Slide blank, and process for producing a slide therefrom
REFERENCE TO RELATED APPLICATION 
     Attention is directed to copending application Ser. No. 08/226,657 filed simultaneously herewith and assigned to the same assignee as this application; this copending application describes and claims a slide blank generally similar to that of the present invention but having an imageable layer at a specified position within the slide blank. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a slide blank, a slide and a process for producing a slide. The term &#34;slide blank&#34; is used herein to refer to a unit which resembles a slide lacking an image, and which upon imaging will form a ready-mounted slide suitable for projection. 
     Hitherto, slides have typically been produced by exposing a roll of silver halide film using either a camera or a film recorder (for example that sold as the CI-5000 film recorder by Polaroid Corporation), which receives a digital image from a computer or similar image processing equipment and exposes the film. In either case, only a latent image is produced upon the film, which requires development and fixing to produce visible images. After development and fixing, the various images on the film are separated from one another and each imaged film portion is mounted by placing it in a slide mount. Conventional slide mounts typically consist of two rectangular sheets of plastic, card or other relatively rigid material, each sheet having a rectangular central cut-out or &#34;window.&#34; The developed and fixed film portion is sandwiched between the two sheets of the slide mount so that its image can be viewed in transmission through the two windows, which are aligned with each other and with the image, and the two sheets of the slide mount and the film portion are all secured together. 
     Such conventional slides suffer from several discrete problems, most of which are felt acutely by users making presentation graphics slides. As with any silver halide roll film, each roll of slide film can produce a number (typically 12, 20, 24 or 36) of images, and one must either expose the whole roll before processing or waste the unexposed portion of the roll. In addition, the development and fixing of the latent images require substantial investment in processing equipment, or the delays inherent in the use of independent photographic processors. Even those who regularly produce presentation graphics slides, and have &#34;in-house&#34; access to film recorders, typically rely upon such processors to develop and fix the film, thus incurring delays of a few hours to a day between the exposure of the film and the availability of the finished slide. 
     Polaroid Corporation sells, under the Registered Trade Mark &#34;POLACHROME,&#34; slide films comprising diffusion transfer film units (&#34;instant films&#34;); these slide films, and apparatus for their processing, are described, for example, in Liggero et al., The Polaroid 35 mm Instant Slide System, J. Imaging Technology, 10, 1-9 (1984), and Sturge, J., Walworth, V., and Shepp, A. (eds.), Imaging Processes and Materials (Neblette&#39;s Eighth Edition), Van Nostrand Reinhold, New York (1989), pages 194-95 and 210-11. These slide films comprise a plurality of photosensitive elements, which are exposed in the same manner as conventional silver halide films. After as many of the photosensitive elements as desired have been exposed, the whole film is run through a specially designed apparatus, which causes development and formation of images on image-receiving elements. The image-receiving elements are then peeled from the photosensitive elements, separated from one another and mounted in the same manner as conventional slide films. Although this type of slide film does eliminate the delays inherent in the processing of conventional slide films, it still requires that all the photosensitive elements in a film be exposed before any are developed, or the remainder wasted, and the mounted slides produced are similar to those produced from conventional slide films, and thus suffer from the disadvantages of conventional mounted slides discussed below. 
     Conventional slides also suffer from problems associated with the physical form of the finished slide. It is not easy to secure the film portion securely between the two pans of the slide mount in a manner that will prevent movement of the film portion during heavy use of the slide, such as may occur when the slide is used for repeated presentations or in an automated slide changer at an exhibition. Even slight movement of the film portion relative to the slide mount causes an objectionable strip of white to appear along one edge of the projected image. Furthermore, in a conventional slide the fragile film portion is exposed through the windows in the slide mount and is easily damaged or marked, for example by the fingerprints of a user during handling. Furthermore, the heating which the exposed, relatively flexible film portion undergoes during projection tends to cause the film portion to buckle out of the focal plane of the projector lens, and such buckling may adversely affect the quality of the projected image. To prevent or reduce such marking or buckling, so-called &#34;glass mounts&#34; are sometimes used. These glass mounts resemble conventional slide mounts but sandwich the film portion between two thin, transparent sheets of glass, which extend across the windows in the slide mount. Although glass mounts do reduce the risk of accidental marking or buckling of the film portion, the glass sheets are themselves fragile and are readily broken. In addition, dirt or other particles can become trapped between the glass sheets and the film portion, causing unwanted artifacts on the image seen when the slide is projected. 
     Whether or not glass mounts are used, the difference in thickness between the window and the remaining portions of the mounted slide leaves a &#34;step&#34; extending around the image. This step tends to trap dirt, fibers and other detritus, which are difficult to remove without damaging the film portion, and which may produce undesirable artifacts when the slide is projected. 
     Conventional slides place restrictions on the shape of the images that can be produced. Slide mounts are normally only produced with windows having a fixed aspect ratio, and the image must either conform to this aspect ratio or part of the window must be covered by an opaque area, thus reducing the size of the image seen upon projection. Obviously, if desired, images can be produced in either portrait or landscape orientation, but if a presentation includes slides in both orientations, the user must manually place the slides in the projector in their correct orientation, and most frequent users of slides are familiar with the embarrassment that results when a slide is inadvertently shown in the wrong orientation. 
     Perhaps the worst disadvantage of conventional slides, however, is the lack of any facility for keeping one or more identifying indicia (for example, time and date of production, number of the slide in a series, or the name of the data file used to produce the image) associated with the image and visible on the mounted slide. Cameras are known having backs that can place the time and date, or other user-defined indicium, on a small area of a negative as it is exposed, so that a reflection print produced from the negative will display the indicium, usually in an inconspicuous corner of the print. Provision of such a visible indicium is not practical in slides, since the user needs to be able to read the indicium on the slide before he places it in the projector, and an indicium large enough to be legible in these circumstances would occupy so large a proportion of the slide as to be highly objectionable when projected. Although it is possible to provide appropriate indicia on mounted slides by writing, printing or securing adhesive labels on the surface of the slide mount, there remains the difficulty of matching up the indicia with each slide after the slide has been returned from processing. This problem is especially difficult for frequent users of presentation graphic slides, who may have several sets of slides being processed at any one time, and may have several slides of the same general type (for example, pie charts), or several revisions of the same slide, which are easily confused and thus subject to mislabeling. The risk of mislabeling is increased by the ease with which the order of a series of slides may be disturbed by the many handling operations needed in conventional processing. 
     One commercial form of slide mount attempts to overcome this problem by providing a small cut-out on one half of the slide mount adjacent its window, this cutout serving to expose a non-image area of the film so that any indicia on this non-image area can be read in reflection against a background provided by the other half of the slide mount. When such a slide mount is used with a conventional silver halide film, the non-image area exposed is that containing one set of the sprocket holes of the film, and conventional cameras and film recorders will not print in this area. Furthermore, the area available is extremely limited, since the edge of the film must be secured in the slide mount, and the area available is interrupted by the sprocket holes themselves. In practice, the only indicium which can be visible in the cut-out is the frame number of the image on the film, and while the use of such a slide mount serves to prevent placing a series of slides in the wrong order, the user is still left with the problem of associating each frame number with the appropriate caption or other indicium. Moreover, the visible frame numbers do not assist the user in identifying the roll of film from which the slide is derived. 
     Use of slides in presentations would be greatly simplified if a system could be developed by which a caption or other identifying indicium could be associated with an image as it is created (normally by means of computer software) such that a slide produced from the image would display the caption in a legible size on the slide mount outside the window. 
     In recent years, various &#34;direct-imaging media&#34; have been developed which allow direct formation of a visible positive image on the medium without requiring development or fixing steps. Such media include dye diffusion thermal transfer media (in which a dye donor sheet is heated imagewise to transfer a diffusible dye from the donor sheet to a dye receiving sheet, on which an image is formed), the media described in U.S. Pat. Nos. 4,602,263; 4,720,449; 4,720,450; 4,745,046; 4,818,742; 4,826,976; 4,839,335; 4,894,358 and 4,960,901 (in which heating of the medium causes a chemical and color change in a thermally sensitive material) and the media described in U.S. Pat. Nos. 5,278,031, 5,286,612 and 5,334,489, and application Ser. Nos. 08/141,852 and 08/141,860 (both filed Oct. 22, 1993) (which media when exposed to radiation generate acid, which changes the color of an indicator dye). The last two types of medium may hereinafter be called &#34;direct-imaging single sheet media.&#34; 
     U.S. Pat. No. 5,234,886, issued Aug. 10, 1993 on application Ser. No. 07/722,810 filed Jun. 28, 1991, describes a slide blank intended for imaging by dye diffusion thermal transfer. This slide blank comprises a rectangular piece of dye receiving material secured in the aperture of a conventional plastic slide mount. Although this slide blank can be imaged and displayed immediately after imaging without any post-imaging mounting steps, it is ill-suited for mass production, since it requires insertion and securing of individual pieces of dye receiving material within the apertures in the slide mounts, and does nothing to solve the problem of associating identifying indicia with each slide. 
     The present invention relates to a slide blank which can be imaged to produce a slide which is already mounted. Preferred forms of the slide blank of the invention incorporating at least one direct-imaging layer can overcome or reduce each disadvantage of conventional slides discussed above. 
     SUMMARY OF THE INVENTION 
     This invention provides a slide blank comprising: 
     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 which is not substantially photosensitive but 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. 
     This invention also provides a slide comprising: 
     a support; 
     a mask layer having a substantially transparent central portion and a non-transparent peripheral portion surrounding the central portion; and, 
     an image layer beating an image which can be viewed in transmission, 
     the support, mask layer and image layer being secured together so that the support and the image 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, so that the image can be projected by passing light through the image layer, the transparent central portion of the mask layer and the support. 
     Finally, this invention provides a process for producing a slide, this process comprising providing a slide blank of the invention and forming in its imageable layer an image which can be viewed in transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic section through a first slide blank of the invention incorporating a direct-imaging single sheet medium, and the section being taken along the vertical center line of the slide blank (the line I--I in FIG. 2); 
     FIG. 2 is a front elevation of the slide blank shown in FIG. 1, looking from the right in that Figure; 
     FIG. 3 is a schematic section through the imageable layers of a direct-imaging single sheet medium as described in copending application Ser. No. 08/065,350, filed May 20, 1993, this medium being usable in the slide blank shown in FIGS. 1 and 2; 
     FIG. 4 is a schematic section through the imageable layers of a direct-imaging single sheet medium as described in copending application Ser. No. 08/141,852, filed Oct. 22, 1993 (and in the corresponding International Application No. PCT/US93/10215, filed on the same day), this medium being usable in the slide blank shown in FIGS. 1 and 2; 
     FIG. 5 is a schematic section through a second slide blank of the invention incorporating an imaging medium as shown in FIG. 3 or FIG. 4, the slide blank being shown as the various layers thereof are being assembled; and 
     FIG. 6 is a schematic section, similar to that of FIG. 5, through a third slide blank of the invention incorporating a modified form of the imaging medium shown in FIG. 3 or FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As already mentioned, the present invention provides a slide blank comprising a support, a mask layer and an imageable layer, all secured together. The mask layer has a substantially transparent central portion and a non-transparent, preferably opaque, peripheral portion surrounding its transparent central portion. Thus, the mask layer mimics the appearance of a conventional slide mount, having a central window and a non-transparent periphery. The transparent portion of the mask layer may be formed of transparent material or may simply have the form of an aperture extending through the mask layer. Both the support and the imageable layer extend across essentially the entire transparent central portion of the mask layer, and at least the portion of the support adjacent the transparent central portion of the mask layer is transparent, so that an image formed in the imageable layer can be viewed in transmission through the transparent central portions of the support and the mask layer, in the same manner as a conventional mounted slide. The imageable layer is not substantially photosensitive; &#34;not substantially photosensitive&#34; is used herein to indicate that the imageable layer is not imaged by approximately two minutes exposure to conventional indoor artificial lighting, so that the present slide can be handled without the need for light-tight enclosures. 
     Desirably, the support, mask layer and imageable layer are of substantially the same dimensions and are secured together so that the mask layer and the imageable layer extend across substantially the whole area of the support. Such a slide blank is convenient to manufacture, since sheets of material appropriate to form the support, mask layer and imageable layer of a plurality of slides can be laminated together by conventional techniques and the laminated sheets then cut to produce individual slide blanks. Also, such a slide blank is readily made in the form of a flat lamina having two substantially planar major surfaces on opposed sides thereof, thus essentially eliminating the step between the thin film portion and the thick slide mount in a conventional slide, and the tendency for this step to gather dust, fibers and other detritus, or to catch on projections adjacent which the slide blank or slide passes. Although the slide blank can be made in any desired size, conveniently it is in the form of a substantially square lamina having an edge length of from about 40 to about 70 mm and a thickness of from about 0.8 to about 1.7 mm, preferably about 1 to about 1.2 mm; slide blanks having these dimension can produce slides that are compatible with conventional slide projectors. 
     The layers of the present slide blank can be arranged in any convenient order consistent with the requirements for imaging of the imageable layer used. For example, the mask layer can be in contact with one face of the support and the imageable layer superposed upon the mask layer. This arrangement places the mask layer and the imageable layer close together, thus minimizing any potential problems which may be caused by separation of these two layers during projection of the slide produced from the blank; such problems may, at least in theory, include an indistinct edge of the mask layer caused by its separation from the focal plane of the projector lens, since the user of the slide naturally aligns this focal plane with the imaged layer. However, placing the mask layer within the slide blank in this manner may cause problems if it is desired to use a mask layer having a central aperture, since this aperture will cause a void within the slide, which could distort the projected image. Even if the central aperture is filled with adhesive during manufacture of the slide blank, undesirable optical artifacts could be produced by bubbles, dirt or changes in refractive index within the adhesive layer. In addition, sometimes it may be difficult to place a thin imageable layer over the mask layer without producing undesirable distortion of the imageable layer, which may cause difficulty in imaging this layer. Accordingly, in general it is preferred that the slide blank of the present invention have the mask layer disposed on the opposed side of the support from the imageable layer. In slide blanks having the preferred thickness of about 1 to 1.2 mm, it has been found that the separation of the mask layer from the imageable layer by the support does not, in practice, cause an objectionable degree of fuzziness in the edges of the mask layer seen in the projected image, and the fact that the imageable layer and the mask layer are placed upon different faces of the support, rather than the imageable layer being placed upon the mask layer, or vice versa, facilitates the attachment of both layers to the support. Any slight degree of fuzziness in the edges of the mask layer caused by the separation between the mask layer and the imageable layer may be dealt with by imaging a black border around the image, this black border forming, visually, an extension of the mask layer; the use of such borders is discussed in more detail below. Although placing a mask layer having a central aperture on one external surface of the slide blank does leave a small step around the central aperture, with the preferred printed form of mask layer (discussed in detail below), this step is very small (of the order of microns) and is thus much less likely to gather dirt, or to catch on projection apparatus, than the much larger steps found in conventional slides. 
     The mask layer of the present slide blank can be formed from any material which is sufficiently opaque, and which possesses the requisite physical properties, to form a dark, well-defined &#34;frame&#34; when a slide produced from the blank is projected using a conventional slide projector. For example, the mask layer may be formed from a layer of opaque plastic, but is preferably formed by printing a layer of ink or other pigment on to one face of the support, conveniently by silk screening. Alternatively, the mask layer may be formed from a metal foil, preferably applied by a hot stamping process. Such metal foils are inexpensive and readily available commercially. Furthermore, such printed layers or foils can be made extremely thin (about 1 to about 2 μm) yet still opaque, so that when such a printed layer or foil is used as a mask layer on an external surface of the slide blank the step between the central aperture and the mask layer is essentially eliminated. Printed layers and metal foils also have the advantage that they can be colored and patterned so that the appearance of the slide blank can be customized as desired. Thus, for example, the primed layer or foil can display a corporate logo, or other identifying indicium indicating its source or ownership. Whether or not a printed layer or foil is used to form the mask layer, advantageously the two major surfaces of tho mask layer differ in color, thus assisting the user to place the completed slide in a projector in the proper manner without turning it over and producing an image that is left-right reversed. 
     In the present slide blank, the support serves to control the physical properties of the blank. Typically, the mask layer and the imageable layer are thin and comparatively fragile, and the physical properties of the slide are essentially those of the support. The support should be chosen to render the slide sufficiently rigid that it can be handled by conventional automated slide projectors without damage, but not so rigid that excessive forces are required to cause the slide to undergo the slight bending which is sometimes required during passage of the slide through automatic projectors, and which may also be desirable in apparatus used for imaging the slide blank. Indeed, it is an important advantage of the present slide blank that it can be deformed substantially during printing, but will return to a planar form after printing. Typically, the present slide blank will be primed by one or more spots of radiation (for example focussed laser beams) which are scanned in a raster pattern over at least the central portion of the imageable layer and modulated to form the image. Conveniently, movement of the spots in the fast scan direction of the raster pattern is achieved by deflecting the beam with an oscillating mirror. However, if the slide blank has to be maintained planar during printing, the variation in distance between the axis of oscillation of the mirror and the slide blank will result in some parts of the image being out of focus unless an expensive, aspherical, f(θ) lens is used to focus the beam. If, on the other hand, the slide blank can be deformed so that the imageable layer has the form of part of the surface of a cylinder having its axis coincident with the axis of oscillation of the mirror, each part of a scan line can be at the same distance from the axis of oscillation and an inexpensive spherical lens can be used to focus the beam. (It is not necessary to bend the slide blank in both dimensions, since movement of the spots in the slow scan direction can readily be effected by moving the entire slide relative to the mirror, for example by means of a stepper motor.) Bending of the present slide to a constant radius in this manner is facilitated by the essentially constant thickness of the slide; a structure resembling a conventional slide with a central window containing a section of imaging medium and surrounded by a much thicker frame cannot readily be bent to a curve of constant radius. (The preferred metal foil and printed types of mask layer discussed above do result in a very small difference between the thickness of the portion of the slide blank containing the central portion of the mask layer and that containing the peripheral portion of the mask layer, this difference in thickness is too small to affect the bending properties of the slide blank.) 
     The central portion of the support adjacent the central portion of the mask layer must be substantially transparent so that the image on the final slide can be projected therethrough, and although it is not essential that the peripheral portions of the support (which are covered by the mask layer) be transparent, conveniently the support is formed from a transparent plastic. Polycarbonate plastics are preferred, since they possess the requisite transparency and have physical properties that render them very suitable for use in the present slide blanks. 
     As discussed below, the present slide blank is well adapted to mass production by formation of the slide blanks in large sheets or on continuous webs, followed by separation of individual slide blanks from these sheets or webs, and the sheets or webs of slide blanks are conveniently prepared by laminating sheets or webs of support material, mask layer material and imageable layer material together. However, it is difficult to obtain commercially polycarbonate webs (continuous rolls) having a thickness of about 0.8-1 mm required to produce slides having the preferred thickness of about 1-1.2 mm, and, even if procurable, such polycarbonate webs are so rigid as to present handling difficulties with conventional web-handling machinery; for example, webs of this thickness cannot readily be wound on rolls, as required for use with roll-fed laminators, without roll set problems. Accordingly, it will often be convenient to form the support of the present slide blank from a plurality of sheets or webs of plastic or other material, these sheets or webs being secured to one another during manufacture of the slide blank. Any method providing a bond of sufficient strength to prevent delamination of the slide blank during imaging and use may be employed to attach the sheets or webs together to form the support (or indeed to attach the mask layer and/or imageable layer to the support). Appropriate methods for securing the sheets or webs together include solvent bonding, heat sealing and other forms of adhesive bonding, for example the use of epoxy or silicone adhesives, pressure-sensitive adhesives or adhesives cured with ultraviolet or other radiation. It should be noted that the present slide blank imposes stringent requirements upon adhesives used to secure its various layers together, especially during projection of the final slide; during projection, large amounts of heat are generated within the slide by absorption of the projector radiation by the colored areas of the image and the mask layer, and unless the adhesive used is carefully chosen the heat generated may cause formation of bubbles or other artifacts within the adhesive layers, and such artifacts may show up on the projected image. When polycarbonate layers are used to form the support, it is presently preferred to bond the layers to each other by solvent bonding, for example using ketones as the solvents, as described in more detail below with reference to the drawings. When a plurality of sheets or webs are secured together to form the support, it is desirable that these sheets or webs be composed of the same material to avoid curl problems caused by differences in coefficients of thermal expansion. 
     The imageable layer of the present slide blank may be of any type which is not substantially photosensitive but is imageable to form an image that can be viewed in transmission. However, preferably the imageable layer of the present slide blank comprises a color-forming composition which, upon exposure to actinic radiation, forms a colored material. Desirably, the color-forming composition comprises a radiation absorber capable of absorbing actinic radiation (preferably infra-red radiation having a wavelength in the range of about 700 to about 1200 nm, since infra-red lasers having wavelengths within this range are excellent sources of imaging radiation) and a leuco dye which, upon absorption of radiation by the radiation absorber, forms the colored material. In one type of such compositions described, for example, in the aforementioned U.S. Pat. Nos. 4,602,263; 4,720,449; 4,720,450; 4,745,046; 4,818,742; 4,826,976; 4,839,335; 4,894,358 and 4,960,901, the radiation absorber generates heat within the imageable layer, and the leuco dye undergoes a thermal reaction to form the colored material. In this type of composition, the leuco dye may be, for example: 
     a. an organic compound capable of undergoing, upon heating, an irreversible unimolecular fragmentation of at least one thermally unstable carbamate moiety, this organic compound initially absorbing radiation in the visible or the non-visible region of the electromagnetic spectrum, the unimolecular fragmentation visibly changing the appearance of the organic compound (see U.S. Pat. No. 4,602,263); 
     b. a substantially colorless di- or triarylmethane imaging compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom to form a 5- or 6-membered ting, the moiety possessing a nitrogen atom bonded directly to the meso carbon atom and the nitrogen atom being bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position that cannot bond to the meso carbon atom, whereby the di- or triarylmethane compound is rendered colored (see U.S. Pat. No. 4,720,449); 
     c. a colored di- or triarylmethane imaging compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a thermally unstable urea moiety, the urea moiety undergoing a unimolecular fragmentation reaction upon heating to provide a new group in the ortho position that bonds to the meso carbon atom to form a ring having 5 or 6 members, whereby the di- or triarylmethane compound becomes ring-closed and rendered colorless (see U.S. Pat. No. 4,720,450); 
     d. in-combination, a substantially colorless di- or triarylmethane compound possessing on the meso carbon atom within its di- or triarylmethane structure an aryl group substituted in the ortho position with a nucleophilic moiety which is ring-closed on the meso carbon atom, and an electrophilic reagent which upon heating and contacting the di- or triarylmethane compound undergoes a bimolecular nucleophilic substitution reaction with the nucleophilic moiety to form a colored, ring-opened di- or triarylmethane compound (see U.S. Pat. No. 4,745,046); 
     e. a compound of the formula ##STR1## wherein M&#39; has the formula: ##STR2##  wherein R is alkyl; --SO 2  R 1  wherein R 1  is alkyl; phenyl; naphthyl; or phenyl substituted with alkyl, alkoxy, halo, trifluoromethyl, cyano, nitro, carboxyl, --CONR 2  R 3  wherein R 2  and R 3  each are hydrogen or alkyl, --CO 2  R 4  wherein R 4  is alkyl or phenyl, --COR 5  wherein R 5  is amino, alkyl or phenyl, --NR 6  R 7  wherein R 6  and R 7  each are hydrogen or alkyl, --SO 2  NR 8  R 9  wherein R 8  and R 9  each are hydrogen, alkyl or benzyl; Z&#39; has the formula: ##STR3## wherein R&#39; is halomethyl or alkyl; X is --N═, --SO 2  -- or --CH 2  --; D taken with X and M&#39; represents the radical of a color-shifted organic dye; q is 0 or 1; and p is a whole number of at least 1; Z&#39; being removed from M&#39; upon the application of heat to effect a visually discernible change in spectral absorption characteristics of the dye (see U.S. Pat. No. 4,826,976); f. a substantially colorless di- or triarylmethane compound of the formula: ##STR4##  wherein ring B represents a carbocyclic aryl ring or a heterocyclic aryl ring; C 1  represents the meso carbon atom of the di- or triarylmethane compound; X represents --C(═O)--; --SO 2  -- or --CH 2  -- and completes a moiety ring-closed on the meso carbon atom, the moiety including the nitrogen atom bonded directly to the meso carbon atom; Y represents --NH--C(═O)--L, wherein L is a leaving group that departs upon thermal fragmentation to unmask --N═C═O for effecting intramolecular acylation of the nitrogen atom to open the N-containing ring and form a new group in the ortho position of ring B that cannot bond to the meso carbon atom; E is hydrogen, an electron-donating group, an electron-withdrawing group or a group, either an electron-donating group or an electron-neutral group that undergoes fragmentation upon heating to liberate an electron-withdrawing group; s is 0 or 1; and Z and Z&#39; taken individually represent the moieties to complete the auxochromic system of a diarylmethane or triarylmethane dye when the N-containing ring is open, and Z and Z&#39; taken together represent the bridged moieties to complete the auxochromic system of a bridged triarylmethane dye when the N-containing ring is open (see U.S. Pat. No. 4,960,901); 
     g. a colorless precursor of a preformed image dye substituted with (a) at least one thermally removable protecting group that undergoes fragmentation from the precursor upon heating and Co) at least one leaving group that is irreversibly eliminated from the precursor upon heating, provided that neither the protecting group nor the leaving group is hydrogen, the protecting and leaving groups maintaining the precursor in its colorless form until heat is applied to effect removal of the protecting and leaving groups whereby the colorless precursor is converted to an image dye; 
     h. a mixed carbonate ester of a quinophthalone dye and a tertiary alkanol containing not more than about 9 carbon atoms (see U.S. Pat. No. 5,243,052); 
     i. a leuco dye represented by: ##STR5##  wherein: E represents a thermally removable leaving group; 
     tM represents a thermally migratable acyl group; 
     Q, Q&#39; and C taken together represent a dye-forming coupler moiety wherein C is the coupling carbon of the coupler moiety; 
     and, (Y) taken together with N represents an aromatic amino color developer, 
     one of Q, Q&#39; and (Y) containing an atom selected from the atoms comprising Group 5A/Group 6A of the Periodic Table, the groups E and tM maintaining the leuco dye in a substantially colorless form until the application of heat causes the group E to be eliminated from the leuco dye and the group tM to migrate from the N atom to the Group 5A/Group 6A atom thereby forming a dye represented by: ##STR6## wherein the dotted lines indicate that the tM group is bonded to the Group 5A/Group 6A atom in one of Q, Q&#39; and (Y) (see U.S. Pat. No. 5,236,884). 
     In another type of composition, described in the aforementioned U.S. Pat. Nos. 5,278,031, 5,286,612 and 5,334,489, and application Ser. Nos. 08/141,852; 08/141,860; and 08/141,920, and in the corresponding International Applications Nos. PCT/US93/10093, PCT/US93/10224 and PCT/US93/10215 (Publication Nos. WO 94/09992, WO 94/10607 and WO 94/10606 respectively), upon absorption of the actinic radiation, the radiation absorber generates acid within the imageable layer, and, upon exposure to this acid, the leuco dye forms the colored material. The acid may be generated by direct thermal breakdown of an acid generating material, for example a squaric acid derivative or a sulfonate (see International Application No. PCT/US93/10093), or by direct decomposition of a superacid precursor by actinic (typically ultra-violet) radiation followed by &#34;amplification&#34; of the superacid produced by superacid-catalyzed thermal decomposition of a secondary acid generator (see International Application No. PCT/US93/10224). Alternatively, (see International Application No. PCT/US93/10215), the color-forming composition may comprise a superacid precursor capable of being decomposed, by radiation of a wavelength shorter than that of the actinic radiation absorbed by the radiation absorber, to form a superacid, the superacid precursor, in the absence of the radiation absorber, not being decomposed by the actinic radiation absorbed by the radiation absorber but, in the presence of the radiation absorber and the actinic radiation absorbed by the radiation absorber, decomposing to form a protonated product derived from the radiation absorber, the color-forming composition further comprising a secondary acid generator capable of being thermally decomposed to form a second acid, the thermal decomposition of the secondary acid generator being catalyzed in the presence of the superacid derived from the superacid precursor. This type of medium is first imagewise exposed to radiation (typically infra-red radiation) of a wavelength which is absorbed by the radiation absorber, thereby producing, in the exposed regions, a protonated product derived from the absorber; in effect, the absorber causes decomposition of the superacid precursor with the formation of superacid buffered by the dye. The medium is then given a second exposure to radiation (typically ultra-violet radiation) of a wavelength which causes decomposition of the superacid precursor. The second exposure is controlled so that in the areas exposed during the first exposure, unbuffered superacid is present after the second exposure, whereas in the areas not exposed during the first exposure, only buffered superacid is present following the second exposure. Thus, the double exposure effectively produces an image in unbuffered superacid. Following the second exposure, the imaging medium is normally heated so that the unbuffered superacid can catalyze the thermal breakdown of a secondary acid generator, thereby producing, in the areas exposed during the first exposure, a large concentration of a secondary acid, which produces color in an acid-sensitive leuco dye. 
     Any or the aforementioned types of imaging medium comprising a color-forming composition which, upon exposure to actinic radiation, forms a colored material, may be rendered capable of producing multicolored images by providing a plurality of imageable layers, each of these imageable layers being capable of generating a different color, and each of these imageable layers having a radiation absorber capable of absorbing actinic radiation of a wavelength different from that of the radiation absorbed by the radiation absorber present in each of the other imageable layers. Such an imaging medium can be imaged using multiple lasers (or other light sources) having wavelengths arranged so that each laser is only absorbed by one of the imageable layers, thereby enabling the various imageable layers to be imaged independently of one another. 
     As indicated above, the order of the layers in the present slide blank may vary, and may be dependent upon the type of imageable layer present. For example, if the imageable layer is a dye receiving layer imaged by dye diffusion thermal transfer, the imageable layer must be present on, or at least very closely adjacent, an external surface of the slide blank so that dye can diffuse into the imageable layer during imaging of the slide blank. However, in an imaging medium comprising a color-forming composition which, upon exposure to actinic radiation, forms a colored material, the imageable layer(s) do not need to be present on an external surface of the slide blank, so radiation can be focussed to image an internal imageable layer through any overlying layers which are substantially transparent to the imaging radiation. Indeed, in this type of slide blank it is generally preferred that the imageable layer not be on an external surface of the slide blank, since such imageable layers are typically rather fragile and easily damaged when exposed on an external surface. Instead, it is preferred that a protective layer be provided overlying the imageable layer, so that the imageable layer is disposed between the protective layer and the support, with the protective layer thus serving to protect the imageable layer from damage during printing and other handling of the slide blank and the slide produced therefrom. As explained in more detail in the aforementioned copending application Ser. No. 08/226,657, the thickness of the layer must be chosen carefully, since when a slide having an internal imaged layer is projected, substantial heat is produced as the partially-opaque portions of the image absorb the projector radiation, and this heat must be dissipated through the surface of the slide without raising the temperature of the imaged layer, or other layers of the slide, to levels sufficient to cause damage. Empirically it has been found that the protective layer desirably has a thickness not greater than about 0.2 mm, and preferably not greater than about 0.15 mm. Also, of course, if the imageable layer is to be imaged through the protective layer, the protective layer must be substantially transparent to the imaging radiation, and have properties (e.g., lack of birefringence, and lack of optical heterogeneities) which do not interfere with the imaging process. 
     As already mentioned, the slide blank of the present invention is well-adapted to mass production since the support, imageable layer and mask layer can be assembled and secured to each other in large sheets or webs, and individual slides thereafter cut from these sheets or webs by conventional processes, for example die cutting. (Obviously, the cutting of the sheets must be done so that the transparent central portion of the mask layer is in the correct position in the finished slide blank, but it is well within the skill of the art to provide automated detection of the position of the central portion of the mask layer and to control the cutting process accordingly.) Moreover, since the imageable layer in the present slide blank extends beyond the central portion of the mask layer, and this peripheral part of the imageable layer is available for imaging, at least part of this peripheral part of the imageable layer can be used as a legend portion on which can be formed an image which can be viewed in reflection against the background provided by the mask layer. This legend portion is very convenient for providing identifying indicia on the slide, since (as those skilled in the electronic imaging an will be aware) software can readily be written to print both the image within the central portion of the imageable layer and the image on the legend portion in a single imaging operation, thus permanently associating the identifying indicia in the legend portion with the main image on the central portion. Moreover, the size of the legend portion can be substantial, sufficient to accommodate 2 or 3 lines of 10-12 point type, and thus the identifying indicia could comprise, for example, a slide number, a date and several words of description, thus facilitating identification and use of the slide. 
     The visible image formed on the present slide blank during printing need not occupy the entire central portion of the mask layer; if desired (and assuming that the imageable layer can achieve a maximum optical density sufficient to render a black portion of the image indistinguishable from the mask layer during projection) one or more portions of the imageable layer adjacent the periphery of the transparent central portion of the mask layer may be rendered substantially opaque during formation of the image, so that the image as seen in transmission differs in at least one of size, shape and aspect ratio from the transparent central portion of the mask layer. For example, a slide blank of the present invention may be provided with a large, square central portion of the mask layer and during printing either top and bottom areas, or left and right side areas, of this central portion could be colored solid black during printing, thereby allowing the slide blank to accommodate rectangular images in both landscape and portrait orientations, while still keeping the image the same way up on the slide. This form of &#34;dual mode&#34; slide blank allows the use of images in both orientations without the user worrying about whether any specific slide needs to be turned sideways before projection. Obviously, such a slide blank might also be useful for adapting to rectangular images with aspect ratios differing from those of conventional portrait or landscape images, and non-rectangular or unusually shaped images, for example, heartshaped wedding photographs. Also, as mentioned above, the image to be projected may be surrounded by a black border to avoid any problem of fuzziness in the edge of the mask layer as seen during projection of the slide. 
     Preferred slide blanks of the present invention, and processes for their preparation, will now be described in more detail, though by way of illustration only, with reference to the accompanying drawings. 
     The first slide blank of the invention, shown in FIGS. 1 and 2 and generally designated 10, is intended for laser imaging and comprises a support 12 formed from two transparent sheets 12a and 12b, each of which is formed of polycarbonate, the two sheets 12a and 12b being solvent bonded to one another. (In FIGS. 1 and 3-6, for ease of illustration the thicknesses of the various layers of imaging media and slide blanks are exaggerated compared with their lengths and widths.) The first sheet 12a is 20 mil (0.5 mm) thick, while the second sheet 12b is 15 mil (0.38 mm) thick. To the outer surface of the sheet 12a is adhesively secured a mask layer 14 having a substantially transparent, rectangular central portion 16 and a non-transparent peripheral portion 18 surrounding the central portion. 
     To the outer surface of the sheet 12b (the surface remote from the sheet 12a) is adhesively secured an imageable layer 20 in the form of a direct imaging single sheet medium. The support 12, the mask layer 14 and the imageable layer 20 are secured together so that the support and the imageable layer extend across the entire central portion 16 of the mask layer. Also, since the imageable layer 20 extends across one entire face of the slide 10, portions of the imageable layer 20 lying adjacent the peripheral portion 18 of the mask layer 14, for example the portions within the dashed areas 28 in FIG. 2, can be imaged (in the same scan as the portion of the imageable layer 20 lying adjacent the central portion 18 of the mask layer 14) to provide legend areas bearing identifying indicia for the slide. 
     It will be seen from FIG. 2 that the first slide blank has an appearance substantially mimicking that of a conventional mounted slide, except of course that the slide blank lacks an image thereon. Since the imageable layer 20 and the support 12 are essentially transparent, an observer viewing the elevation of the slide blank shown in FIG. 2 (which is the view normally regarded as the front of a conventional slide, i.e., the side which faces the projector bulb during projection) sees the central portion 16 of the mask layer 14 as a central &#34;window&#34; or piece of film surrounded by a slide mount or &#34;frame&#34; provided by the peripheral portion 18 of the mask layer 14. In a slide produced by printing on such a slide blank, any legend printed in the legend areas 28 is seen in reflection against the peripheral portion 18, and thus appears to be printed on the frame of the slide. 
     It will be seen from FIG. 1 that both the mask layer 14 and the imageable layer 20 comprise a plurality of layers in this embodiment of the invention. The mask layer 14 is formed by successively silk screen printing on to the first sheet 12a three separate layers, namely a white layer 14a, a blue layer 14b and a grey layer 14c; the transparent central portion 16 is formed simply by not printing the layers 14a, 14b and 14c on the central portion of the slide blank. The white and grey layers 14a and 14c respectively cause the appearance of the slide blank to resemble closely that of a normal mounted slide, which typically is white on one surface and grey on the other; since the polycarbonate sheets 12a and 12b are transparent, as are non-imaged portions of the imageable layer 20, a user viewing the slide blank 10 from the side bearing the imageable layer sees mainly the white layer 14a. The difference in color between the two faces of the slide assists the user in correctly orienting the slide, with the white face and the imageable layer 20 facing the projector bulb. The provision of the white layer facing the projector bulb reduces heat generation within the slide during projection, since the white layer reflects most of the projector radiation striking it, and thus minimizes any chance of heat buildup within the slide affecting a thermally sensitive imaging layer. The central aperture in the blue layer 14b is made slightly smaller than that in the white layer 14a, since it has been found that having a blue layer present avoids esthetic problems which might otherwise result from slight misregistration between the grey and white layers, i.e., the appearance of a narrow strip of white on the grey side of the slide, or a narrow strip of grey on the white side of the slide. If desired, portions of the grey layer 14c can be imagewise omitted so that portions of the blue layer 14b appear through the grey layer 14c, thereby presenting any desired image (for example, a corporate logo) on the rear surface of the slide. Also, a transparent protective layer may be applied over the grey layer 14c to protect the mask layer 14 from damage during imaging and handling of the slide blank or slide produced therefrom. 
     The imageable layer or imaging medium 20 comprises a base (or support) 22 having a thickness of 5 mil (0.13 mm) and formed from the same polycarbonate as the sheets 12a and 12b, this base 22 is solvent bonded to the second sheet 12b so that it effectively becomes part of the support in the finished slide blank 10. The imageable layer further comprises color-forming layers, which are shown as a single layer 24 in FIG. 1 for ease of illustration, and a topcoat 26. This topcoat 26 of the imaging medium forms one external surface of the slide blank, and serves as a protective layer protecting the relatively fragile color-forming layers 24 from damage caused by handling of the slide blank. 
     The slide blank 10 can conveniently be mass produced from sheets or, preferably, continuous webs of material. The imaging medium 20 is first prepared by coating and lamination in the manner described below. The mask layer 14 is silk screen printed on to a web of the first sheet 12a, and the resultant printed web is solvent bonded to a web of the second sheet 12b using methyl ethyl ketone. The sheets thus joined are immediately solvent bonded to the support 22 of the imaging medium 20 using methyl propyl ketone, which has been found to produce more uniform lamination than methyl ethyl ketone in this case. Finally, individual slide blanks are cut from the resultant web. It has been found empirically that the slide blank produced in this manner is sufficiently rigid to resemble a conventional mounted slide, and be usable in conventional slide projectors without modification of the projector, but sufficiently flexible to allow some bending of the slide blank during printing. 
     The location of the color-forming layers 24 adjacent the external surface of the slide formed by the topcoat 26 allows for efficient dissipation of heat caused by absorption of projector radiation in the imaged color-forming layers when a slide produced from the slide blank is projected, and thus prevents overheating and possible damage to the color-forming layers. 
     As noted above, the slide blank 10 is designed so that the base 22 of the imaging medium 20 effectively becomes part of the support in the finished slide blank, and thus the base 22 is formed from the same polycarbonate as the first and second sheets 12a and 12b respectively. It will be appreciated that the base 22 need not be of the same material as the sheets 12a and 12b; if desired, the sheet 12b could be made thicker and a much thinner material, which need not be polycarbonate, used as the base 22, provided of course that the material chosen for the base 22 can form a strong bond to the polycarbonate sheet 12b. Also, the topcoat 26 need not be incorporated into the imaging medium 20 but could be a separate layer applied over and bonded to the imaging medium as the imaging medium is incorporated into the slide blank (see the description of FIGS. 5 and 6 below). 
     The front elevations of the second and third slide blanks of the invention shown in FIGS. 5 and 6 respectively are essentially identical to that of the first slide blank shown in FIG. 2, and hence these additional front elevations will not be separately illustrated herein. 
     FIGS. 3 and 4 of the accompanying drawings illustrate imaging media which can be used as the imageable layer 20 in the slide blank shown in FIGS. 1 and 2. The imaging medium (generally designated 30) shown in FIG. 3 is of the type described in the aforementioned copending application Ser. No. 08/065,350, and is designed so that the various layers thereof can be coated without the use of organic solvents. The imaging medium 30 comprises a substantially transparent base 32 formed of 5 mil (126 μm) polycarbonate film incorporating an ultra-violet absorber; it is this base 32 which forms the base 22 of the imageable layer in the slide blank 10 shown in FIGS. 1 and 2. (The thicknesses of the layers 34-52 (described below) are exaggerated in FIG. 3 relative to the thickness of the base 32.) Appropriate polycarbonate films are readily available commercially. 
     On the base 32 is coated, from an aqueous polyurethane dispersion, a compression layer 34, which is approximately 6 μm thick. The compression layer 34 is designed to prevent cracking of the relatively fragile imaging layers (described below) when a slide blank incorporating the imaging medium 30 is bent, for example during printing of the slide blank. It has been found that the presence of a soft, flexible compression layer 30 reduces the tendency for the imaging layers to crack during bending of the slide blank. 
     A cyan imaging layer 36 is in contact with the compression layer 34. To prepare the cyan imaging layer 36, 52.24 parts by weight of a leuco dye of formula: ##STR7## (this leuco dye may be prepared by the methods described in U.S. Pat. Nos. 4,720,449 and 4,960,901), 2.37 parts by weight of an infra-red dye of formula: ##STR8## (prepared as described in the aforementioned application Ser. No. 08/065,350), 1.6 parts by weight of a hindered amine light stabilizer (HALS-63, sold by Fairmount Chemical Co., Inc., 117 Blanchard Street, Newark N.J. 07105), 7.84 parts by weight of di-tert-butyl hydroquinone (a light stabilizer), 12.82 parts by weight of a surfactant (Aerosol TR-70, supplied by American Cyanamid Co., Wayne, N.J. 07470, with pH adjusted to 5.6 using a 1.0M aqueous solution of sodium hydroxide) and 31.32 parts by weight of a poly(ethyl methacrylate) binder (Elvacite 2043, sold by E. I. DuPont de Nemours and Company, Wilmington, Del.) were dissolved in 1282 parts by weight of dichloromethane. 1134 Parts by weight of deionized water were added to this solution, and the resulting mixture was homogenized. The dichloromethane was then removed by rotary evaporation under reduced pressure to leave a dispersion in water of particles whose size was in the 100-300 nm range. A water-soluble binder, poly(vinyl alcohol) (Airvol 540, supplied by Air Products, Allentown, Pa. 18195, 219.3 parts by weight of a 9.8% aqueous solution) was added to 1200 parts by weight of the dispersion prepared above, followed by a fluorinated surfactant (FC-120, supplied by the Minnesota Mining and Manufacturing Corporation, Minneapolis, Minn., 1.23 parts by weight of a 25% aqueous solution) to provide the coating fluid. To form the cyan color-forming layer 36, this coating fluid was coated to a dried coating weight of 360 mg/ft 2 . 
     The next layer of the imaging medium 30 is an interlayer 38, which is formed from a 2:1 w/w mixture of two water-soluble acrylic polymers, (Carboset XL-37 and Carboset 526, both sold by B.F. Goodrich Co., Akron Ohio 44313). The interlayer is coated on to the cyan layer 36 from aqueous solution at a dried coating weight of 437 mg/ft 2 . This interlayer 38 serves as a thermal insulator to prevent coloration of the cyan imaging layer by heat generated during exposure of the magenta imaging layer (and vice versa). The interlayer 38 also serves to reduce or eliminate migration of dye compound from the cyan and magenta imaging layers, and to increase adhesion between these layers. 
     Superposed on the interlayer 38 is a magenta imaging layer 40. To prepare the magenta imaging layer 40, 45 parts by weight of a leuco dye of formula: ##STR9## (this leuco dye may be prepared by the methods described in the aforementioned U.S. Pat. Nos. 4,720,449 and 4,960,901), 1.875 parts by weight of an infra-red dye of formula: ##STR10## (prepared as described in the aforementioned application Ser. No. 08/065,350), 1.725 parts by weight of a hindered amine light stabilizer HALS-63, 11.275 parts by weight of a surfactant (Aerosol TR-70, with pH adjusted to 5.6 using a 1.0M aqueous solution of sodium hydroxide) and 33.9 parts by weight of a poly(ethyl methacrylate) binder (Elvacite 2043) were dissolved in 1060 parts by weight of dichloromethane. 1125 Parts by weight of deionized water were added to this solution, and the resulting mixture was homogenized. The dichloromethane was then removed by rotary evaporation under reduced pressure to leave a dispersion in water of particles whose size was in the 100-300 nm range. A water-soluble binder, poly(vinyl alcohol) (Airvol 540, 195.3 parts by weight of a 9.8% aqueous solution) were added to 1145 parts by weight of the dispersion prepared above, followed by a fluorinated surfactant (FC-120, 1.07 parts by weight of a 25% aqueous solution) to provide the coating fluid. To form the magenta imaging layer 40, this coating fluid was coated to a dried coating weight of 334 mg/ft 2 . 
     The next layer of the imaging medium 30 is an interlayer 42, which is identical in composition, function and dried coating weight to the interlayer 38 described above. 
     Superposed on the interlayer 42 is a yellow imaging layer 44. To prepare the yellow imaging layer 44, 61.6 parts by weight of a leuco dye of formula: ##STR11## in which R&#39; is a tertiary butyl group (the compounds in which R&#39; is an isobutyl or benzyl group may alternatively be used), 1.54 parts by weight of an infra-red dye of formula: ##STR12## (prepared as described in the aforementioned application Ser. No. 08/065,350), 1.715 parts by weight of a hindered amine light stabilizer HALS-63, 15.435 parts by weight of a surfactant (Aerosol TR-70, with pH adjusted to 5.6 using a 1.0M aqueous solution of sodium hydroxide) and 46.2 parts by weight of a poly(ethyl methacrylate) binder (Elvacite 2043) were dissolved in 1235 parts by weight of dichloromethane. 1116 Parts by weight of deionized water were added to this solution, and the resulting mixture was homogenized. The dichloromethane was then removed by rotary evaporation under reduced pressure to leave a dispersion in water of particles whose size was in the 100-300 nm range. A water-soluble binder, poly(vinyl pyrrolidone) (PVP K-120, supplied by International Specialty Products, Wayne, N.J. 07470, 220.7 parts by weight of a 9.2% aqueous solution) was added to 875 parts by weight of the dispersion prepared above, followed by a fluorinated surfactant (FC-120, 1.14 parts by weight of a 25% aqueous solution) to provide the coating fluid. To form the yellow imaging layer 44, this coating fluid was coated to a dried coating weight of 415 mg/ft 2 . 
     The next layer of the imaging medium 30 is an interlayer 46, which is identical in composition, function and dried coating weight to the interlayers 38 and 42 described above. 
     As already indicated, the layers 32-46 of the imaging medium 30 are produced by coating on to the transparent base 32. However, the remaining layers of the medium 30 are coated on to a disposable support 52 (described below) and then laminated to form the final imaging medium 30. 
     The disposable support 52 is conveniently 3 mil (76 μm) poly(ethylene terephthalate) film (Melinex 505, supplied by ICI Films, Hopewell, Va. 23860). 0n to this support 52 is coated a durable layer 50. To form this durable layer 50, 350 parts by weight of ethyl cellulose (Ethocel, 10 cps, Standard Grade, supplied by Dow Chemical, Midland, Mich. 48674) and a fluorinated surfactant (FC-431, supplied by the Minnesota Mining and Manufacturing Corporation, Minneapolis, Minn., 3.5 parts by weight of a 50% solution in ethyl acetate) were dissolved in a mixture of 2205 parts by weight of ethyl acetate and 945 parts by weight of toluene to provide the coating solution. To form the durable layer 50, this coating solution was coated to a dried coating weight of 988 mg/ft 2 . 
     On to the durable layer 50 is coated an ultra-violet filter layer 48, which serves to protect the imaging layers 44, 40 and 36 from the effects of ambient ultraviolet radiation. It has been found that the leuco dyes are susceptible to undergoing color changes when exposed to ultraviolet radiation during storage before or after imaging; such color changes are obviously undesirable since they increase the D min  of the image and may distort the colors therein. To prepare the filter layer 48, 350 parts by weight of ethyl cellulose (Ethocel, 10 cps, Standard Grade), 35 parts by weight of Tinuvin 328 (an ultra-violet filter) and a fluorinated surfactant (FC-431, 3.5 parts by weight of a 50% solution in ethyl acetate) were dissolved in a mixture of 2205 parts by weight of ethyl acetate and 945 parts by weight of toluene to provide the coating solution. To form the filter layer 48, this coating solution was coated to a dried coating weight of 991 mg/ft 2 . 
     In combination, the durable layer 50, the filter layer 48 and the interlayer 46 are sufficiently thick to serve as a bubble-suppressant layer to suppress the formation of bubbles in the imaging layers during imaging of the medium 30, as described in International Patent Application No. PCT/US92/02055 (Publication No. WO 92/19454), and serve as a protective layer for the fragile imaging layers in the final slide blank. 
     The structure comprising the disposable layer 52, the durable layer 50 and the filter layer 48 is laminated under heat (250° F., 121° C.) and pressure to the structure comprising the layers 32-46, and then the disposable layer 52 is peeled away to form the final imaging medium 30. 
     The medium 30 may be imaged by exposing it simultaneously to the beams from three infra-red lasers having wavelengths in the ranges of 780-815 nm, 840-870 nm and 900-930 nm. The 900-930 nm beam images the cyan imaging layer 36, the 840-870 nm beam images the magenta imaging layer 40 and the 780-815 nm beam images the yellow imaging layer 44. Thus, a multicolor image is formed in the imaging medium 30, and this multicolor image requires no further development steps. Furthermore, the medium 30 may be handled in normal room lighting before exposure, and the apparatus in which the imaging is performed need not be light-tight. 
     FIG. 4 shows a second imaging medium, generally designated 60, which can alternatively be used as the imageable layer 20 in the slide blank shown in FIGS. 1 and 2. The imaging medium 60 is of the type described in the aforementioned U.S. Pat. No. 5,286,612 and comprises a support 62, which is identical to the support 32 shown in FIG. 3. On the support 62 is disposed an acid-generating layer 64 comprising a superacid precursor, an infra-red sensitizing dye and a secondary acid generator, which undergoes a superacid-catalyzed thermal decomposition to form a second acid. On the opposed side of the acid-generating layer 64 from the support 62 is disposed a color-forming layer 66 comprising an acid-sensitive material, which is colorless in the absence of acid, but turns yellow in the presence of acid, and a small amount of a base. The acid-generating layer 64 and the color-forming layer 66 both contain a binder having a glass transition temperature substantially above room temperature. 
     Superposed on the color-forming layer 66 is an acid-impermeable layer 68, which serves to prevent acid generated in the acid-generating layer 64 during imaging penetrating beyond the color-forming layer 66. Superposed on the acid-impermeable layer 68 are a second acid-generating layer 70 and a second color-forming layer 72, which are similar to the layers 64 and 66 respectively, except that the infra-red sensitizing dye in the layer 70 absorbs at a wavelength different from that of the infra-red sensitizing dye in the layer 64, and that the acid-sensitive material in the layer 72 turns cyan in the presence of acid. The remaining layers of the imaging medium 60 are a second acid-impermeable interlayer 74, identical to the layer 68, a third acid-generating layer 76 and a third color-forming layer 78 (which are similar to the layers 64 and 66 respectively, except that the infra-red sensitizing dye in the layer 76 absorbs at a wavelength different from that of the infra-red sensitizing dyes in the layers 64 and 70, and that the acid-sensitive material in the layer 78 turns magenta in the presence of acid), and an abrasion-resistant topcoat 80. 
     As described in the aforementioned U.S. Pat. No. 5,286,612, the imaging medium 60 is first exposed in a manner similar to the imaging medium 30 discussed above, by writing on selected areas of the medium with three infra-red lasers tuned to the wavelengths of the infra-red sensitizing dyes in the acid-generating layers 64, 70 and 76. Within the exposed regions of each acid-generating layer, the exposure to infra-red radiation causes breakdown of the superacid precursor, with formation of the corresponding superacid buffered by the sensitizing dye. After this infra-red exposure, the imaging medium 60 is passed beneath a mercury lamp and given a blanket ultraviolet exposure; this exposure may use three different ultra-violet wavelengths, with each acid-generating layer 64, 70 and 76 being sensitized to one of these three ultra-violet wavelengths, but in some cases it may be possible to use only a single ultra-violet wavelength for all three acid-generating layers. The ultra-violet exposure causes formation of unbuffered superacid in the infra-red exposed areas of each acid-generating layer. Finally, the imaging medium 60 is passed between heated rollers; the heat applied by these rollers causes the superacid present in the infra-red exposed regions of the acid-generating layers 64, 70 and 76 to cause catalytic breakdown of the secondary acid generator therein, thereby causing formation of a quantity of second acid substantially greater than the quantity of unbuffered superacid generated by the ultra-violet exposure. The heat and pressure applied by the heated rollers also raise the acid-generating layers 64, 70 and 76 and the color-forming layers 66, 72 and 78 above their glass transition temperatures, thereby causing the components present in each acid-generating layer to intermix with the components present in the associated color-forming layer, so that, in infra-red exposed regions, the second acid produced in the acid-generating layer effects the color change of the acid-sensitive material, thereby forming an image. 
     The second slide blank 90 of this invention shown in FIG. 5 differs from that shown in FIGS. 1 and 2 in that the imaging medium 30&#39; or 60&#39; is modified to eliminate the support 32 or 62 and to provide a carrier 92 in contact with the durable layer 50 or topcoat 80 but peelable therefrom. This modified imaging medium 30&#39; or 60&#39;  is formed by coating its various layers on to the carrier 92, the layers of course being coated in the reverse order from that used to form the imaging medium 30 or 60, as described above. If necessary, as is well known to those skilled in the coating art, a release layer may be coated on to the carrier 92 to render this carrier readily peelable from the remaining layers of the imaging medium 30&#39; or 60&#39;. To compensate for the absence of the support 32 or 62, the thickness of the second polycarbonate sheet 12b is increased to 20 mil (0.5 mm). 
     As shown in FIG. 5, the slide blank 90 is assembled in a manner similar to that of the slide blank 10 shown in FIG. 1, except that the imaging layers of the imaging medium are laminated directly to the second sheet 12b, and after this bonding has been completed, the carrier 92 is peeled away from the durable layer or topcoat to leave the finished slide blank. 
     The third slide blank 100 of this invention shown in FIG. 6 closely resembles that shown in FIG. 5 except that in the slide blank 100 the durable layer 50 or topcoat 80 is coated on a first carrier 102, while the imaging layers of the imaging medium are coated on a second carder 104 (conveniently, when the imaging medium 30 shown in FIG. 3 is used in this type of slide blank, the filter layer 48 is coated on the first carder with the durable layer 50). As in the second slide blank shown in FIG. 5, the support 32 or 62 is eliminated (the imaging layers being coated directly on to the second carrier 104) and to compensate for the absence of the support 32 or 62, the thickness of the second polycarbonate sheet 12b is increased to 20 mil (0.5 mm). The slide blank 100 is assembled in a manner very similar to the slide blank 90, except that two laminations are required; the imaging layers 34-46 or 64-78 are first laminated to the second sheet 12b, the second carrier 104 is peeled away from the resultant structure, then the durable layer 50 or topcoat 80 is laminated over the imaging layers and finally the first carrier 102 is peeled from the top coat to leave the finished slide blank 100. 
     Although the invention has been illustrated above with slide blanks using direct-imaging single sheet media, it will be apparent to those skilled in the imaging art that any type of imageable layer that does not require extensive post-imaging processing with developing or fixing solutions may be used in the present slide blank. (Imageable layers that require heating for imaging and/or development may be used in the slide blank, since, provided appropriate materials are chosen, the slide blank can be heated without damage.) For example, the imageable layer may be a dye receiving layer imageable by dye diffusion thermal transfer. Such an imageable layer is imaged in the conventional manner by bringing a dye donor sheet comprising a diffusible dye adjacent the slide blank and heating the dye donor sheet, thus transferring the dye from the dye donor sheet to the imageable layer, and thereby forming the image. Desirably, the heating of the dye donor sheet is effected by a focussed beam of radiation (for example, from a laser) rather than by a thermal head, since the resolution of a dye diffusion thermal transfer image produced by a thermal head, while adequate for prints, is generally insufficient for slides which are enlarged many times when projected. The imageable layer may also comprise a layer that can be imaged by ink jet printing, thermal wax transfer or laser ablation. The imageable layer could also be imaged by a differential adhesion imaging process, such as those described in International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237) and in U.S. Pat. Nos. 5,200,297 and 5,170,261. In this type of imaging process, a layer of a porous or particulate image-forming substance is provided on a heat-activatable image-forming surface of a first sheet-like element, this layer of the image-forming substance having a cohesive strength greater than the adhesive strength between the layer and the first element. A second sheet-like element is provided on the opposed side of the layer of image-forming substance from the first element, the image-forming substance having an adhesion to the second element greater than its adhesion to the first element. Upon imagewise exposure of the medium to appropriate brief and intense radiation (typically from an infra-red laser), the radiation is absorbed at or near the heat-activatable image-forming surface, and converted into heat, which activates the heat-activatable image-forming surface, thus firmly attaching exposed portions of the image-forming substance to the first element. The first and second elements are then separated, leaving those portions of the image-forming substance not exposed to the radiation attached to the second element and those portions of the image-forming substance exposed to the radiation attached to the first element, and thus forming a pair of images on the first and second elements. In a monochrome slide blank using such a differential adhesion imaging process, the support could serve as one sheet-like element. In a polychrome slide, the three colored images would be formed separately from the slide blank and later secured to one surface of the slide blank. 
     From the foregoing it will be seen that the slide blank of the present invention overcomes numerous disadvantages associated with the use of conventional slides. A single slide blank of this invention can be imaged individually; it is not necessary to expose a whole roll of slide film before processing and mounting the slides, and the delays inherent in processing and mounting steps are avoided, as are the physical difficulties involved in handling small, fragile unmounted slides. Since the imaged portion of a slide of the present invention is integral with the &#34;mount,&#34; the imaged portion cannot slip relative to the mount and the image will always project in the intended manner. The present slide can provide good protection to the image by including layers of plastic or similar material on both sides of the imaged layers. The present slide blank eliminates the substantial &#34;step&#34; on the external surfaces of conventional mounted slides, and the problems associated with the collection of dust, fibers and detritus in this step. The slide of the present invention can include a large legend area to carry permanent identifying indicia that cannot become detached from the slide, and can be printed at the same time as the slide is imaged, thus avoiding the problems involved in associating already-printed slides with appropriate indicia. Finally, as discussed above the present slide blank can allow for variation in the shape of the image projected, and can allow portrait and landscape images, and images with other aspect ratios and shapes, to be printed in the same orientation on the same slide blank.