Apparatus and method of imaging UV-sensitive media
FIELD OF THE INVENTION 
     The present invention relates to methods of obtaining useful images on ultraviolet-sensitive media with the use of xerographic laser printers. 
     BACKGROUND OF THE INVENTION 
     Various imaging media, most notably those which involve a photopolymerization reaction, require ultraviolet light for exposure. Examples of such media include printing plates, printed circuit boards, and 3M&#39;s Dynamark sign material. 
     Obtaining imagewise exposure of such media is costly and may require the services of a service bureau of another company which specializes in the use of appropriate imaging equipment. 
     Office printers have generally been viewed as unsuitable for preparing the images used in creating these signs, printing plates, or printed circuit boards. 
     Customary usage of ultraviolet-sensitive media is as follows. Computer data is prepared describing the desired image to be created. This data is then converted to raster format using a Raster Image Processor, or RIP, and sent to a laser imager. This imager, typically using a He-Ne laser, Argon ion laser, or laser diode, images onto silver halide film sensitive to the wavelength of the laser. The resultant film is black in regions where the final media is intended to be imaged. The film is then contact printed to obtain a wrong-reading, reversed image of the original film. The original film, when viewed facing the emulsion side, is readable. The contact print, when viewed facing the emulsion side, is wrong reading, i.e. lines of text read from right to left. Moreover, the areas to be imaged are white, rather than black. These reversals of image sense and color allow the contact print to then be placed in contact with the final media (e.g. sign material) and exposed in a contact printer using ultraviolet light. Following the exposure of this media, it is chemically processed to remove areas which were not exposed to the light. 
     It should be noted that the contact print (film to film) and the final imaging step (film to sign or other material) occur with the two media in pressurized contact with each other (e.g. in a vacuum frame contact printer) and includes emulsion to emulsion contact. Emulsion to emulsion imaging is desirable inasmuch as minimal spreading of the image occurs. 
     The original recording of the image onto silver halide film requires expensive equipment, this including not only the imager but also a chemical processor required for development of the film. A film recorder, such as the Linotype Linotronic L300™, costs about $50,000. A film processor may cost another $10,000, and may create various environmental problems. Companies which make signs often cannot afford such equipment, and/or may wish to avoid the environmental problems, and so must send floppy disks containing the image information out to a service bureau for preparation of films and negatives. After these are returned to the company making the sign, the contact exposure of the sign material is made. This process is both time-consuming and expensive. 
     Xerographic laser printers, e.g. the Hewlett-Packard Laserjet IIIP™, are usually capable of imaging the same information as is used by the service bureau. Such printers are usually less than 1/10 as expensive as the equipment used to create images onto film. If such printers could be used for creating signs, they could be made much more economically and quickly. 
     SUMMARY OF THE INVENTION 
     This invention addresses ways of creating images on ultraviolet light-sensitive media, using ordinary xerographic office printers for the creation of the image. 
     Another area addressed by this invention is to provide optimal sharpness of the resultant image and to minimize the exposure time required. 
     Another area addressed by this invention is to minimize the presence of artifacts in the resultant image. 
     Another area addressed by this invention is to accomplish the exposure of the photosensitive material automatically, and at low cost, and with equipment of small size. 
     Software modifications are made either within the host computer or within the Raster Image Processor (RIP) or within the xerographic laser printer itself to convert the image from right reading (text reads from left to right) and normal video (areas to be black on the final media are black in the xerographic image) to wrong reading (text appears as a mirror image, reading right to left) and reverse video (areas to be black on the final media are white in the xerographic image, and vice versa). In some printers, the Raster Image Processor is packaged to reside within the printer itself. 
     The paper produced by the xerographic printer is placed, toner side to emulsion side, in contact with the ultraviolet sensitive media. The non-toner side of the paper is exposed to ultraviolet light, while the paper and media are kept in intimate physical contact. 
     In a first embodiment, a feature that utilizes the characteristic of the paper that permits the paper to transmit some ultraviolet light is used, and the duration of exposure or intensity of exposure is increased so as to compensate for the absorption and reflection of ultraviolet light by the paper. 
     In a second embodiment, the paper is saturated with a chemical which increases its translucency to ultraviolet light. This being done, the amount of exposure required or the intensity of exposure is closer to that which would be required if a film negative were used. 
     In a third embodiment, similar to the second embodiment, a very thin film intermediate is used between the paper and the UV-sensitive media, so as to prevent chemical changes in the latter as a result of the chemical used to saturate the paper. 
     In a fourth embodiment, nip rollers are used to apply a chemical to increase translucency and insure good contact between the two. This being done, the photosensitive material and paper are exposed in an automatic device. The liquid holds the photosensitive material and the paper in contact with the other while the exposure is being made. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial elevation showing a first preferred embodiment of a device embodying the principles of the present invention; 
     FIG. 2 is a partial elevation showing a second preferred embodiment of a device embodying the principles of the present invention; 
     FIG. 3 is a partial elevation showing a third preferred embodiment of a device embodying the principles of the present invention; and 
     FIG. 4 is a partial elevation showing a fourth preferred embodiment of a device embodying the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Ordinary xerographic printing is well adapted to the use of paper as a medium. It is true that overhead transparencies can be made on most xerographic printers using a special mylar substrate. The surface of the mylar is roughened so as to improve its receptivity for toner. Nevertheless, upon close examination it can be seen that black areas on the mylar contain many pinholes. These pinholes would cause objectionable artifacts to occur if the mylar were used as an imaging master. Paper is preferable to mylar for imaging inasmuch as it has the porosity and roughness (i.e. high surface area on a micro scale) to provide for a thick layer of toner on the surface. The fibrous structure of paper is ideal for enabling black areas to be imaged without pinholes or voids. 
     It should also be understood that paper transmits considerable amounts of light. While the amount is significantly less than might be transmitted by clear film, nevertheless enough light is transmitted to enable photosensitive materials to be imaged. The way the photosensitive material (e.g. sign media) is imaged is illustrated in FIG. 1. 
     The photosensitive media, 101, is placed, emulsion side, 120 to the right, next to a sheet of paper 102 which has been imaged with its toner side, 121, to the left. Toner exists in regions such as 103 on the paper 102. The two sheets 101 and 102 are held together by glass plate 106 and pressure pad, 105. Alternatively or in addition to the use of a pressure pad 105, a vacuum hold down system could be used (not shown). The paper is illuminated by ultraviolet light source 110 with optional reflector 111. The light passes through the paper 102 via, for example, paths 122 so as to cause areas 104 not obstructed by toner 103 to receive light. The exposure time is adjusted as needed to compensate for the amount of light absorbed and reflected by the particular type of paper being used. 
     The nature of the ultraviolet source 110 is determined to some extent by the spectral response curve of the photosensitive medium 101. For most photopolymer materials, significant illumination in the 350 nm wavelength region is desired. Ultraviolet lights such as the ones provided in EPROM erasers (the devices which erase the computer chips which hold data) are good choices. The party lights which cause some fluorescence of some materials, but are safe to the eye, are poor choices since the wavelengths which are harmful to the eye (350 nm and below) are best at generating photopolymerization. 
     After the exposure is completed, the photosensitive media 101 is removed and chemically processed in a manner consistent with practices well known in the state of the art. 
     In most situations the image created on the paper should be reversed, both in terms of right and left and in terms of black and white. This could be done in any of a number of ways, including the following: 
     A. Within the printer (not shown), an inverter (not shown) is inserted in the data path so as to reverse the &#34;laser on&#34; and &#34;laser off&#34; states. This causes whites and blacks to be reversed. Moreover, the direction of mirror movement is reversed and the &#34;start of scan&#34; sensor within the printer is moved to the opposite side of the scan line. This causes left and right sides to be reversed. Alternatively, the way data is removed from a data buffer is reversed, or opposite the order that would normally be used. 
     B. Within the raster image processor, the software is arranged so as to initially start with an entirely black page, in memory, with lines and fonts being represented as white areas on a black background, rather than the reverse. The details of how this would be done would depend on the Page Description Language used. For example, some computer programs support the Hewlett-Packard printers by use of a language called HPGL (Hewlett-Packard Graphics Language). Apple computers tend to use a much more complex, albeit slower, language called Postscript. A programmer would write modified application programs which would produce wrong reading and reverse video. Once that was done, the changes would be transparent to the user. 
     In the HPGL language, for bitmap graphics, the changes would be very straightforward. The order of the bits would simply be reversed, and the 1&#39;s and 0&#39;s would be reversed. For text apart from bitmap graphics the procedure would be somewhat more complex. The font library would be processed to make the fonts wrong reading and reversed. Moreover, instructions would be sent to the printer for placement of every character, so as to make the characters flow from right to left rather than the reverse. A record would be maintained for those areas which had been imaged with bitmap graphics or characters. Once the page is finished, font fill characters (black rectangles of various sizes) would be imaged to blacken the areas not otherwise imaged. In the Postscript language, the needed changes can be represented by variations in the &#34;setgray&#34; commands for white and black transformations, and by variations in a transformation matrix, which controls the way left, right, up, and down directions are interpreted. If the changes are applied within the application code, the modifications would be quite simple to implement. Apart from the application, in many cases the needed transformation matrices could simply be inserted at appropriate points into the file. Searches in the output file for transformation matrices and for setgray commands could be done, with appropriate changes made to those commands. Such modifications are well understood by those skilled in the art. 
     C. Within the host computer, the application software is modified so as to reverse the whites and blacks, and left and right senses. One of the first commands to be output would be one which would represent the entire page as black. This could be done as follows: 
     0 setgray 
     0 0 moveto 
     0 792 lineto 
     612 792 lineto 
     612 0 lineto 
     0 0 lineto fill 
     Subsequent to that, for example, a command which normally would have generated a Postscript code of 
     0 setgray 
     x1 y1 moveto 
     x2 y2 lineto 
     (which would draw a black line) would be changed to 
     1 setgray 
     x1 y1 moveto 
     x2 y2 lineto 
     which would draw a white line. 
     The second preferred embodiment is similar to the first, except that the paper is saturated with a chemical so as to improve its transmission of ultraviolet light. FIG. 2 illustrates this second embodiment. 
     It should be understood that a primary reason for paper&#39;s opacity is that the fibers of which it is composed have a significantly different refractive index from that of air. Internally within the paper is an extremely high surface area per unit volume, consisting of these fibers. Consider a ray of light passing into the paper. Light impinging at a more or less normal angle to a fiber will experience some reflection due to the difference in refractive index, but most of the light will pass into the fiber. It is likely that in some instances the light will exit from within the fiber at an angle such as to cause total internal reflection, in which case the light will usually stay within the fiber until it is absorbed. In other cases, the light will approach the fiber at such an angle so as to cause total reflection, in which case the light will bounce around within the paper until it either returns on the side of light incidence, or pass through the paper and exit on the toner side. 
     When a liquid is added which has a refractive index that is not very different from that of the fibers, significantly less reflection occurs as light enters the fiber, and the critical angles for total external reflection and total internal reflection are changed to make such circumstances much less likely. 
     FIG. 2 illustrates one way imaging can be done. As in the first embodiment, there is a pressure pad, 205, photosensitive media, 201, (emulsion side, 220, to the right), paper, 202, with toned side, 221, to the left, and toner areas 203. Glass plate 206 holds the components together, and light source 210 and reflector 211 provide illumination. 
     Liquid 207 saturates the paper, 202. Moreover, the areas between the paper 202 and the photosensitive media 201 are filled with this same liquid 207, these regions illustrated by 208. This liquid provides not only translucency of the paper, but also by virtue of its surface tension and viscosity, once air pockets are removed, holds the paper and photosensitive material together. Thus glass plate 206 and pressure pad 205 become optional rather than essential. 
     The application of the liquid to the paper can be done in a variety of ways, including putting the liquid in a squeeze bottle and manually squirting about 0.5 ml of liquid on the paper, with spreading of the liquid via a sponge if total coverage is not otherwise obtained. 
     The desirable qualities for the liquid, 207 and 208, are the following: 
     Refractive index similar to that of paper. 
     Surface tension characteristics so as to readily permeate the paper. 
     Viscosity low enough to not take too long to saturate the paper. 
     Nonreactive chemically with the paper, toner, and photosensitive material. 
     With traditional paper-based 3M Dynamark, ordinary mineral spirits satisfy all of these requirements. Other materials may work as well or better. Soapy water is poor inasmuch as it tends to weaken the paper, and takes too long, because of its surface tension characteristics, to penetrate the paper. Cooking oil is poor inasmuch as it takes too long to saturate the paper, because of its viscosity. 
     Mineral spirits (paint thinner) is good in that it readily saturates the paper, does not cause any wrinkling of the paper, does not loosen the toner, and provides good translucency. It is less than optimal, however, in that it has an objectionable smell. It is also less than optimal in that with very long exposure times (as would be required if the lamp is other than optimal), some chemical reaction with one, but not all, of the 3M Dynamark materials occurs. It is likely that some additional investigation would produce a better chemical which would not have any objectionable odor or health hazard, and yet have the desirable characteristics of the mineral spirits. 
     Referring to FIG. 3, the third preferred embodiment is similar to the second, except that a very thin film barrier is inserted between the paper and the photosensitive material. This film barrier allows the use of chemicals for saturating the paper which satisfy all of the desired characteristics except for not being unreactive with the photosensitive emulsion. One mil mylar is one example of such a material. It is thin enough that minimal image distortion occurs because of its use. One example of such a film is the original Saran Wrap which was not wrinkled. (The wrinkling is done to make it easier to pull off the roll if the leading edge is lost.) More specifically, 0.5 mil to 1.5 mil mylar or polyester film is suitable. 
     In FIG. 3, items 301 thru 308 correspond to items 201 thru 208 in FIG. 2. 
     Specifically, 301 is the photosensitive media, with emulsion side 320 to the right, facing paper 302 with toner 303 to the left. This left side is indicated by 321. Light from lamp 310 and reflector 311 passes through glass plate 306 and paper 302 with liquid 307, exposing photosensitive media 301 in areas not blocked by toner 303. The liquid within the paper is indicated by 308, and the pressure pad by 305. The item which is not present in the second preferred embodiment is film 309, which sits between the paper and photosensitive material. In this case, the pressure pad 305 and glass 306 are required, because the liquid 307 does not prevent separation of the materials as it does in the previously discussed second preferred embodiment. 
     In the fourth preferred embodiment a degree of automation is added, as illustrated in FIG. 4. The user places paper 402 containing toner, toner side 421 down, on top of photosensitive material (e.g. sign material) 401, with emulsion side 420 up, next to the toner side 421 of the paper. These two items are fed into slowly rotating nip rollers 432 and 433. The nip rollers are turned by a gear motor (not shown), with motion of the material between them being to the right. Nip roller 432 is porous, e.g. an open cell foam. A controlled amount of liquid, 431, stored within bottle 430, is allowed by solenoid or other automatic valve 429 to flow into the nip roller 432, saturating the roller and causing enough of this liquid to contact the paper, 402, to saturate it. As the paper and photosensitive material pass through the nip rollers, the two are in intimate contact so that air bubbles are forced out of their interface. As they emerge to the right of the nip rollers, they are illuminated by lamp 410 and optional reflector 411. The rate of rotation of the nip rollers is set to produce optimal exposure. Following exposure, the materials pass into exit slot 412. Such an automatic exposure unit as this is desirable inasmuch as it occupies very little table space. This is particularly important if large signs or other material are being imaged. The surface tension of the liquid insures that the paper and photosensitive material stay in intimate contact during exposure. 
     It will be obvious to those skilled in the art that the requirement for wrong reading and reverse video can be eliminated by imaging of a film intermediate first, and then imaging from that onto the final photosensitive media. While this requires an additional step, this would still fall within the scope of this invention. Such an imaging scheme would simplify the software requirements, at the expense of greater media cost and time to create the final image.