid salt                                                     
                 1   PhOAc            3.25                                
__________________________________________________________________________
 
    
     The second preferred deprotonation process of the invention shown in Table 2 begins in exactly the same manner as the first preferred deprotonation process described above with reference to Table 1; the same amounts of the same components are initially present in the imaging medium, and the same initial ultraviolet exposure is effected to generate 0.75 mole of buffered superacid throughout the imaging medium, thus converting 0.75 mole of sensitizing dye to its protonated form. Accordingly, FIG. 2A is essentially identical to FIG. 1A. However, in the next step, namely the imagewise exposure, the exposure is preferably continued until a further 1 mole of superacid precursor is decomposed by the sensitizing dye, so that 1.75 moles of acid are present in the acid-generating layer, as shown in FIG. 2B. If the sensitizing dye is of the type which is &#34;consumed&#34; by reaction with one molecule of superacid precursor, so that each mole of sensitizing dye can only generate one mole of superacid, the acid-generating reaction is self-limiting at this point. It is not essential that the imagewise exposure be continued to this point, since the only requirement for the later stages of the process is that, after the imagewise exposure, there be a substantial difference in acid concentration between the exposed and non-exposed areas. Accordingly, it may be convenient to continue the imagewise exposure to this point, thus rendering the imaging process less susceptible to temporal or areal variations in the radiation source used for the imagewise exposure. As shown in Table 2, at the conclusion of the imagewise exposure, the exposed areas of the acid-generating layer contain 0.75 mole of doubly protonated sensitizing dye species, 0.25 mole of singly protonated sensitizing dye species, 10 (unchanged) moles of secondary acid generator, and 3.25 moles of superacid precursor. (As in the first preferred process, obviously the imagewise exposure causes no change in the composition of the non-exposed areas of the medium.) 
     At this point in the process, all of the superacid is still buffered by the sensitizing dye and no unbuffered superacid is available to catalyze thermal decomposition of the secondary acid generator. Accordingly, the imagewise exposure is followed by a second blanket exposure with radiation in the second wavelength range, namely ultra-violet radiation in the process illustrated in Table 2 and FIGS. 2A-2E. As shown in FIG. 2C, this second ultra-violet exposure causes further decomposition of the superacid precursor and a uniform increase in acid concentration throughout the acid-generating layer. The amount of ultra-violet radiation applied in this blanket exposure is chosen so that, in the non-exposed areas, the acid concentration will not exceed the 2T threshold, so that all the acid will remain buffered by the sensitizing dye, but so that, in the exposed areas, the acid concentration will exceed the 2T threshold and unbuffered superacid will be present. FIG. 2C and Table 2 show the results obtained using a second ultra-violet exposure sufficient to decompose one mole of superacid precursor. Besides the unchanged secondary acid generator, the exposed areas of the acid-generating layer contain 1 mole of doubly protonated sensitizing dye species, 0.75 mole of unbuffered superacid and 2.25 moles of superacid precursor, while the non-exposed areas contain 0.75 mole of doubly protonated sensitizing dye species, 0.25 mole of singly protonated sensitizing dye species and 3.25 moles of superacid precursor. 
     The remaining two steps of the second preferred process are essentially identical to the corresponding steps of the first preferred process. First, the imaging medium is heated so that in the exposed areas the unbuffered superacid will catalyze the thermal decomposition of the secondary acid generator, thus converting the 10 moles of secondary acid generator to (in theory) 10 moles of secondary acid. In the non-exposed areas, however, since no unbuffered superacid is present, essentially no generation of secondary acid occurs. FIG. 2D shows the result of the heating step graphically. Finally, the components of the acid-generating and color-change layers become intermixed. As in the first preferred process described above with reference to Table 1, Table 2 assumes that the color-change layer contains 10 moles of an indicator image dye, 1 mole of copper compound, 1 mole of a reducing agent (the products produced by oxidation of this reducing agent are again ignored in Table 2 for simplicity) and 6 moles of a base, shown as potassium acetate in Table 2. Table 2 also again assumes that the image dye is more basic than the sensitizing dye; the contrary case is discussed below. In the non-exposed areas, the copper, the reducing agent and the reactive material decompose all remaining superacid precursor, with generation of phenyl acetate (Table 2 assumes), phenyl iodide (omitted from Table 2) and potassium hexafluoroantimonate. In the exposed areas, the potassium acetate is protonated by the superacid and by some of the secondary acid. The copper reagent catalyzes decomposition of the remaining superacid precursor with the formation of phenyl cations, which react with the most nucleophilic species remaining, here assumed to be the image dye. (In practice, the decompostion of the superacid precursor is probably somewhat more complicated and other products may be produced; however, the exact type of decomposition products produced does not affect the essential nature of the present process.) 
     In the exposed areas, as in the first preferred process, the unbuffered superacid and 3.25 moles of the secondary acid are immediately neutralized by the potassium acetate, which also deprotonates all protonated forms of the sensitizing dye to produce the corresponding unprotonated forms. The decomposition of the superacid precursor is catalyzed by the copper compound, leading to the formation of 2.25 moles of phenylated image dye. 6.75 Moles of secondary acid reversibly protonate and form a salt with the image dye. Both the phenylated and the protonated image dyes are colored. 1 Mole of image dye remains unprotonated and in its leuco form. 
     In the non-exposed areas, the potassium acetate deprotonates all protonated forms of the sensitizing dye to produce the corresponding unprotonated forms, and 1 mole of potassium acetate remains (this free base is shown as -1 moles of acid if FIG. 2E), so that all the image dye stays in its unprotonated, leuco form and no color change occurs. 
     One advantage of the present deprotonation process is that, at least in many preferred embodiments of the invention, it is possible to compensate for any premature breakdown of the superacid precursor which may occur before use of the imaging medium. Such premature breakdown may occur, for example, by exposure of the imaging medium to radiation during transportation and storage or because the mixture of the superacid precursor and the sensitizing dye in the acid-generating layer undergoes slow decomposition on protracted storage. If, as in the two preferred processes described above, the first step of the process is blanket exposure of the imaging medium to radiation in the second wavelength range to generate superacid and convert the sensitizing dye to its protonated (first) form, the blanket exposure can be adjusted to ensure that the present process works properly, even if some decomposition of the superacid precursor has occurred earlier. 
     For example, to take an extreme case purely for purposes of illustration, suppose that the imaging medium shown in Table 1 is exposed to so much ultra-violet radiation during storage and transport that premature breakdown of 0.5 mole of superacid precursor occurs. At the beginning of imaging, all areas of the medium thus contain 0.5 mole of sensitizing dye, 10 moles of secondary acid generator, 4.5 moles of superacid precursor and 0.5 mole of protonated sensitizing dye. After spectral analysis to determine the amount of protonated sensitizing dye already present, the initial ultra-violet exposure may be adjusted so that, in exposed areas, only a further 0.25 mole of superacid precursor is decomposed. After this exposure, the medium will contain 0.75 mole of protonated sensitizing dye, and will thus be in exactly the same condition as the medium used in the first preferred process described above (at the stage represented in FIG. 1A), in which no premature breakdown of the superacid precursor occurred before imaging, but the initial ultra-violet exposure generated 0.75 mole of superacid. Also, provided that no substantial breakdown of superacid precursor occurs during transportation and storage, a deprotonation medium of the present invention which is produced with the sensitizing dye in its unprotonated form is, prior to imaging, essentially insensitive to radiation of the wavelength used for the imagewise exposure, since the unprotonated sensitizing dye, even when exposed to such radiation, does not cause substantial decomposition of the superacid precursor. 
     For similar reasons, all variants of the present process are relatively insensitive to variations in the radiation used for the imagewise exposure, such as variations in laser output, differences between individual lasers in an array used to form the imaging beam, timing errors in laser drivers, etc. For example, in the process shown in Table 1, the imagewise exposure causes decomposition of 0.75 mole of superacid precursor. If the imaging radiation delivered to the imaging medium varies by ±20%, some exposed areas will experience decomposition of 0.6 mole of superacid precursor, while others will experience decomposition of 0.9 mole. Thus, after the imagewise exposure, the concentration of unbuffered superacid in the exposed areas will vary from 0.35 to 0.65 mole. With appropriate control of the heating step, this range of variation in unbuffered superacid concentration will have minimal effects on the final image in cases where the medium is designed to be essentially binary, i.e., any specific pixel is either at D min  or at D max . 
     From Tables 1 and 2 and the related description above, it will be seen that, after the present medium has been imaged and fixed, in both the exposed and non-exposed areas the sensitizing dye has been returned to its unprotonated form. This is always the case in the non-exposed areas, and is also the case in the exposed areas if the image dye is substantially more basic than the sensitizing dye. If this is not so, in the exposed areas the sensitizing dye will remain protonated (or possibly phenylated) and the absorption in the first wavelength range is a combination of that due to the protonated (or phenylated) image dye and that due to the protonated (or phenylated) sensitizing dye. In such cases, the sensitizing dye should be chosen so that the presence of its protonated (or phenylated) form in the D max  areas does not cause objectionable effects on the image. This is especially important in color media having a plurality of different acid-generating layers and color-change layers since if, for example, the protonated (or phenylated) form of the sensitizing dye used in the acid-generating layer associated with the magenta color-change layer has a yellow color, crosstalk will result between the magenta and yellow components of the image. To reduce or eliminate such objectionable effects, it is desirable that the protonated (or phenylated) form of the sensitizing dye have a color similar to that of the colored form of the associated image dye. Sometimes it may be possible to use the same (or a chemically similar) dye as both the sensitizing dye and the image dye. 
     The potential problem discussed in the preceding paragraph is confined to the deprotonation process of the present invention. However, a related problem occurs in the nucleophile process of the invention if the nucleophile is more basic than the image dye. 
     The sensitizing dye used in the deprotonation process of the present invention may be any molecule, the absorption spectrum of which depends reversibly upon its state of protonation and which can cause decomposition of the superacid precursor used, provided of course that the dye is compatible with the other components of the imaging medium. The state of the sensitizing dye called herein the &#34;unprotonated form&#34; need not necessarily be a neutral molecule; the unprotonated form may be anionic but capable of being protonated to a neutral or cationic form. For example, fluorescein monomethyl ether can exist in a non-aqueous medium in anionic (deprotonated), neutral or cationic (protonated) forms; both the anionic and cationic forms are yellow, while the neutral form is colorless to the eye but absorbs strongly in the mid-ultra-violet region (around 320 nm). The spectral shift of the sensitizing dye upon protonation may be either hypsochromic (to shorter wavelength) or bathochromic (to longer wavelength). Fluorescein monomethyl ether exhibits both behaviors; the first protonation of the anionic form causes a hypsochromic shift, while the second protonation to the cationic form causes a bathochromic shift. 
     Preferred indicator sensitizing dyes for use in the deprotonation process include fluoran dyes, phthalide dyes, xanthene dyes, acridine dyes, hydroxypyrylium dyes, hydroxythiopyrylium dyes, styrylpyridinium dyes, styrylquinolinium dyes, and other substituted quinolinium, isoquinolinium and pyridinium dyes, with triarylpyridinium, quinolinium and xanthene dyes being especially preferred. Specific triarylpyridinium dyes which have been found useful in the present invention are protonated forms of: 
     2,4,6-tris(4-methoxyphenyl)pyridine; 
     2,6-bis(4-methoxyphenyl)-4-(2-thienyl)pyridine; 
     2,6-bis(4-methoxyphenyl)-4-(2-(4-bromophenyl)pyridine; 
     2,6-bis(4-methoxyphenyl)-4-(2-naphthyl)pyridine; 
     2,4-bis(4-methoxyphenyl)-6-(2-naphthyl)pyridine; 
     2,4,6-tris(2,4,6-trimethoxyphenyl)pyridine; and 
     2,6-bis(4-methoxyphenyl)-4-(2-(1,4-dimethoxy)naphthyl)pyridine. 
     A specific preferred triarylpyridinium dye is the protonated form of 2,4,6-tris-(2,4-dimethoxyphenyl)pyridine. 
     A specific preferred quinolinium dye is the protonated form of 2-[2-[2,4 bis[octyloxy]phenyl]ethen-1-yl]quinoline (the unprotonated form of this dye is available from Yamada Chemical Co., Kyoto, Japan), while a specific preferred xanthene dye is the protonated form of 3&#39;,6&#39;-bis[N-[2-chlorophenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9&#39;-(9H)xanthene]] (which may be prepared as described in U.S. Pat. No. 4,345,017). 
     Methods for the preparation of triarylpyridinium dyes are described in the literature. One convenient method for the preparation of such dyes beating identical substituents at the 2- and 6-positions is described in Weiss, J. Am. Chem. Soc., 74, 200 (1952) and comprises heating a mixture of an acetophenone, an aldehyde (that containing the desired 4-substituent) and ammonium acetate in acetic acid. A dihydropyfidine is produced as the penultimate intermediate, but is oxidized to the pyridine by the intermediate chalcone. A second method is similar to the first, but uses hydroxylamine or unsymmetrical dimethylhydrazine in place of ammonium acetate; the penultimate intermediate in these cases are the N-hydroxydihydropyridine or N,N-dimethylaminodihydropyridine, which undergo elimination and aromatization without the need for an oxidizing agent. A third method, described in Krohnke, Synthesis, 1976, 1, can produce asymmetric triarylpyridinium dyes. In this third method, an aryl aldehyde containing the desired 4 substituent and an acetophenone containing the desired 2-substituent are reacted to form an intermediate chalcone, which is then reacted with the phenacylpyridinium salt derived from the acetophenone containing the desired 6-substituent. The resultant pyridinium-substituted dihydropyridine undergoes loss of pyridine with aromatization. All three methods are illustrated in Examples 1-3 below. 
     The prior art describes various combinations of nucleophiles and sensitizing dyes which can be used in the nucleophile process of the present invention; see, for example, U.S. Pat. Nos. 5,258,274 and 5,314,795 (although note that in the present process the imaging medium may contain the nucleophile itself rather than a nucleophile-generating species as in these patents, since the nucleophile can be kept in a layer or phase separate from the acid-generating layer until the final heating step when the nucleophile converts the sensitizing dye to its second form). The nucleophile used in the present process may be a neutral molecule, for example a primary or secondary amine, a stabilized carbanion, for example a carbanion derived from a malonate ester or a nitroalkane, or a charged nucleophile, for example a thiolate. 
     The preferred sensitizing dyes for use in the nucleophile process are hemicyanine dyes; the term &#34;hemicyanine dyes&#34; is used herein its broad sense as meaning any dye in which a quaternary nitrogen-containing heterocycle is in charge resonance with an electron-donating moiety within the same molecule. Hemicyanine dyes which may be used in the present process include those described in the aforementioned U.S. Pat. Nos. 5,258,274 and 5,314,795; preferred hemicyanine dyes are those of the formula: ##STR1## wherein: 
     G is a CR c  R d  group, a CR c  =R d  group, an oxygen or sulfur atom, or an NR b  group; 
     R a  and R b  are each an alkyl group containing from about 1 to about 20 carbon atoms; 
     R c  and R d  are each a hydrogen atom or an alkyl group containing from about 1 to about 20 carbon atoms; 
     n is 1 or 2; 
     Ar is an aryl or heterocyclyl group; and 
     X&#39; is an anion. 
     The anion X of the dye should be chosen with care having regard to the superacid which will be generated during the imaging process. For example, it is inadvisable to use iodide, or another anion derived from a weak acid, as the anion of the dye, since the presence of such an anion in the acid-generating layer during imaging will cause the superacid generated to protonate the anion, thus leading to the formation of HI, or an acid which is similarly weak in a polymeric medium of low dielectric constant (such as those typically used in the imaging media of the present invention), which cannot effectively protonate the secondary acid generator, and thus does not initiate the acid amplification process. Conveniently, the anion X is chosen to be the same as that of the superacid precursor; thus, for example, when the preferred superacid precursor diphenyliodonium hexafluoroantimonate is used, the anion X is conveniently hexafluoroantimonate. Specific preferred hemicyanine sensitizing dyes which have been found useful in the present nucleophile process include: 
     1-methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinolinium hexafluoroantimonate; 
     1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinolinium hexafluoroantimonate; 
     3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indolium hexafluoroantimonate; and 
     3,3-dimethyl -1-methyl-2-[2-[9-ethylcarbazol-3-yl]ethen-1-yl]-3H-indolium hexafluoroantimonate. 
     In the present process, it is desirable that the layer or phase containing the sensitizing dye also comprise a cosensitizer which is a reducing agent less basic than the secondary acid generator, since it has been found that the presence of such a cosensitizer greatly improves the quantum efficiency of the reaction between the photoexcited sensitizing dye and the superacid precursor (i.e., the quantum efficiency of superacid generation and thus the sensitivity of the imaging medium). Preferred cosensitizers include triarylamines (for example, triphenylamine) and hydroquinones. 
     Since the present process relies upon the production of unbuffered superacid, it is highly desirable that the process be conducted under essentially anhydrous conditions; as chemists are well aware, the most powerful acidic species that can exist in the presence of more than one equivalent of water is the hydroxonium (hydronium) ion, [H 3  O] + . Accordingly, if the medium in which the present process is conducted contains water, at least part of the superacid produced by the present process will simply generate hydroxonium ion. However, in the absence of water, the superacid yields an acidic species much stronger than hydroxonium ion, and this acidic species can effect the acid-catalyzed decomposition of various secondary acid generators which hydroxonium ion cannot. Typically, the present process is carded out with the superacid precursor and the sensitizing dye dispersed in a polymeric binder, and such binders can readily be chosen to provide an essentially anhydrous environment for the process. 
     In its most general form, the present process requires only that, following the formation of the imagewise distribution of the secondary acid, the sensitizing dye be converted to its second form in at least the non-exposed areas of the medium. Although, at least in theory, the appropriate reagent for effecting this conversion could be supplied from an external source (for example, by spraying a solution of the reagent on to the exposed medium), for practical reasons it is normally desirable to have the reagent present in the imaging medium prior to exposure, so that the imaging medium provides a self-contained imaging system not requiring the use of liquid reagents. Usually, it is convenient to have the reagent capable of converting the first form of the sensitizing dye to its second form present in the color-change layer or phase of the imaging medium, so that the mixing of the components of the acid-generating and color-change layers or phases introduces the reagent into the non-exposed areas of the medium, thus converting the sensitizing dye to its second form. However, there are certain applications of the invention where it may be desirable to include the reagent in a layer of the imaging medium other than the color-change layer. In particular, for practical reasons, it is generally convenient to prepare imaging media of the invention by first depositing an acid-generating layer from a non-aqueous medium on to a support, and then to deposit a color-change layer from an aqueous medium on to the acid-generating layer. If this process is used to prepare a &#34;dupe&#34; film for use in the graphic arts industry, it is necessary that the medium be exposed from the color-change layer side, since exposing through the relatively thick support may lead to a loss in resolution. However, exposing through the color-change layer may require that the image dye (which in a write-white medium such as dupe film must be colored in base and essentially colorless in acid) be present in its colorless form during the exposure, since otherwise it may absorb the radiation intended for exposure of the underlying acid-generating layer. To ensure that the image dye is coated in a form which does not prevent imaging by absorbing the imaging radiation, a medium of this type conveniently comprises: 
     (a) a color-change layer comprising an image dye which has substantial absorption (i.e., is &#34;colored,&#34; where the &#34;color&#34; in question will typically be in the near ultra-violet) in base but has low absorption in acid, the image dye being present in its protonated (colorless) form; 
     (b) an acid-generating layer; and 
     (c) a layer containing a conversion reagent (typically a base) interposed between the acid-generating layer and the color-change layer, the quantity of the conversion reagent being sufficient to cause coloration of all the image dye in non-exposed regions of the medium after heating, plus any additional amount necessary for fixation. 
     In principle, in the present process the mixing of the components of the acid-generating and color-change layers should be effected after the generation of the secondary acid from the secondary acid generator. However, in practice both the generation of the secondary acid in the acid-generating layer and the mixing of the components of the two layers may be effected in a single heating step, since the superacid-catalyzed decomposition of the secondary acid generator will typically be essentially complete before mixing of the two layers becomes significant. 
     Obviously, it is important that the components of the acid-generating layer and the color-change layer not mix prematurely. In practice, as already noted, the present imaging medium will typically be formed by coating both layers from a solution or dispersion on to a support. To prevent premature mixing, it is usually desirable to coat one layer from an aqueous medium and the other from a non-aqueous medium. Typically, the acid-generating layer is coated from an organic medium and the color-change layer from an aqueous medium. 
     Any of the known superacid precursors, for example diazonium, phosphonium, sulfonium and iodonium compounds, may be used in this invention, but iodonium compounds are preferred. Especially preferred superacid precursors are diphenyliodonium salts, specifically (4-octyloxyphenyl)phenyliodonium hexafluorophosphate and hexafluoroantimonate, bis(n-dodecylphenyl)iodonium hexafluoroantimonate and (4-(2-hydroxytetradecan-1-yloxy)phenyl)phenyl iodonium hexafluoroantimonate. 
     Any secondary acid generator capable of superacid-catalyzed breakdown to give a secondary acid may be used in the present process. One preferred group of secondary acid generators is 3,4-disubstituted-cyclobut-3-ene-1,2-diones (hereinafter for convenience called &#34;squaric acid derivatives&#34;) capable of generating squaric acid or an acidic derivative thereof, since squaric acid and its acidic derivatives are strong acids well suited to effecting color changes in acid-sensitive materials. Especially preferred squaric acid derivatives are those in which there is bonded to the squaric acid ring, via an oxygen atom, an alkyl or alkylene group, a partially hydrogenated aryl or arylene group, or an aralkyl group. The acid-catalyzed decomposition of these squaric acid derivatives causes replacement of the original alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group of the derivative with a hydroxyl group, thus producing squaric acid or an acidic squaric acid derivative having one hydroxyl group. 
     The exact mechanism by which squaric acid or an acidic derivative thereof is formed in the present process may vary depending upon the type of squaric acid derivative employed. In some cases, for example di-t-butyl squarate, one or both groups attached via oxygen atoms to the squaric acid ring may thermally decompose to yield an alkene or arene, thus converting an alkoxy or aryloxy group to a hydroxyl group and forming the squaric acid or acidic derivative thereof. In other cases, for example 3-amino-4-(p vinylbenzyloxy)cyclobut-3-ene-1,2-dione, there is no obvious mechanism for formation of a corresponding alkene or arene, and it appears that the mechanism of acid formation is migration of the vinylbenzyl carbocation or similar group to a different position within the molecule (probably to the amino group), and protonation of the remaining oxygen atom to form a hydroxyl group at the position from which the group migrates. In other cases, neither of these pathways is possible. However, in all cases the net effect is the replacement of the alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group present in the original derivative with a hydroxyl group to form squaric acid or an acidic derivative thereof. 
     Those skilled in the art of organic chemistry will appreciate that the susceptibility to thermal decomposition of the squaric acid derivatives preferred for use in the present process is related to the stability of the cation produced from the ester grouping during the decomposition process. Although the stability of specific cations may be influenced by a variety of factors, including steric factors, which may be peculiar to a particular ester, in general it may be said that the squaric acid esters preferred for use in the present process are: 
     (a) primary and secondary esters of squaric acid in which the α-carbon atom (i.e., the carbon atom bonded directly to the -O- atom of the squarate ring) bears a non-basic cation-stabilizing group. This cation-stabilizing group may be, for example, an sp 2  or sp hybridized carbon atom, or an oxygen atom; 
     (b) tertiary esters of squaric acid in which the α-carbon atom does not have an sp 2  or sp hybridized carbon atom directly bonded thereto; and 
     (c) tertiary esters of squaric acid in which the α-carbon atom does have an sp 2  or sp hybridized carbon atom directly bonded thereto, provided that this sp 2  or sp hybridized carbon atom (or at least one of these sp 2  or sp hybridized carbon atoms, if more than one such atom is bonded directly to the α-carbon atom) is conjugated with an electron-withdrawing group. 
     It will be apparent to skilled organic chemists that, provided one of the aforementioned types of ester groupings is present in the squaric acid derivative to produce one hydroxyl group after thermal decomposition, the group present in place of the other hydroxyl group of squaric acid is of little consequence, provided that this other group does not interfere with the thermal decomposition. Indeed, the wide variation possible in this other group has the advantage that this group can be varied to control other properties of the derivative, for example its compatibility with other components of the imaging medium, or its solubility in solvents used to form coating solutions used in the preparation of the imaging medium. 
     Examples of squaric acid derivatives useful in the present processes include: 
     (a) those of the formula: ##STR2## in which R 1  is an alkyl group, a partially hydrogenated aromatic group, or an aralkyl group, and R 2  is a hydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino, alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino, dialkylphosphoxy or trialkylsilyl group, subject to the proviso that either or both of the groups R 1  and R 2  may be attached to a polymer. Among the derivatives of Formula I, especially preferred groups are those in which (a) R 1  is an unsubstituted or phenyl substituted alkyl group containing a total of not more than about 20 carbon atoms, and R 2  is an alkyl group containing not more than about 20 carbon atoms, or a phenyl group (which may be substituted or unsubstituted); and (b) R 1  is a benzyl group and R 2  is an amino group. 
     (b) those of the formula: ##STR3## in which R 1  and R 3  independently are each an alkyl group, a partially hydrogenated aryl group or an aralkyl group, subject to the proviso that either or both of the groups R 1  and R 3  may be attached to a polymer. Among the derivatives of Formula II, an especially preferred group are those in which R 1  and R 3  are each independently an unsubstituted or phenyl substituted alkyl group containing a total of not more than about 20 carbon atoms. Specific preferred compounds of Formula II are those in which R 1  and R 3  are each a tertiary butyl group, a benzyl group, an α-methylbenzyl group or a cyclohexyl group, namely di-tertiary butyl squarate, dibenzyl squarate, bis(α-methylbenzyl) squarate and dicyclohexyl squarate. 
     (c) those of the formula: ##STR4## in which n is 0 or 1, and R 4  is an alkylene group or a partially hydrogenated arylene group. Among the derivatives of Formula III, an especially preferred group are those in which n is 1 and R 4  is an alkylene group containing not more than about 12 carbon atoms. 
     (d) those having at least one unit of the formula: ##STR5## in which n is 0 or 1, and R 5  is an alkylene or partially hydrogenated arylene group. Besides the fragmentable groups R 5 , the compounds may also contain one or more units in which a non-fragmentable group is attached to a squarate ring, directly or via an oxygen atom. 
     The squaric acid derivatives of Formula IV include not only high polymers, but also dimers, trimers, tetramers, etc. including at least one of the specified units. The terminating groups on the derivatives of Formula IV may be any of groups OR 1  or R 2  discussed above with reference to Formula I. Thus, for example, Formula IV includes the squaric acid dimer derivative of the formula: ##STR6## 
     The squaric acid derivatives of Formulae I and II are usually monomeric. However, these derivatives of Formulae I and II can be incorporated into polymers by having at least one of the groups R 1 , R 2  and R 3  attached to a polymer. Attachment of the squaric acid derivatives to a polymer in this manner may be advantageous in that it may avoid incompatibility and/or phase separation that might occur between a monomeric squaric acid derivative of Formula I or II and a polymeric binder needed in an imaging medium. 
     The attachment of the groups R 1 , R 2  and R 3  to a polymer may be effected in various ways, which will be familiar to those skilled in the art of polymer synthesis. The squaric acid derivatives may be incorporated into the backbone of a polymer, for example in a polymer similar to the dimer of the formula given above. Alternatively, the squaric acid derivatives may be present as sidechains on a polymer; for example, one of the groups R 1 , R 2  and R 3  could contain an amino group able to react with a polymer containing carboxyl groups or derivatives thereof to form an amide linkage which would link the square acid derivative as a sidechain on to the polymer, or these groups may contain unsaturated linkages that enable the squaric acid derivatives to be polymerized, either alone or in admixture with other unsaturated monomers. 
     In the present process, it is generally undesirable to form substantial quantities of gas during the acid-catalyzed decomposition of the secondary acid generator since such gas may distort the imaging medium or form vesicles therein, and such distortion or vesicle formation may interfere with proper image formation. Accordingly, if the decomposition of the squaric acid derivative yields an alkene, it is desirable that the groups R 1 , R 3 , R 4  and R 5  be chosen so that this alkene is a liquid at 20° C., and preferably higher, since some heating of the alkene will inevitably occur during the acid-catalyzed decomposition. Sometimes, however, the alkene liberated may be sufficiently soluble in the medium containing the squaric acid derivative that liberation of a highly volatile alkene will not result in distortion of, or vesicle formation in, the medium. 
     Another preferred group of secondary acid generators for use in the present process are oxalic acid derivatives that undergo acid-catalyzed breakdown to give oxalic acid or an acidic derivative thereof, for example an oxalic acid hemiester. Although oxalic acid and its acidic derivatives are not quite such strong acids as squaric acid and its acidic derivatives, oxalic acid and its derivatives are sufficiently strong acids for use with most image dyes. Also, oxalic acid derivatives are, in general, less costly than squaric acid derivatives. 
     The types of oxalic acid derivatives preferred for use in the present process are rather more diverse in structure than the squaric acid derivatives, and the choice of oxalic acid derivative for any specific process may be governed more by the thermal breakdown properties of the derivative than its exact chemical structure; in general, for practical reasons such as the limited temperature range to which other components of the imaging medium may safely be exposed, it is preferred that the oxalic acid derivative be one which begins to decompose thermally at a temperature in the range of about 140° to about 180° C., as measured by differential scanning calorimetry in a nitrogen atmosphere at a 10° C./minute temperature ramp, in the absence of any catalyst. Since the presence of an acid catalyst lowers the thermal decomposition temperature of oxalic acid derivatives by at least about 20° C. and potentially significantly more, derivatives which decompose uncatalyzed at about 140° to about 180° C., will, in the presence of acid, decompose at temperatures as low as about 65° C., temperatures to which other components of the imaging medium can in general be exposed. 
     The factors affecting the ability of the oxalic acid derivatives to undergo acid-catalyzed thermal decomposition are similar to those affecting the ability of the aforementioned squaric acid derivatives to undergo the same reaction, and thus the preferred ester groups are of the same types. Accordingly, preferred oxalic acid derivatives for use in the present process include: 
     (a) primary and secondary esters of oxalic acid in which the α-carbon atom (i.e., the carbon atom bonded directly to the -O- atom of the oxalate grouping) bears a non-basic cation-stabilizing group. This cation-stabilizing group may be, for example, an sp 2  or sp hybridized carbon atom, or an oxygen atom; 
     (b) tertiary esters of oxalic acid in which the α-carbon atom does not have an sp 2  or sp hybridized carbon atom directly bonded thereto; and 
     (c) tertiary esters of oxalic acid in which the α-carbon atom does have an sp 2  or sp hybridized carbon atom directly bonded thereto, provided that this sp 2  or sp hybridized carbon atom (or at least one of these sp 2  or sp hybridized carbon atoms, if more than one such atom is bonded directly to the α-carbon atom) is conjugated with an electron-withdrawing group. 
     (d) an ester formed by condensation of two moles of an alcohol with the bis(hemioxalate) of a diol, provided that the ester contains at least one ester grouping of types (a), (b) or (c) above. One example of an ester of this type is that of the structure: ##STR7## which can be regarded as formed from two moles of menthol (2-methylethyl-4-methylcyclohexanol) and one mole of the bis(hemioxalate) of 1,6-bis-(4-hydroxymethylphenoxy)hexane. Since the structure of the central residue of the diol in such esters can vary widely, the solubility and other properties of the esters can be &#34;tuned&#34; as required for compatibility with other components of the imaging medium, while the nature of the end groups, which undergo the acid-forming thermal decomposition, can be varied independently of the nature of the central residue. 
     (e) polymeric oxalates derived from polymerization of oxalate esters having an ethylenically unsaturated group, provided that the ester contains at least one ester grouping of type (a), (b) or (c) above. As with the squaric acid derivatives discussed above, use of a polymeric oxalate rather than a monomeric one may be advantageous in that it may avoid incompatibility and/or phase separation that might occur between a monomeric derivative and a polymeric binder needed in an imaging medium. Use of a polymeric derivative also tends to inhibit diffusion of the oxalate through the imaging medium during storage before imaging. Although polymeric oxalates can be formed in other ways, at present we prefer to form such oxalates by first forming an oxalate ester in which one of the ester groupings comprises an ethylenically unsaturated group, and then polymerizing this ester using a conventional free radical polymerization initiator, for example azobis(isobutyronitrile) (AIBN). The ethylenically unsaturated group is conveniently an acrylate or methacrylate group, while the other ester grouping in the monomeric oxalate can be any of the types discussed above. 
     (f) Condensation polymers of oxalates, provided that the ester contains at least one ester grouping of type (a), (b) or (c) above. This type of polymer also possesses the advantages discussed under (e) above. 
     Methods for the synthesis of the preferred secondary acid generators described above are given in the aforementioned U.S. Pat. No. 5,286,612, application Ser. No. 08/141,852, and International Application No. PCT/US93/10215. 
     The image dye used in the present invention may be any material that undergoes a color change in the presence of the secondary acid. Thus any conventional indicator dye may be used as the acid-sensitive material, as may the leuco dyes disclosed in U.S. Pat. Nos. 4,602,263; 4,720,449 and 4,826,976, which are also sensitive to acid. 
     As will be apparent to those skilled in the imaging art, a medium of the present invention which contains a plurality of color-change layers (for example, a full color medium containing three or four color-change layers) need not use the present process in all of the color-change layers; one or more of the color-change layers can use the present process, while the other color-change layer(s) use other color-forming mechanisms, for example the &#39;612 process or conventional sensitization of superacid precursors with non-basic polycyclic aromatic sensitizers. For example, the specific preferred embodiment of the invention described below with reference to FIG. 3 uses two acid-generating and two color-change layers using the present process, and one acid-generating and color-change layer using a conventional non-basic polycyclic aromatic sensitizer. 
     It should be noted that the present process may allow the use of combinations of superacid precursor and indicator sensitizing dye in which the combination of the precursor and the absorbing (i.e., first) form of the sensitizing dye is unstable on long term storage. Provided that the combination of the precursor and the non-absorbing (i.e., second) form of the sensitizing dye is stable on long term storage, the relevant acid-generating layer can be prepared with the dye in its second (or other precursor) form, and the first form of the dye produced immediately before the imagewise exposure; in a deprotonation process, the conversion of the second form of the sensitizing dye to its first form is conveniently accomplished by exposing the imaging medium to actinic radiation effective to generate superacid within the acid-generating layer, as described above with reference to Table 1. After the imagewise exposure, the conversion of the sensitizing dye to its second form in accordance with the present invention and/or fixing effected by destruction of the remaining superacid precursor will ensure that the unstable combination of the superacid precursor and the first form of the sensitizing dye is not present after imaging. 
     Preferred uses of the present process include: 
     (a) the use of visible imagewise exposure to produce a visible image, which may be positive or negative; 
     (b) a true- or false-sensitized full color image exposed at three different wavelengths (for example, a print paper) 
     (c) the use of near infra-red (700-1200 nm) radiation to produce a visible image having good D min  when viewed in reflection (in this process, the first from of the sensitizing dye has a near infra-red absorption peak and the second form of the dye has a substantially lower visible absorption than the first form); 
     (d) the use of ultra-violet exposure to form an ultra-violet photomask; and 
     (e) the formation of a full color image using a single source (preferably a laser) at a single visible or near infra-red wavelength to effect imagewise exposure of all three acid-generating layers of the medium. 
     Process (e) above conveniently uses a deprotonation process of the invention with an imaging medium having three associated pairs of acid-generating layers and color-change layers (each pair comprising an acid-generating layer and a color-change layer may hereinafter be called a &#34;bilayer&#34;), with each adjacent pair of bilayers being separated by an acid-impermeable interlayer. This type of imaging medium comprises: 
     a first acid-generating layer comprising a sensitizing dye in its protonated form, optionally a cosensitizer, a superacid precursor and a secondary acid generator; 
     a first color-change layer disposed adjacent the first acid-generating layer and comprising a reactive material, a copper compound and a first image dye undergoing a change in its absorption of radiation upon contact with the secondary acid generated upon acid-catalyzed decomposition of the secondary acid generator in the first acid-generating layer; 
     a first acid-resistant interlayer superposed on the first acid-generating layer and the first color-change layer; 
     a second acid-generating layer disposed on the opposed side of the first acid-resistant interlayer from the first acid-generating layer and the first color-change layer, the second acid-generating layer comprising a sensitizing dye in its unprotonated form, optionally a cosensitizer, a superacid precursor and a secondary acid generator, the second acid-generating layer further comprising a first auxiliary sensitizer which renders the superacid precursor therein susceptible to decomposition by actinic radiation of a first wavelength in the second wavelength range, but not susceptible to decomposition by actinic radiation of a second wavelength in the second wavelength range; 
     a second color-change layer disposed adjacent the second acid-generating layer and on the opposed side of the first acid-resistant interlayer from the first acid-generating layer and the first color-change layer, the second color-change layer comprising a reactive material, a copper compound and a second image dye undergoing a change in its absorption of radiation upon contact with the secondary acid generated upon acid-catalyzed decomposition of the secondary acid generator in the second acid-generating layer, the absorption change undergone by the second image dye being different from that undergone by the first image dye; 
     a second acid-resistant interlayer disposed on the opposed side of the second acid-generating layer and second color-change layer from the first acid-resistant interlayer; 
     a third acid-generating layer disposed on the opposed side of the second acid-resistant interlayer from the second acid-generating layer and second color-change layer, the third acid-generating layer comprising a sensitizing dye in its unprotonated form, optionally a cosensitizer, a superacid precursor and a secondary acid generator, the third acid-generating layer further comprising a second auxiliary sensitizer which renders the superacid precursor therein susceptible to decomposition by actinic radiation of the second wavelength in the second wavelength range, but not susceptible to decomposition by actinic radiation of the first wavelength in the second wavelength range; and 
     a third color-change layer disposed adjacent the third acid-generating layer and on the opposed side of the second acid-resistant interlayer from the second acid-generating layer and the second color-change layer, the third color-change layer comprising a reactive material, a copper compound and a third image dye undergoing a change in its absorption of radiation upon contact with the secondary acid generated upon acid-catalyzed decomposition of the secondary acid generator in the third acid-generating layer, the absorption change undergone by the third image dye being different from those undergone by the first and second image dyes. Very conveniently, in this preferred form of imaging medium, the same sensitizing dye, superacid precursor and secondary acid generator are present in each of the three acid-generating layers. If the image is to be fixing, the same fixed reagent is also preferably used in each of the three color-forming layers. 
     This type of imaging medium is imaged in the following manner. First, the medium is imagewise exposed, from the surface closer to the third acid-generating layer, to actinic radiation in the first wavelength range, thereby causing, in the exposed areas of the first acid-generating layer or phase, the sensitizing dye to decompose at least part of the superacid precursor, with formation of unbuffered superacid in the first acid-generating layer, without substantial production of unbuffered superacid in the second and third acid-generating layers. Thereafter, the whole imaging medium is exposed to radiation of the first wavelength in the second wavelength range, thus decomposing part of the superacid precursor in the second acid-generating layer to produce superacid and convening at least part of the sensitizing dye in the second acid-generating layer to its protonated form, without substantial production of superacid in the third acid-generating layer. The medium is then imagewise exposed to actinic radiation in the first wavelength range, thus causing, in the exposed areas of the second acid-generating layer or phase, the sensitizing dye to decompose at least part of the superacid precursor, with formation of unbuffered superacid in the second acid-generating layer, without substantial production of unbuffered superacid in the first and third acid-generating layers. Thereafter, the whole imaging medium is exposed to radiation of the second wavelength in the second wavelength range, thus decomposing part of the superacid precursor in the third acid-generating layer to produce superacid and converting at least part of the sensitizing dye in the third acid-generating layer to its protonated form. The medium is then imagewise exposed to actinic radiation in the first wavelength range, thus causing, in the exposed areas of the third acid-generating layer or phase, the sensitizing dye to decompose at least part of the superacid precursor, with formation of unbuffered superacid in the third acid-generating layer, without substantial production of unbuffered superacid in the first and second acid-generating layers. The last two stages of the imaging process are heating the medium to cause, in the exposed areas of the first, second and third acid-generating layers, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid, and admixing the components of the first acid-generating layer with those of the first color-change layer, the components of the second acid-generating layer with those of the second color-change layer, and the components of the third acid-generating layer with those of the third color-change layer, thus causing, in the areas of the medium exposed to the three imagewise exposures, the secondary acids to bring about the changes in absorption of the first, second and third image dyes and thus form a trichrome image, and the copper compound and reactive material to destroy the remaining superacid precursor in each of the three acid-generating layers, thus fixing the image. If the same sensitizing dye is used in each of the three acid-generating layers, all three imagewise exposures can be effected using radiation of the same wavelength (for example, a single laser) thus avoiding, for example, the need for three separate sources of imaging radiation all of which must be scanned across the imaging medium. 
     Besides the acid-generating and color-change layers or phases, the imaging media of the present invention may comprise a support and additional layers, for example, a subbing layer to improve adhesion to the support, acid-impermeable interlayers (as discussed above) for separating multiple bilayers from one another, an anti-abrasive topcoat layer, and other auxiliary layers. 
     The support employed may be transparent or opaque and may be any material that retains its dimensional stability at the temperature used for image formation. Suitable supports include paper, paper coated with a resin or pigment, such as, calcium carbonate or calcined clay, synthetic papers or plastic films, such as polyethylene, polypropylene, polycarbonate, cellulose acetate and polystyrene. The preferred material for the support is a polyester, desirably poly(ethylene terephthalate). 
     Usually the acid-generating and color-change layers or phases will each also contain a binder; typically these layers are formed by combining the active materials and the binder in a common solvent, applying a layer of the coating composition to the support and then drying. Rather than a solution coating, the layer may be applied as a dispersion or an emulsion. The coating composition also may contain dispersing agents, plasticizers, defoaming agents, coating aids and materials such as waxes to prevent sticking. 
     The binder used for the acid-generating layer(s) must of course be non-basic, such that the superacid is not buffered by the binder. Examples of binders that may be used include styrene-acrylonitrile copolymers, polystyrene, poly(α-methylstyrene), copolymers of styrene and butadiene, poly(methyl methacrylate), copolymers of methyl and ethyl acrylate, poly(vinyl butyral), polycarbonate, poly(vinylidene chloride) and poly(vinyl chloride). It will be appreciated that the binder selected should not have any adverse effect on the superacid precursor, sensitizing dye, secondary acid generator, fixing reagent (if any) or image dye incorporated therein. Also, the binder should be heat-stable at the temperatures encountered during the heating step and should be transparent so that it does not interfere with viewing of the image. The binder must of course transmit the actinic radiation used in the exposure steps. 
     The imaging media of the present invention may be used in any of the ways in which the aforementioned &#39;489 and &#39;612 media have been used. Specifically, the imaging media of the present invention are very suitable for use in slide blanks similar to those described in copending applications Ser. Nos. 08/226,452 and 08/226,657, both filed Apr. 12, 1994 and assigned to the same assignee as the present application (now U.S. Pat. Nos. 5,422,230 and 5,451,478 respectively) and the corresponding International Applications Nos. PCT/US95/04401 and PCT/US95/04395 respectively (Publication Nos. WO 95/27623 and WO 95/27622 respectively). 
     Thus, one preferred slide blank of the present invention comprises: 
     a support; 
     a mask layer having a substantially transparent central portion and a non-transparent peripheral portion surrounding the central portion; and, 
     an imageable layer comprising an imaging medium of the present invention which is imageable to form an image which can be viewed in transmission, 
     the support, mask layer and imageable layer being secured together so that the support and the imageable layer extend across essentially the entire transparent central portion of the mask layer, at least the portion of the support adjacent the central portion of the mask layer being substantially transparent. 
     A second preferred slide blank of the present invention comprises: 
     a support at least part of which is essentially transparent; 
     an imageable layer superposed on one face of the support, the imageable layer comprising an imaging medium of the present invention which is imageable to form an image which can be viewed in transmission; and 
     a protective layer superposed on the imageable layer on the opposed side thereof from the support, at least part of the protective layer being essentially transparent; 
     the support, imageable layer and protective layer being secured together to form a slide blank having a thickness of at least about 0.8 mm, and the thickness of the protective layer being such that no part of the imageable layer containing the color-forming composition is more than about 0.2 mm from one external surface of the slide blank. 
     For further details of these preferred slide blanks, methods for their use and slides produced therefrom, the readier is referred to the aforementioned applications Ser. Nos. 08/226,452 and 08/226,657, and International Applications Nos. PCT/US95/04401 and PCT/US95/04395. 
     A preferred embodiment of the invention will now be described, though by way of illustration only, with reference to FIG. 3 of the accompanying drawings, which shows a schematic cross-section through a full color deprotonation type imaging medium (generally designated 10) of the invention as the image therein is being fixed by being passed between a pair of hot rollers 12. 
     The imaging medium 10 comprises a support 14 formed from a plastic film. Typically the support 14 will comprise a polyethylene terephthalate film 3 to 10 mils (76 to 254 mμ) in thickness, and its upper surface (in FIG. 3) may be treated with a sub-coat, such as are well-known to those skilled in the preparation of imaging media, to improve adhesion of the other layers to the support. 
     On the support 14 is disposed a first acid-generating layer 16 comprising: 
     (a) a superacid precursor, namely (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate; 
     (b) an indicator sensitizing dye of the formula: ##STR8## (the unprotonated form is available from Yamada Chemicals, Kyoto, Japan); this sensitizing dye sensitizes the superacid precursor to visible radiation at approximately 450 nm); 
     (c) a secondary acid generator, which undergoes a superacid-catalyzed thermal decomposition to form a secondary acid, this secondary acid generator being of the formula: ##STR9## 
     (d) a cosensitizer, preferably triphenylamine; and 
     (e) a polystyrene binder. 
     On the opposed side of the acid-generating layer 16 from the support 14 is disposed a first color-change layer 18 comprising: 
     (a) a first image dye, of the formula: ##STR10## (available from Hilton Davis Co., 2235 Langdon Farm Road, Cincinnati, Ohio 45237 under the tradename &#34;Copikem 37&#34;), which changes from colorless to yellow in the presence of an acid; 
     (b) copper(II) acetate; 
     (c) a reactive material, namely potassium acetate; and 
     (d) a binder comprising Acryloid B82 (available from Rohm &amp; Haas, Philadelphia, Pa. 19104) and poly(vinyl alcohol); the poly(vinyl alcohol) acts as both a binder and a reducing agent for the fixing process. 
     The acid-generating layer 16 and the color-change layer 18 both contain a binder having a glass transition temperature substantially above room temperature. 
     Superposed on the first color-change layer 18 is an acid-impermeable layer 20, which serves to prevent acid generated in the second acid-generating layer 22 (see below) during imaging penetrating to the first color-change layer 18. Superposed on the acid-impermeable layer 20 is a second acid-generating layer 22, which contains the same superacid precursor, secondary acid generator and binder as the first acid-generating layer 16. However, the second acid-generating layer 22 contains, in its protonated form, as an indicator sensitizing dye, 2,4,6-tris(2,4-dimethoxyphenyl)pyridine, which sensitizes the superacid precursor to visible/near ultra-violet radiation of approximately 400 nm wavelength. 
     Superposed on the second acid-generating layer 22 is a second color-change layer 24 which is identical to the first color-change layer, except that the Copikem 37 is replaced by a second image dye, of the formula: ##STR11## (available from Hilton Davis Co. under the tradename &#34;Copikem 35&#34;), which changes from colorless to magenta in the presence of an acid. 
     The next layer of the imaging medium is a second acid-impermeable interlayer 26, identical to the layer 20. Superposed on the acid-impermeable layer 26 is a third acid-generating layer 28, which contains the same superacid precursor, secondary acid generator and binder as the first and second acid-generating layers 16 and 22 respectively. However, this third acid-generating layer 28 does not contain an indicator sensitizing dye, but instead contains a conventional non-basic polycyclic aromatic sensitizer, namely 1-vinylpyrene, which sensitizes the superacid precursor to ultra-violet radiation of approximately 350 nm wavelength. Superposed on the third acid-generating layer 28 is a third color-change layer 30 which is identical to the first color-change layer, except that the Copikem 37 is replaced by a third image dye, of the formula: ##STR12## (see U.S. Pat. No. 4,345,017) which changes from colorless to cyan in the presence of an acid. Finally, the imaging medium 10 comprises an abrasion-resistant topcoat 32. 
     The imaging medium 10 is exposed by writing on selected areas of the medium with three radiation sources having wavelengths of 450, 400 and 350 nm respectively. The three radiation sources may be applied simultaneously or sequentially; for example, the medium may be scanned in a raster pattern simultaneously by the focused beams from three lasers of appropriate wavelengths, or the medium may be exposed sequentially through three masks to radiation from lamps generating radiation of appropriate wavelengths. The 450 nm radiation, which carries the yellow channel of the desired image, images the first acid-generating layer 16, the 400 nm radiation, which carries the magenta channel, images the second acid-generating layer 22 and the 350 nm radiation, which carries the cyan channel, images the third acid-generating layer 28. Thus, as described above with reference to FIGS. 1A-1D, since the sensitizing dyes in the first and second acid-generating layers 16 and 22 respectively are present in protonated form (i.e., these two layers are, prior to imaging, as shown in FIG. 1A, except that all the sensitizing dye, not merely 75%, is present in protonated form), latent images in unbuffered superacid are formed in the first and second acid-generating layers 16 and 22. A latent image in unbuffered superacid is also present in the third acid-generating layer 28, since the vinylpyrene sensitizer used in this layer does not buffer the superacid produced by decomposition of the superacid precursor. 
     The imaging medium 10 is passed between the heated rollers 12; the heat applied by these rollers causes the unbuffered superacid present in the exposed areas of the acid-generating layers 16, 22 and 28 to cause catalytic breakdown of the secondary acid generator therein, thus causing formation of a quantity of secondary acid substantially greater than the quantity of unbuffered superacid generated by the imagewise exposures. The heat and pressure applied by the heated rollers 12 also raise the acid-generating layers 16, 22 and 28 and the color-change layers 18, 24 and 30 above their glass transition temperatures, thus causing the components present in each acid-generating layer to intermix with the components present in its associated color-change layer. Accordingly, the three associated pairs of acid-generating and color-change layers are &#34;developed&#34; and fixed as described above with reference to Table 1; i.e., the copper compound decomposes the remaining superacid precursor and the base neutralizes the unbuffered superacid present. In these exposed areas, the secondary acid produced in the acid-generating layer effects the color change of the image dye in the associated color-change layer, thereby forming yellow, magenta and cyan images in the layers 18, 24 and 30. In the non-exposed areas, excess base remains and the image dye remains uncolored. The acid-impermeable interlayers 20 and 26 prevent the unbuffered superacid or the secondary acid generated in the second and third acid-generating layers 22 and 28 respectively migrating to the first and second color-change layers 18 and 24 respectively, thus preventing crosstalk among the three images. The mixing of the components present in each bilayer also causes the base present in each of the color-change layers to deprotonate the protonated forms of the sensitizing dye (in the layers using indicator sensitizing dye) present in the non-exposed areas of its associated acid-generating layer, thus removing the visible absorption due to the protonated sensitizing dye, and reducing the D min  of the images to a low level. 
     The following Examples are now given, though by way of illustration only, to show details of preferred reagents, conditions and techniques for use in the process and medium of the present invention. 
     EXAMPLES 1-7: Deprotonation Processes 
     EXAMPLES 1-3: Preparation of sensitizing dyes for deprotonation processes 
     EXAMPLE 1: Preparation of 2,4,6-tris(4-methoxyphenyl)pyridine 
     This Example illustrates the first of the three preferred methods for the preparation of triarylpyridinium dyes described above. 
     A mixture of p-anisaldehyde (4.08 g, 30 mmole), p-methoxyacetophenone (9.01 g, 60 mmole) and ammonium acetate (30 g) in acetic acid (75 mL) was stirred at reflux for one hour, then at 20° C. for 16 hours. The precipitate formed was collected by filtration, washed with a 70:30 acetic acid/water mixture, and dried in vacuo to provide 3.50 g (30% yield based upon starting p-methoxyacetophenone) of crude product. Recrystallization from ethanol provided fine colorless matted needles, m. pt. 132°-133.5° C. (Dilthey, J. Pract. Chim. [2], 102, 239 gives the melting point as 133° C.). 
     EXAMPLE 2: Preparation of 2,4,6-tris(2,4-dimethoxyphenyl)pyridine 
     This Example illustrates the second of the three preferred methods for the preparation of triarylpyridinium dyes described above. 
     A mixture of 2,4-dimethoxybenzaldehyde (1.66 g, 10 mmole), 2,4-dimethoxyacetophenone (3.60 g, 20 mmole) and potassium acetate (10.0 g) in acetic acid (35 mL) was stirred with heating until reflux was reached, then allowed to cool to 20° C. To the resultant yellow-orange solution was added hydroxylamine hydrochloride (1.4 g, 20 mmole). The solution was then stirred at reflux for 1 hour, an additional 5.6 g (80 mmole) of hydroxylamine hydrochloride was added, and the solution was again refluxed for 1 hour. The dark reaction mixture containing suspended solid thus produced was quenched into water (200 mL) and the resultant solution neutralized with potassium hydroxide. The slurry which resulted was extracted with methylene chloride (2×50 mL aliquots) and the organic layer evaporated to a brown oil, which was triturated with methanol (80 mL) to provide a cream-colored solid weighing 1.58 grams. A second crop weighing 0.41 g was obtained by evaporation of the liquors. Total yield of crude product was 1.99 g (42%). Recrystallization from ethyl acetate provided an analytically pure sample, m. pt. 160.5°-162° C. 
     EXAMPLE 3: Preparation of 2,6-bis(2,4-dimethoxyphenyl)-4-(2-(1,4-dimethoxy)naphthyl)pyridine 
     This Example illustrates the third of the three preferred methods for the preparation of triarylpyridinium dyes described above. 
     A mixture of 1,4-dimethoxy-2-naphthaldehyde (1.08 g, 5 mmole, prepared as described in N. P. Buu-Hoi and D. Lavit, J. Chem. Soc. 1956, 1743), p-methoxyacetophenone (0.75 g, 5 mmole) and methanesulfonic acid (12 drops) in acetic acid (10 mL) was stirred at reflux for 70 minutes, then cooled to 20° C. Ammonium acetate (4.0 g) and 4-methoxyphenacylpyridinium bromide (1.54 g, 5 mmole) were added and the reaction mixture was refluxed for a further 2 hours. The mixture was then cooled to 20° C. and poured into water. The resultant solution was neutralized with sodium hydroxide, and extracted with methylene chloride (20 mL). The organic layer was washed with water (90 mL) and evaporated to a brown solid which was chromatographed (on silica gel, eluted with methylene chloride) to provide the main fraction as a pale tan solid weighing 1.48 g (62%). Recrystallization from ethyl acetate provided an analytically pure sample as colorless fine matted needles, m. pt. 185.5°-186° C. 
     EXAMPLES 4-7: Preparation and use of deprotonation imaging media 
     The following indicator dyes are used in the Examples below: 
     Dye A: 2-[2-octyloxyphenyl]-4-[4-dimethylaminophenyl]-6-phenylpyridine (available from Hilton Davis Co.) 
     Dye B: 2,4,6-tris[4-methoxyphenyl]pyridine (prepared in Example 1 above) 
     Dye C: 2,4,6-tris[2,4-dimethoxyphenyl]pyridine (prepared in Example 2 above) 
     Dye D: 2,6-bis[4-methoxyphenyl]-4-[thienyl]pyridine (prepared by the method of Example 1 above) 
     Dye E: 2,6-bis[4-methoxyphenyl]-4-[4-bromophenyl]pyridine (prepared by the method of Example 1 above) 
     Dye F: 2,6-bis[4-methoxyphenyl]-4-[2-naphthyl]pyridine (prepared by the method of Example 1 above) 
     Dye G: 2,4-bis[4-methoxyphenyl]-6-[2-naphthyl]pyridine (prepared by the method of Example 3 above) 
     Dye H: 2,4,6-tris[2,4,6-trimethoxyphenyl]pyridine (prepared by the method of Example 1 above) 
     Dye I: 2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinoline (available from Yamada Chemical Co., Kyoto Japan) 
     Dye J: 2-[2-[4-dodecyloxy-3-methoxyphenyl]ethen-1-yl]quinoline (available from Yamada Chemical Co.) 
     Dye K: 3&#39;,6&#39;-bis[N-[2-chlorophenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9&#39;-(9H)xanthene]](prepared by the method of U.S. Pat. No. 4,345,017) 
     Dye L: 3&#39;,6&#39;-bis[N-[2-[methanesulfonyl]phenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9&#39;-(9H)xanthene]](prepared by the method of U.S. Pat. No. 4,345,017) 
     Dye M: 9-diethylamino[spiro[12H-benzo(a)xanthene-12,1&#39;(3&#39;H)-isobenzofuran)-3&#39;-one] (available from Hilton Davis Co.) 
     Dye N: 2&#39;-di(phenylmethyl)amino-6&#39;-[diethylamino]spiro[isobenzofuran-1(3H),9&#39;-(9H)xanthen]-3-one (available from Hilton Davis Co.) 
     Dye O: 3-[butyl-2-methylindol-3-yl]-3-[1-octyl-2-methylindol-3-yl]-1-(3H)-isobenzofuranone (available from Hilton Davis Co.) 
     Dye P: 6-[dimethylamino]-3,3-bis[4-dimethylamino]-phenyl-(3H)-isobenzofuranone (available from Hilton Davis Co.) 
     EXAMPLE 4 
     This example illustrates the quantum yields of acid generation which may be achieved using indicator sensitizing dyes in their protonated forms. 
     The general method for measurement of quantum yields involved coating the sensitizing indicator dye, a superacid precursor, and optionally another acid indicator dye, in a polymeric binder. In some experiments, a non-basic, polycyclic aromatic sensitizer (for example 2-t-butylanthracene) was also present in the film to facilitate generation of an initial amount of acid used to protonate the sensitizing indicator dye when the film is initially exposed to ultra-violet radiation, as described below. The film was then irradiated near the long wavelength absorption maximum of the protonated sensitizing indicator dye. The absorbance at the wavelength of maximum absorption due to the protonated sensitizing indicator dye was measured before and after this exposure. From the change in absorption, and measurement of the exposure received by the sample, the quantum yield of acid production could be calculated as described below. 
     By way of example, Dye A was tested in the following manner. A coating fluid was prepared from Dye A (5.0 mg), [4-[2-hydroxytetradec-1-yloxy]phenyl]iodonium hexafluoroantimonate (5.0 mg; this superacid precursor was used for all the quantum yield determinations) and 0.5 mL of a 7.5% (w/v) solution of polystyrene (molecular weight 45,000, available from Aldrich Chemical Co., Milwaukee, Wis.) in 2-butanone. This solution was coated onto transparent poly(ethylene terephthalate) film base 4 mil (101 μm) in thickness (ICI Type 3295, supplied by ICI Americas, Inc., Wilmington, Del.) using a #18 wire-wound coating rod. After drying overnight (under reduced pressure, in the dark, at room temperature), a portion of the film was briefly exposed to ultra-violet radiation from a low-power mercury lamp sufficient to generate enough acid in the film to partially protonate the Dye A (the absorbance achieved at 460 nm was 0.16). The film was then exposed to light at 460 nm (0.48 mW/cm 2 ) for 60 seconds, using the output of a 150 W Xenon lamp, which had been spatially homogenized by passing it through about 5-7 m of single core optical fiber, and then passed through a three-cavity interference filter with a bandpass of about 10 nm to select the wavelength. The light intensity at the sample was measured with a calibrated photodiode. After exposure, the absorbance of the film at 456 nm (the wavelength of maximum absorption of protonated Dye A) was measured and found to have increased by 0.205 absorbance units due to acid generation during exposure, which protonated more Dye A. The quantum yield of acid photogeneration under these exposure conditions was 0.13. 
     In this experiment, the protonated Dye A served as a photosensitizer of the superacid precursor, and the unprotonated Dye A in the film served as an acid indicator dye, since its increased absorbance at 456 nm upon protonation (ε 80  =33,000M -1  cm -1 ) is used to quantify the acid formed. 
     The amount of acid generated (Δn, in moles) in a given area of the film, is: ##EQU1## where a ex  is the area of the film exposed (in cm 2 ), ΔA.sup.λ  is the change in the absorbance of the film at the absorbance maximum of the protonated indicator dye (or, if present, the alternative acid indicator dye), and e.sup.λ  is the molar absorptivity of the protonated sensitizer or indicator dye at that wavelength in L mol -1  cm -1 . 
     This calculation assumes that every photogenerated proton protonates an unprotonated sensitizing indicator dye or alternative acid indicator dye molecule, which will be the case if (1) the concentration of the sensitizing indicator dye or alternative acid indicator dye is large; (2) the sensitizing dye or alternative acid indicator dye is more basic than other major components of the film; and (3) there is relatively little of any basic impurity in the sample (which would act as a base threshold, to &#34;buffer&#34; the sample). Even if these conditions do not all hold, the calculation can still give valid relative results for similar samples containing, for example, different sensitizing indicator dyes or superacid precursors. 
     Quantum yields of acid photogeneration (Φ acid ) are calculated according to the relation: 
     
         Φ.sub.acid =Δn/ζ.sub.abs 
    
     where ζ abs  is the number of photons absorbed by the sensitizer, in einsteins (moles of photons). ζ abs  is determined by the relation: 
     
         ζ.sub.abs =8.359×10.sup.-12 [Pt.sub.ex λ.sub.ex a.sub.ex (1-10.sup.-A.sbsp.av)] einstein.sup.-1 nm.sup.-1 mW.sup.-1 sec.sup.-1 
    
     where P is the incident (monochromatic or quasi-monochromatic) light power in mW cm -2 , t ex  is the duration of the irradiation in seconds, λ ex  is the exposure wavelength in nm, and A av  is the average (over the exposure period) absorbance of the sample at the exposure wavelength. The term in parentheses is the average absorptance of the sample. In performing the measurement, care must be taken that (1) the area of the film probed by the spectrophotometer light is fully within the exposed area; and (2) that exactly the same area of the film is probed by the spectrophotometer before and after the exposure (because the films which are hand-coated are generally not very uniform in thickness). 
     The efficiency of the sensitization of superacid precursor decomposition is strongly dependent on the extent of the reaction, with the sensitization yield highest initially and decreasing as the exposure proceeds. 
     The quantum yields obtained are shown in Table 3 below, in which λ U  denotes the longest unprotonated absorption wavelength, λ P  denotes the longest protonated absorption wavelength, λ IRR  denotes the irradiation wavelength and QY denotes the quantum yield. 
     
                       TABLE 3                                                     
______________________________________                                    
Dye  Class of dye                                                         
                 λ.sub.U, nm                                       
                          λ.sub.P, nm                              
                                 λ.sub.IRR, nm                     
                                        QY, %                             
______________________________________                                    
A    Triarylpyridine                                                      
                 334      458    460    6.8                               
B    Triarylpyridine                                                      
                 330      370    400    7.8                               
C    Triarylpyridine                                                      
                 320      382    400    11.1                              
D    Triarylpyridine                                                      
                 340      372    400    4.8                               
E    Triarylpyridine                                                      
                 332      388    400    7.0                               
F    Triarylpyridine                                                      
                 332      386    400    5.5                               
G    Triarylpyridine                                                      
                 &lt;320     374    400    6.2                               
H    Triarylpyridine                                                      
                 &lt;320     366    400    3.5                               
I    Styrylquinoline                                                      
                 368      450    460    1.9                               
J    Styrylquinoline                                                      
                 366      438    460    2.3                               
K    Xanthene    358      554    560    0.23                              
L    Xanthene    356      552    544    0.13                              
M    Xanthene    356      528    560    0.21                              
N    Fluoran     380      582    580    0.16                              
O    Phthalide   358      536    544    0.18                              
P    Phthalide   358      616    620    0.065                             
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     EXAMPLE 5 
     This example illustrates the use of a triarylpyridine indicator sensitizing dye which is coated in its protonated form as the hexafluoroantimonate salt. The protonated indicator dye is used to sensitize an imaging medium of the present invention to near ultra-violet exposure, to produce a final image having, in D max  areas, significant absorption in the near ultra-violet and visible regions but, in D min  areas, negligible absorption in either the near ultra-violet or visible regions. 
     Two coating fluids were prepared as follows: 
     Fluid A 
     An indicator sensitizing dye (2,4,6-tris[2,4-dimethoxyphenyl]pyridinium hexafluoroantimonate, prepared by washing a dichloromethane solution of Dye C with aqueous hexafluoroantimonic acid, then separating and drying the dichloromethane layer, 10 mg), a superacid precursor ([4-octyloxyphenyl]phenyliodonium hexafluoroantimonate, 20 mg, prepared as described in U.S. Pat. No. 4,992,571), a secondary acid generator (2,2-dimethyl-1-[4-benzyloxybenzyloxalyloxy]prop-3-yl[4-benzyloxybenzyl]oxalate (prepared as described in the aforementioned copending application Ser. No. 08/141,852, 80 mg) and the polystyrene binder as in Example 4 (800 mg of a 20% solution in 2-butanone) were combined together and heated at about 50° C. until complete dissolution of all components had been achieved. 
     Fluid B 
     A dispersion was first prepared as follows. A magenta indicator dye (3-[butyl-2-methylindol-3-yl]-3-[1-octyl-2-methylindol-3-yl]-1-(3H)-isobenzofuranone, available from Hilton Davis Co. under the tradename Copikem 35, 1.0 g) and an acrylate polymeric binder (Acryloid B-82, supplied by Rohm and Hass Corporation, Philadelphia, Pa. 19104, 1.25 g) were dissolved in ethyl acetate (10 g), and the resultant solution was added to 7 g of a 2.55% solution of poly(vinyl alcohol) (Vinol 540, available from Air Products Corporation, Allentown, Pa.) in water. The mixture was then sonicated, after which evaporation of ethyl acetate afforded the required dispersion (14.5% solids by weight). 
     To 1.0 g of this dispersion was added 100 mg of a 20% aqueous solution of potassium acetate and 10 mg of a 5% aqueous solution of a surfactant (Igepal CO-630 (available from GAF Corporation, 1361 Alps Road, Wayne N.J. 07470) to form Fluid B. 
     Fluid A was coated onto transparent poly(ethylene terephthalate) base 4 mil (101 μm) in thickness (ICI Type 3295, supplied by ICI Americas, Inc., Wilmington, Del.) using a #4 coating rod, to form an acid-generating layer, and the UV/visible absorption spectrum of this coating was measured using a piece of the uncoated film base as the reference. Fluid B was next coated on top of the acid-generating layer using a #3 coating rod, to form a color-change layer. 
     A portion of imaging medium so produced was exposed to ultraviolet radiation at an exposure of 4.3 mJ/cm 2  using a nuArc 26-1K Mercury Exposure System (available from nuArc Company, Inc., 6200 West Howard Street, Niles, Ill. 60648; the output of this device was measured using an IL390A &#34;Light Bug&#34; radiometer, available from International Light, Inc., 17 Graf Road, Newburyport, Mass. 01950); after this exposure the entire imaging medium was heated first at 45° C. for 20 seconds and then at 120° C. for 30 seconds. The UV/visible absorption spectrum of the exposed and unexposed regions were then measured, again using a piece of uncoated film base as the reference. 
     Table 4 below shows the absorbances at one wavelength in the near ultra-violet and one wavelength in the visible region at the various stages of imaging, with uncoated film base as the reference. 
     
                       TABLE 4                                                     
______________________________________                                    
          Acid-generating         Non-exposed                             
Absorption                                                                
          layer before                                                    
                      Exposed area                                        
                                  area after                              
wavelength, nm                                                            
          imaging     after heating                                       
                                  heating                                 
______________________________________                                    
366       0.312       0.468       0.023                                   
544       0.005       1.105       0.004                                   
______________________________________                                    
 
    
     As can be seen in Table 4, the absorbance in the near ultra-violet at 366 nm in the D min  region of the imaged film is 0.023, whereas before imaging the absorbance at this wavelength (due to protonated Dye C) was 0.312. On the other hand, in the D max  region the absorbance at this wavelength is 0.468, due partly to protonated Dye C and partly to the ultra-violet absorbance of the magenta image dye, whose maximum absorbance is at 544 nm (with an absorbance of 1.105). 
     EXAMPLE 6 
     This example illustrates the use, in a process of the present invention similar to that described above with reference to Table 1, of a styryl quinoline indicator sensitizer dye which is coated in its unprotonated form. A non-basic, polycyclic aromatic sensitizer which absorbs significantly only below 370 nm (in the near ultra-violet) is also coated. The protonated form of the indicator sensitizer dye absorbs in the blue visible region. The protonated form of the image dye used absorbs in the green region of the visible spectrum. The imaging medium used is first blanket exposed to near ultra-violet radiation, generating acid from the non-basic polycyclic aromatic sensitizer and the superacid precursor which protonates the indicator sensitizing dye. The medium is then imagewise exposed to blue light, which is absorbed by the protonated form of the indicator sensitizing dye. After heating, a final image having significant absorption in the green region of the spectrum in D max  areas but negligible absorption in either the blue or green regions of the spectrum in D min  areas is obtained. 
     Two coating fluids were prepared as follows: 
     Fluid A 
     Indicator sensitizing dye Dye I (1.5 mg), a non-basic polycyclic aromatic sensitizer 1-[prop-1-enyl]pyrene (10 mg), a secondary acid generator 1-[4-benzyloxybenzyloxalyloxy]oct-2-yl[4-benzyloxybenzyl]oxalate (prepared according to the methods described in the aforementioned copending application Ser. No. 08/141,852, 60 mg) and the same superacid precursor (15 mg) and polystyrene binder (600 mg of a 20% solution in 2-butanone) as in Example 5 above were combined and heated at about 50° C. until complete dissolution of all components had been achieved. 
     Fluid B 
     To 1.0 g of the dispersion prepared by the method described in Example 5 above (17% solids) was added 100 mg of a 2% aqueous solution of potassium acetate and 50 mg of a 5% aqueous solution of surfactant Igepal CO-630. 
     Fluid A was coated onto reflective Melinex base 4 mil (101 μm) in thickness using a #4 coating rod, to form an acid-generating layer. Fluid B was coated on top of this layer using a #3 coating rod, to form a color-change layer. 
     The entire imaging medium so produced was exposed to ultraviolet radiation using 4.5 &#34;Units&#34; of exposure from the same nuArc apparatus as in Example 5 above filtered through a U360 colored-glass bandpass filter (available from Hoya Corporation, Fremont, Calif.). The exposure received by the film was measured as 7.6 mJ/cm 2  using the same radiometer as in Example 5 above. After this exposure, a portion of the imaging medium was exposed to filtered radiation for various lengths of time from a 150 W Xenon lamp, using the apparatus described in Example 4 above with a three-cavity interference filter having a bandpass of about 10 nm centered at 440 nm. After this Xenon lamp exposure, the whole imaging medium was heated first at 45° C. for 20 seconds and then at 120° C. for 30 seconds. The blue and green reflection optical densities were measured at each stage of this process using an X-Rite 310 photographic densitometer, supplied by X-Rite, Inc., Grandville, Mich., equipped with the appropriate filter. The results obtained are shown in Table 5 below. 
     
                       TABLE 5                                                     
______________________________________                                    
Exposure                                                                  
       Exposure at                                                        
                  Heating time,                                           
at 360 nm                                                                 
       440 nm     sec. at 45° C.                                   
                             Blue OD Green OD                             
(mJ/cm.sup.2)                                                             
       (mJ/cm.sup.2)                                                      
                  (90° C.)                                         
                             (reflection)                                 
                                     (reflection)                         
______________________________________                                    
0      0          0 (0)      0.06    0.04                                 
7.6    0          0 (0)      0.21    0.04                                 
7.6    0          20 (30)    0.07    0.10                                 
7.6    40         20 (30)    0.38    0.79                                 
7.6    60         20 (30)    0.49    1.86                                 
7.6    80         20 (30)    0.63    2.59                                 
______________________________________                                    
 
    
     As can be seen from Table 5, after exposure at 360 nm the optical density in the blue region was 0.21, meaning that about 38% of incident blue light was absorbed by protonated Dye I. The imaging medium could then be imaged using blue light to give a green D max  of 2.59 by protonation of the image dye. However, D min  regions of the imaged film exhibited optical density in the blue region of only 0.07, showing that Dye I had been returned to the unprotonated form in which it was originally coated (when it gave a blue optical density of 0.06). 
     EXAMPLE 7 
     This example illustrates the use of a xanthene indicator sensitizing dye which is coated in its unprotonated form. A non-basic, polycyclic aromatic sensitizer which absorbs significantly only below 370 nm (in the near ultra-violet) is also coated. The protonated form of the indicator sensitizing dye absorbs in the blue visible region, while the protonated form of the image dye used also absorbs in the green region of the spectrum. The imaging medium is first blanket exposed to near ultra-violet radiation, generating acid from the polycyclic aromatic sensitizer and the superacid precursor and protonating the indicator sensitizing dye. The medium is then imagewise exposed to green light, which is absorbed by the protonated form of the indicator sensitizing dye. After heating, a final image having significant absorption in the green region of the spectrum in D max , areas but negligible absorption in the green region of the spectrum in D min  areas is obtained. 
     Two coating fluids were prepared as follows: 
     Fluid A 
     Indicator sensitizing dye Dye K (2.4 mg), a non-basic polycyclic aromatic sensitizer (1-[prop-1-enyl]pyrene, 15 mg), and the same superacid precursor (30 mg), secondary acid generator (120 mg) and polystyrene binder (1.2 g of a 20% solution in 2-butanone) as in Example 5 above were combined and heated at about 50° C. until complete dissolution of all components had been achieved. The same Fluid B as in Example 6 above was used. 
     Fluid A was coated onto reflective Melinex base 4 mil (101 μm) in thickness using a #4 coating rod, to form an acid-generating layer. Fluid B was coated on top of this layer using a #3 coating rod, to form a color-change layer. 
     The entire imaging medium so produced was exposed to ultraviolet radiation in the same way as in Example 6 above using 3 &#34;Units&#34; of exposure. The exposure received by the film under these conditions was measured (using the same radiometer as in Example 5) to be 4.1 mJ/cm 2 ) . After this exposure, a portion of the imaging medium was exposed to filtered radiation for various lengths of time from a 150 W Xenon lamp, using the apparatus described in Example 4 above with a three-cavity interference filter having a bandpass of about 10 nm centered at 560 nm. After this exposure, the whole imaging medium was heated first at 45° C. for 20 seconds and then at 120° C. for 30 seconds. The green reflection optical density was measured at each stage of this process using the same densitometer as in Example 6. The results obtained are shown in Table 6 below 
     
                       TABLE 6                                                     
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Exposure at                                                               
         Exposure at  Heating time,                                       
360 nm   560 nm       sec. at 45° C.                               
                                 Green OD                                 
(mJ/cm.sup.2)                                                             
         (mJ/cm.sup.2)                                                    
                      (90° C.)                                     
                                 (reflection)                             
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0        0            0 (0)      0.06                                     
4.1      0            0 (0)      0.34                                     
4.1      0            20 (30)    0.09                                     
4.1      270          20 (30)    0.93                                     
4.1      540          20 (30)    2.48                                     
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     As can be seen from Table 6, after exposure at 360 nm the optical density in the green region was 0.34, meaning that about 54% of incident green light was absorbed by protonated Dye K. The imaging medium could then be imaged using green light to give a green D max  of 2.59 by protonation of the image dye. However, D min  regions of the imaged film exhibited optical density in the green region of only 0.09, showing that Dye K had been substantially returned to the unprotonated form in which it was originally coated (when it gave a green optical density of 0.06). 
     EXAMPLES 8-11: Nucleophile Processes 
     EXAMPLES 8-9: Preparation of sensitizing dyes for nucleophile processes 
     EXAMPLE 8: Preparation of 1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinolinium hexafluoroantimonate 
     A mixture of quinaldinium methiodide (713 mg, 2.5 mmole), diphenylbenzaldehyde (683 mg, 2.3 mmole) and acetic anhydride (8 mL) was stirred with heating to reflux for 15 minutes, then cooled to approximately 100° C. and maintained at that temperature for a further 90 minutes. The reaction mixture was then cooled to 20° C., whereupon a small amount of solid precipitated. The reaction mixture was then diluted with acetone (8 mL) and diethyl ether (20 mL) and filtered to provide 665 mg of brick-red prisms. A 600 mg sample of this crude product was chromatographed on silica gel, eluting with a 5% methanol/dichloromethane mixture, to provide 84.5 mg of the desired pure iodide salt as a red-brown amorphous solid. This solid was dissolved in methanol (8 mL) and stirred with 72 mg (0.2 mmole) of silver hexafluoroantimonate dissolved in methanol (0.7 mL). The resulting precipitate was filtered off and the filtrate diluted with a solution of sodium hexafluoroantimonate (0.5 g) in water (30 mL). The precipitate which resulted was filtered off and dried in vacuo at 25° C. overnight to give the desired hexafluoroantimonate salt as brick-red prisms (41 mg), having λ max  (CH 2  Cl 2 ) 550 μm, ε=35,430. A dichloromethane solution of the product was 67% bleached after two minutes and completely bleached within ten minutes by addition of the nucleophilic amine 3-aminopropanol. 
     EXAMPLE 9: Preparation of 3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indolium hexafluoroantimonate 
     A mixture of fleshly-distilled Fischer&#39;s base (0.70 g, 4.0 mmole) and N-phenylcarbazole-3-carboxaldehyde (1.0 g g, 4.0 mmole) in toluene (3 mL) was heated to approximately 50° C., then phosphorus oxychloride (0.3 mL) was added, resulting in a vigorous (almost violent) exotherm. The resultant mixture was then stirred at ambient temperature for approximately 30 minutes, water (15 mL) was added, and the supernatant was decanted and discarded. The glassy residue remaining was taken up in warm methanol (7 mL) and quenched into water (60 mL) to give a brick-red precipitate, which was collected by filtration, then chromatographed on silica gel, eluting with a 5% methanol/dichloromethane mixture, followed by a 10% methanol/dichloromethane mixture. The purest fractions from the chromatography were combined and evaporated to dryness, then the solid residue was taken up in dichloromethane (12 mL) and washed with 10% hexafluoroantimonic acid (2×4 mL aliquots), then with water (6 mL), and finally evaporated to a brittle foam (1.15 g). A 250 mg sample of this foam was recrystallized from chloroform (1.5 mL) to provide the desired pure product (206 mg) as orange-red prisms, having λ max  (CH 2  Cl 2 ) 512 nm, ε=46,170. A dichloromethane solution of the product was completely bleached within one minute by addition of the nucleophilic amine 3-aminopropanol. 
     EXAMPLES 10-11: Preparation and use of nucleophile imaging media 
     The following nucleophile bleachable sensitizing dyes are used in the Examples below: 
     Dye Q: 1-methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinolinium hexafluoroantimonate (available from methylation of Dye I above) 
     Dye R: 1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinolinium hexafluoroantimonate (prepared in Example 8 above) 
     Dye S: 3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indolium hexafluoroantimonate (prepared in Example 9 above) 
     Dye T: 3,3-dimethyl-1-methyl-2-[2-[9-ethylcarbazol-3-yl]ethen-1-yl]-3H-indolium hexafluoroantimonate (prepared using a method analogous to that described in Example 9 above) 
     The following image dye was used in Example 10 below: 
     Dye U: 3&#39;,6&#39;-bis[2,3-dihydro-2,3,3-trimethylindol-1-yl]spiro[2-[2-methylprop-1-yl]-1,1-dioxo[1,2-benzisothiazole-3(3H),9&#39;(9H)xanthene]] (prepared in a manner analogous to that described in U.S. Pat. No. 4,345,017). 
     EXAMPLE 10 
     This Example illustrates the quantum yields of acid generation which may be achieved using hemicyanine dyes in their first form, such dyes being capable of being converted to a second form, which does not absorb visible light, by addition of a nucleophile. 
     The general method for measurement of quantum yields was very similar to that described in Example 4 above. However, in that Example the formation of acid could be measured by conversion of the unprotonated sensitizing dye itself to its protonated form. The hemicyanine dyes described in this Example do not readily undergo protonation, and in any case such protonation would not have a predictable effect on the dye&#39;s absorption characteristics. Therefore, a separate acid indicator (image) dye was incorporated into the test film with the hemicyanine sensitizing dye and the superacid precursor, and the change in absorption of this image dye, due to protonation, was used to quantify the amount of acid produced from irradiation of the sensitizing dye. The absorption spectrum of the unprotonated and protonated forms of this image dye were selected so as to overlap minimally with the absorption due to the hemicyanine sensitizer. 
     Two different indicator dyes were used in these experiments: Dye A and Dye U. Their extinction coefficients were taken to be, respectively, 33,000 M -1  cm -1  and 98,000 M -1  cm -1 . 
     By way of Example, Dye Q was tested in the following manner. A coating fluid was prepared from Dye Q (3 mg), the same superacid precursor as in Example 4 above (5 mg), Dye U (5 mg) and the same polystyrene as in Example 4 (0.5 mL of a 7.5% w/v solution in 2-butanone). This coating fluid was coated on to the same transparent film base as described in Example 4. The resulting acid-generating layer was not exposed to ultraviolet light, as in Example 4, but instead was directly exposed at 501 nm. After exposure, the layer was heated at 81° C. for 40 seconds, and the change in absorption due to the indicator dye, Dye U, was measured at 656 nm. All of the acid produced photochemically was assumed to have protonated dye U, and therefore the absorption change of the dye U was used in the calculation described in Example 4 to obtain the quantum yield of acid formation. Dye U was used as the image dye, except in the case of Dye R, for which Dye A was used as the image dye. 
     In addition to the assumptions mentioned in Example 4, it is also assumed that the indicator dye (Dye A or Dye U) is photochemically inert. However, it is likely that these dyes may function to some extent as cosensitizers, and the quantum yield measured in a different system may be different from that quoted here. 
     The quantum yields obtained are shown in Table 7 below, in which λ 1  denotes the absorption maximum of the sensitizing dye in the film, λ 2  denotes the irradiation wavelength, and QY denotes the quantum. 
     
                       TABLE 7                                                     
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Dye     λ.sub.1 (nm)                                               
                      λ.sub.2 (nm)                                 
                               QY (%)                                     
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Q       450           501      0.9                                        
R       530           544      1.4                                        
S       488           501      0.65                                       
T       500           501      0.53                                       
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     EXAMPLE 10 
     This Example illustrates the use of a hemicyanine sensitizing dye (Dye S), with and without added cosensitizer, to form an image by a nucleophile process of the present invention. The color-change layer used in this Example contains a nucleophilic primary amine base, which decolorizes the sensitizing dye when the film is heated after imaging. 
     A stock solution was prepared by dissolving the same superacid precursor as in Example 4 above (40 mg), the same secondary acid generator as in Example 6 above (120 mg) and Dye S (20 mg) in a solution of the same polystyrene as in Example 4 (4 g of a 10% w/w solution in 2-butanone). Coating fluids A-F were prepared by adding, to 400 mg of this stock solution, the following various cosensitizers: 
     
         ______________________________________                                    
Fluid A     No cosensitizer (control)                                     
Fluid B     5 mg triphenylamine                                           
Fluid C     9.6 mg tris(p-bromophenyl)amine                               
Fluid D     5.4 mg N-(p-methoxyphenyl)carbazole                           
Fluid E     4.8 mg N-phenylcarbazole                                      
Fluid F     4.4 mg 2,5-di-t-butylhydroquinone.                            
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     These amounts of cosensitizers were chosen so that the molar ratio of cosensitizer to superacid precursor in the coating solutions was approximately 3.7. 
     Coating fluids A-F were coated on to reflective Melinex film base of 5 mil (126 μm) thickness using a #8 coating rod to form an acid-generating layer. The resultant films were overcoated to form a color-change layer using a #12 coating rod with a dispersion prepared as follows. 
     The same Copikem 35 indicator dye as in Example 5 above (2.5 g) and an amine base (1-(3-aminoprop-1-yl)imidazole, 0.275 g) were added to a solution of the same Acryloid B-82 binder as in Example 5 (7.5 g in 40 g of ethyl acetate), and the resultant solution was added to a solution of the same Vinol 540 as used in Example 5 (28.6 g of a 7% solution in water). Water (55 g) was added, and the resultant mixture was sonicated. Evaporation of ethyl acetate afforded the required aqueous dispersion (61.2 g, 16.9% solids by weight). 
     The resultant imaging media were exposed to light at 501 nm (0.93 mW/cm 2  exposure) through a transmission step wedge for 1800 seconds. After exposure, the films were heated at 60° C. for 30 seconds, then at 120° C. for 40 seconds. Optical densities (green) were recorded in D min  (non-exposed) and D max  (exposed) areas using the same densitometer as in Example 6 with the appropriate filter (Status A). Table 8 shows optical densities before imaging and after exposure and heating, the minimum exposure required to attain D max , and the oxidation potential of the cosensitizer (measured in acetonitrile solution against a standard calomel electrode). 
     
                       TABLE 8                                                     
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       OD       D.sub.max                                                 
                         D.sub.min                                        
                                Energy to                                 
                                        Oxidation                         
       before   after    after  D.sub.max                                 
                                        Potential                         
Medium imaging  imaging  imaging                                          
                                (mJ/cm.sup.2)                             
                                        (mV)                              
______________________________________                                    
A      0.7      --       0.10   &gt;1674    1410*                            
B      0.71     0.65     0.08   15.9     956                              
C      0.69     0.69     0.10   82      1109                              
D      0.72     0.64     0.09   293     1260                              
E      0.70     --       0.10   &gt;1674   1312                              
F      0.74     0.68     0.10   293     Un-                               
                                        known                             
______________________________________                                    
 *Since no cosensitizer was added, the oxidation potential quoted is that 
 of the hemicyanine dye itself.                                           
 
    
     From the data in Table 8, it will be seen that in all cases the sensitizing dye was bleached after imaging, as shown by a drop in optical density from about 0.7 before heating to about 0.1 after heating. The sensitivity of the medium, as measured by the energy required to reach D max  was highly dependent upon the cosensitizer used; without a cosensitizer, the medium could not be imaged at all with less than 1674 mJ/cm 2  exposure, whereas with triphenylamine as cosensitizer, D max  was attained at an exposure of 15.9 mJ/cm 2 . Furthermore, the effectiveness of a cosensitizer was inversely related to its oxidation potential 
     From the foregoing, it will be seen that the present invention provides an imaging medium, and a process for producing an image, which overcomes the imitations of the &#39;489 and &#39;612 processes. In particular, the present invention allows imagewise exposure to be effected using radiation in the same wavelength range as that in which the image is to be viewed, and also enables an image to be produced in which the D min  regions do not exhibit the absorption of the sensitizing dye used in the imaging process. Also, certain processes of the present invention can produce a full color image using imagewise exposures at only one wavelength, with blanket exposures at two further wavelengths.