US3554749A - Process for producing photographic images with photosensitive materials and products produced thereby - Google Patents

Abstract

THIS INVENTION RELATES TO A PROCESS OF PRODUCING PHOTOGRAPHIC IMAGES USING PHOTOSENSITIVE MATERIALS WHICH HAVE BEEN PRESENSITIZED BY EXPOSURE TO ACTIVATING RADIATION SO THAT THE PHOTOSENSITIVE SUBSTRATE BECOMES SENSITIVE TO WAVELENGTHS OF RADIATION TO WHICH THE SUBSTRATE IS USUALLY NOT SENSITIVE. THE PHOTOSENSITIVE MATERIAL WHICH BECOMES REVERSIBLY ACTIVATED UPON EXPOSURE TO ACTIVATING RADIATION BUT PREFERABLY REMAINS UNCHANGED AFTER SUCH EXPOSURE IS HEREINAFTER REFERRED TO AS A PHOTOCONDUCTOR. THE PRESENSITIZED PHOTOCONDUCTOR IS EXPOSED TO A PATTERN OF LIGHT TO WHICH THE PRESENSITIZED PHOTOCONDUCTOR IS SENSITIZED TO FORM A LATENT IMAGE WHICH IS DEVELOPED BY KNOWN METHODS, E.G. USING LIQUID REDOX SYSTEMS TO OBTAIN THE VISIBLE IMAGE OF THE PATTERN. THE REDOX SYSTEM MAY BE APPLIED TO THE PHOTOCONDUCTOR EITHER BEFORE OR AFTER THE STEP OF LATENT IMAGE FORMATION. IT HAS ALSO BEEN FOUND THAT THE QUALITY OF THE VISIBLE IMAGE MAY BE SUBSTANTIALLY IMPROVED BY SELECTION OF LIGHT OF SPECIFIC WAVELENGTH TO ENSURE OPTIMUM DENSITY DIFFERENTIALS BETWEEN THE BACKGROUND AND THE VISIBLE IMAGE.

Description

Jan. 12,1971

G. M. FLETCHER ETA!- PROCESS FORPRODUCING PHOTOGRAPHIC IMAGES WITH PHOTOSENSITIVE MATERIALS AND PRODUCTS PRODUCED THEREBY Filed July 13, 1967 I I, I

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BY ma AGENT.

United States Patent 3,554,749 PROCESS FOR PRODUCING PHOTOGRAPHIC IMAGES WITH PHOTOSENSITIVE MATERIALS AND PRODUCTS PRODUCED THEREBY Gerald Matthew Fletcher, Arlington, Amal Kumar Ghosh,

Lexington, and Fahd G. Wakin, Bedford, Mass., assignors to Itek Corporation, Lexington, Mass., a corporation of Delaware Filed July 13, 1967, Ser. No. 653,198 Int. Cl. G03c 5/04 U.S. Cl. 96-27 27 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of producing photographic images using photosensitive materials which have been presensitized by exposure to activating radiation so that the photosensitive substrate becomes sensitive to wavelengths of radiation to which the substrate is usually not sensitive. The photosensitive material which becomes reversibly activated upon exposure to activating radiation but preferably remains unchanged after such exposure is hereinafter referred to as a photoconductor. The presensitized photoconductor is exposed to a pattern of light to which the presensitized photoconductor is sensitized to form a latent image which is developed by known methods, e.g. using liquid redox systems, to obtain the visible image of the pattern. The redox system may be applied to the photoconductor either before or after the step of latent image formation. It has also been found that the quality of the visible image may be substantially improved by selection of light of specific wavelength to ensure optimum density differentials :between the background and the visible image.

BACKGROUND OF THE INVENTION (a) Field of invention This invention relates to processes for production of photographic images using presensitized light-sensitive compositions and products obtained thereby.

(b) Description of the prior art The use of media comprising photoconductors for the production of latent images is described in British Pat. 1,043,250. In the patent, the method generally requires the formation on the media of a latent reversible image corresponding to a pattern of activating light and, at a subsequent time, conversion of the latent image to a visible image using liquid redox system which contact at least the light-activated portions of the media to form a deposit on the media corresponding to the light-activated portions. The product of this aspect of the described process is a negative corresponding to the initial pattern of activating light, which may be used to produce a positive in known manner. A further modification of the described process which results in a positive visible image involves the further step of deactivating those portions of the media which were activated by the initial pattern, after which development leads to the positive.

In the art of electrostatic printing, as described for example in US. Pats. 3,041,168 and 3,121,006, the basic principle involves providing a substantially uniform electrostatic charge on the photoconductive coating of the recording element followed by discharge of selected areas in order to produce an electrostatic image, which is then developed using powdered developing material, which process requires special developer materials and costly recording media.

SUMMARY OF THE INVENTION It has now been unexpectedly found that latent images may be produced utilizing presensitized media comprising a photoconductor. The latent image is produced by exposing such a presensitized medium to a pattern of light to which the medium is sensitive to obtain a latent image corresponding to the pattern. The latent image is then developed, preferably substantially immediately after formation for optimum results. The developer is preferably a liquid redox system which on contact with activated portions of the medium deposits a solid, visible residue on the activated portions. If desired, the redox system may be applied to the medium prior to the latent image formation step in which case the visible image is obtained when the medium is still wet. On the other hand, if the redox system is applied to the medium and the medium then dried prior to latent image formation, the visible image is obtained by immersion of the medium in a suitable solvent system.

Optimum results are obtained when latent image formation is conducted with light of wavelength to provide maximum density dilference between the visible image and the background of the medium. The determination of optimum wavelengths of light for this purpose is readily accomplished by determination of the spectral response of the medium before and after presensitization, as hereinafter described.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductor or photocatalyst is not limited to any group of compounds but may include both organic and inorganic photosensitive materials. Preferred photoconductors useful in this invention are metal containing photoconductors. A preferred group of such photosensitive materials are the inorganic materials such as com pounds of a metal and a nonmetallic element of group VIA of the periodic table 1 such as metal oxides, such as zinc oxide, titanium dioxide, antimony trioxide, aluminum oxide, zirconium dioxide, germanium dioxide, indium trioxide, chromium oxide, magnesium oxide, cerium oxide, hydrated potassium aluminum silicate tin oxide (SnO bismuth oxide (Bi O lead oxide (PbO), beryllium oxide (BeO), silicon dioxide (SiO- barium titanate (BaTiO tantalum oxide (Ta O tellurium oxide (TeO and boron oxide (B 0 metal sulfides such as cadmium sulfide (CdS), zinc sulfide (ZnS) I and tin disulfide (SnS metal selenides such-as cadmium selenide (CdSe). Metal oxides are especially preferred photoconductors of this group. Titanium dioxide is a preferred metal oxide because of its unexpectedly good results.

Also useful in this invention as photoconductors are certain flourescent materials. Such materials include, for example, compounds such as silver activated zinc sulfide, zinc activated zinc oxide, manganese activated zinc phosphate Zn (PO an admixture of copper sulfide, antimony sulfide (SbS) and magnesium oxide (MgO), and cadmium borate.

Organic photoconductors suitable for use in this invention are, for example, the imidazolidinones, the imidazolidinethiones, the tetraarylazacyclooctatetraenes, and thiazines, such as 1,3-diphenyl 4,5 bis(p-methoxyphenyl) imidazolidinone-Z; 4,5-bis(para-methoxyphenyl)imidazolidinone-2; 4 phenyl 5 (paradimethylaminophenyl) imidazolidinone-2; 4,5-bis(para-methoxyphenyl)imidazo- Periodic "table from Langes Handbook of Chemistry, 9th edition, pp. 56-57, 1956.

lidenthione-2; 3,4,7,8-tetraphenyl l,2,5,6 tetraazacyclooctatetraene-2,3,6,8; and methylene blue.

Also useful as photoconductors in this invention are the heteropolyacids such as phosphotungstic acid, phosphosilicic acid, and phosphomolybdic acid.

While the exact mechanism of the present process is not known, it is beileved that the presensitization, i.e. exposure to activating light, e.g. ultraviolet light, causes the transference of electrons of the photoconductor from the valence band to the conductance band, or at least to some similar excited states wherein the electron is loosely held thereby converting the photoconductor from an inactive to an active form. If the photoconductor in the active form is in the presence of an electron-accepting agent, a transfer of electrons will take place between the photoconductor and the electron-accepting agent and the latter will be reduced. Accordingly, a simple test to determine whether photoconductors have reducing properties is to mix the material in question with aqueous silver nitrate. In the absence of light, little, if any, reduction of silver ions should occur. At the same time as exposing the same mixture to light, a control sample of an aqueous silver nitrate solution alone is similarly exposed and if the mixture darkens faster than the control sample, the test material is a photoconductor with reducing properties.

It is evident that the gap between the valence and the conducting band of a compound determines the energy needed to make electron transitions and the light required to provide the needed energy is called bandgap light, as employed herein. The higher energy needed, the higher the frequency to which the photoconductor will respond. It is known in the art that electrons may be present in secondary levels within the band gap due to impurities on defects in the structure of the photoconductor. With light of suitable energy, which in this case would be less than the band gap, electrons from these levels could be raised to the conduction band. A typical example of a secondary level due to a defect in the structure would be an F-center (electrons trapped at negative ion vacancies in an alkali halide crystal). The band gap of KCl is about 8.5 ev. (1460 A.), but the secondary levels due to F-centers are about 2.4 ev (5400 A.) below the conduction band. Electrons could be raised to the conduction band with 5400 A. light. An example of an impurity photoconductivity could be ZnS doped with Cu. The band gap of ZnS is about 3.7 ev. (3350 A.), but by doping it with Cu one could introduce some secondary levels which would result in photoconduction due to 4600 A. light.

As is generally known, the activation of photoconductors, i.e. transference of electrons from valence bands to conductance bands, is not permanent but rather the activation decays primarily as a function of time. The decay is apparently due to the loss of electrons in the conductance bands, the electrons reverting to lower energy levels, many reverting to the original valence band and others to energy levels intermediate between the respective bands, i.e. secondary levels, or traps. After decay of the activated photoconductor, the medium retains little, if any, ability to reduce silver ions, or similar metal ion, due to the fact that there are little, if any, electrons in the conductance band. When reference is made herein to a decayed sensitized medium it is intended that the photoconductor is in a state intermediate between the active and inactive states by virtue of the fact that electrons of the photoconductor are in the secondary levels, or traps.

When the decayed sensitized medium is exposed to an image pattern of light of wavelength longer than the bandgap light, the energy provided is sufficient to raise the electrons in the secondary levels to the conductance band, but not sufiicient to raise electrons from the valence band to the conductance band. This results in a latent image on the medium corresponding to the pattern, and when the medium is brought into contact with an electron acceptor, electron transfer occurs. Accordingly, if the medium is contacted with a liquid re dox system, reduction of the reducible component thereof occurs. If the reducible component, in the reduced form, is a particulate solid, the result obtained is a visible image corresponding to the pattern.

The foregoing theoretical explanation is offered to enable a better understanding of the present invention and is believed to reasonably interpret the phenomenon of this invention. Of course, the applicants are not necessarily bound by this explanation.

In the sensitization of the photoconductor it is preferred to utilize high exposure energies of bandgap light. For example, the exposure energy should be about 10 millijoules/cm. or greater for best results, while energies ranging from 0.05 millijoule/cm. are found operable.

For decaying the photoconductor sensitized with bandgap light it is preferred to allow the medium to stand over a period of time usually of at least about one hour to ensure substantially complete decay. As mentioned hereinbefore, samples of the sensitized medium may be tested using, for example, aqueous silver salt solutions,

' to determine the time required for decay which will of course be dependent on the photoconductor, the exposure energy, and other factors known to those skilled in the art. After sensitization, the medium should be protected from exposure to activating light prior to exposure to a light pattern to form the latent image and subsequent to this step until the medium is developed. Techniques for avoiding unintended exposure are well known and need not be enumerated for those skilled in the art.

The exposure of the decayed sensitized photoconductor to the image pattern may be conducted using standard techniques. The light utilized is preferably visible light, of wavelength longer than the bandgap light of the photoconductor. Such wavelength preferably ranges from about 4200 A. to about 7000 A. and may even include the near infrared. Certain ranges are more effective depending on the photoconductor, among other factors, and a minimum of testing will indicate the optimum range for the specific photoconductor. For example, a range from about 4500 A. to about 6000 A. gives excellent results when titanium dioxide is the photoconductor. The time of exposure may be varied considerably, from fractions of a second to several minutes without appreciable variation in the results.

As should be obvious to those skilled in the art, the latent image at this stage may be stored of course protected from unintentional light activation, but care should be taken to avoid substantial decay of the latent image for which reason long periods of storage should be avoided. As desired, the visible image may be produced, or alternatively, modification of the latent image may be effected, e.g. by erasure, in whole or in part, of the latent image and/or super-imposing of a further image. Such erasure may be accomplished by reactivating the photoconductor with light of wavelength longer than bandgap light.

When it is desired to convert the latent image to a visible image, the medium is preferably immediately developed. Immediate development, while not necessary, minimizes the effect or decay of activation, for which reason it is preferred. In this respect, it may be advisable, but not necessary, to apply the developer to the medium prior to the latent image formation step and the visible image will form on latent image formation. If the developer requires solvent, the visible image will not form unless solvent is present. Therefore, if the developer including solvent is applied to the medium and the solvent is subsequently removed prior to latent image formation exposure of the treated medium to the solvent will result in visible image formation.

As indicated herein, the developer may be applied immediately after the latent image formation or at least within a reasonable period of time before appreciable decay of the activation as will be appreciated by those skilled in the art.

The developing agents preferred for this invention are liquid redox systems preferably comprising heavy metal ions such as silver, gold, copper, mercury, and other noble metal ions. British Pat. 1,043,250 fully describes the intended developing agents and processes for developing and fixing for use in this invention and is incorporated herein by reference.

As mentioned hereinbefore, optimum density difference between the visible image and the background of the medium is attainable by selection of light of specific wavelengths. The determination of the optimum wavelength is readily accomplished by a mere comparison of the spectral response of the photoconductor before and after bandgap light radiation. For example, the spectral response curve of normal titanium dioxide is determined by plotting the activation versus the wavelength of light, the activation being measured by the ability of the lightactivated photoconductor to reduce silver ions from solution as indicated by the density above fog on the thustreated medium. If the density above fog is plotted against the wavelength of light, the resulting curve approaches zero density as the wavelength approaches that of visible light.

When the titanium dioxide is sensitized with bandgap light, decayed and exposed to light of longer wavelength than bandgap light at the same exposure energy, the corresponding curve does not approach zero in the visible light region of the curve.

Typical'spectral response curves are presented in FIG. I which represents the spectral response of titanium dioxide both before and after ultraviolet sensitization and decay.

Curve A represents the spectral response of titanium dioxide (coated in thin layer on a sheet of paper) after exposure to light of wavelength 3660 A. at an intensity of 36.6 .watts/cm. for five minutes, after a wait time of 1.5 hours (to decay the activation). Samples of the medium comprising titanium dioxide so treated were then exposed to light of varying wavelength of exposure energy=15.3 joules/cm. and the density above fog measured for each sample after treatment with alcoholic silver nitrate. The curve was plotted on the basis of the density above fog corresponding to the wavelength of light applied.

Curve B represents the normal spectral response of identically treated samples of titanium dioxide sheets with ultraviolet sensitization omitted.

It is obvious from a comparison of the respective curves that light of from about 4500 A. and even lower to 4200 A. could be used to obtain density differences between the image and the background of the medium to obtain a visible image. It is obvious too that with light of wavelength lower than about -5000 A., the background would be of varying shades of gray but the image would be darker by comparison. Above 5000 A., the background should be relatively free of silver deposits while the image is of considerable density. Choice of optimum wavelength for the exposure to visible light becomes quite obvious and is determined by the desired end result. For absolute image clarity (black on white or near-white), wavelengths in the vicinity of 5000 A. and higher should be employed. Where such considerations are not of paramount interest, any wavelength ranging from 4200 A., up

to about 7000 A. could be employed, as practicality dictates.

When visible light of higher exposure energy is utilized,

the spectral response curve shifts to the higher wavelengthsas is evidenced by curves A and B which are determined in identical manner as curves A and B with the exception ferences are substantial over a longer and higher range of wavelength values. Optimum wavelengths of visible light should be quite apparent in view of the foregoing comments.

For the purpose of the foregoing discussion the curves of the graph represent the image areas and the background of the medium. Curve A corresponds to the area activated by visible light, i.e. the image while Curve B corresponds to the background, which is unaffected by the exposure to visible light to form the latent image.

In the initial sensitizing of the photoconductor, bandgap light is employed. Exemplary photoconductors with corresponding bandgap and absorption edges are listed in Table 1.

The initial sensitizing of the photoconductor may also be augmented by the presence of dyes in the photoconductor medium. As is well known, the sensitivity of the semiconductor may be increased by known sensitization techniques such as admixtures of dyes with the photoconductor, Dye sensitizing permits the use of light of wavelength longer than the bandgap light of the photoconductor.

When employed as data storage media according to the present invention, the photoconductor materials previously discussed herein can suitably be employed in bulk, e.g., in the form of a continuous layer. When used in image forming processes, the photoconductors are conveniently applied to a suitable backing which may be either porous or non-porous, such as of paper, wood, aluminum, glass and the like. The photoconductor, which is suitably used in the form of finely divided particles, may simply be deposited on the surface of such a backing, or can be deposited on such a backing in a hydrophobic or, preferably, a hydrophilic binder known to those skilled in the art of making radiation sensitive papers. Suitable hydrophobic binders, for example, include the polyvinylacetate resin binders commonly used in the preparation of papers for electrostatic printing processes. Typical of the preferred hydrophilic binders having a limited water solubility are gelatin, polyvinyl alcohol, and ethyl cellulose. for example, though many other materials of both types could be mentioned. Particularly advantageous results are employed when the finely divided photoconductor is merely dispersed in the interstices of a fibrous backing such as paper, the fibers of the backing acting to lock in and to hold the photoconductor particles in the finished structure. For example, the photoconductor is easily incorporated in paper during. its manufacture by methods known in the papermaking art.

A better understanding of the invention will be had by reference to the following examples, given by way of illustration of the methods of carrying out the invention.

EXAMPLE 1 A mixture of 4 parts by weight of titanium dioxide and 1 part by Weight of an emulsion of polyvinyl alcohol resin containing about 50 percent solids in water is used to coat paper sheets.

A sheet of the coated paper is exposed to light (ultraviolet) of wavelength 3660 A. for five minutes at an intensity of 50 ,uwatts/cm. The so-treated paper is allowed to decay by dark storage for 30 minutes, after which it is exposed to an image pattern of activating light of wavelength 4800 A. for five minutes at an intensity of 50 ,u.watts/cm. The latent image pattern is developed by dipping in a saturated solution of silver nitrate in methanol followed by dipping into a solution comprising g. phenidone, 40 g. of citric acid monohydrate and one liter of methanol. The resulting visible image bearing print is a negative of the latent image pattern which is fixed by immersion in an aqueous solution of sodium thiosulfate followed by washing in running water.

EXAMPLE 2 The procedure of Example 1 is repeated with the exception that the latent image formation is conducted with light of wavelength 5700 A. at the same intensity and for the same time of exposure, with similar results.

EXAMPLE 3 A sheet of the coated paper described in Example 1 is exposed to light of wavelength 3660 A. at an intensity of 50 watts/cm. for 5 minutes and allowed to decay by dark storage for 30 minutes.

The sheet is exposed to an image pattern of light of wavelength 6000 A. at an intensity of 83.3 ,uwatts/crn. for a period of 5 minutes to form a latent image of the pattern. Developing with silver nitrate as described in Example 1 gives a negative visible image corresponding to the image pattern.

EXAMPLE 4 The procedure of Example 1 is repeated using sheets prepared with an acrylate binder in lieu of polyvinyl alcohol with similar results.

EXAMPLE 5 A mixture of 4 parts by weight of titanium dioxide and one part by weight of an emulsion of polyvinyl alcohol resin containing about 50 percent solids in water is used to coat paper sheets.

A sheet of the coated paper is exposed to light of wavelength 3660 A. at an intensity of 79 watts/cm. for 5 minutes. The so-sensitized sheet is allowed to stand in dark storage for 1.5 hours after which the activation is substantially completed decayed. The decayed medium is then exposed to an image pattern of light of wavelength 5700 A. at intensity of 79 watts/cm. for a period of 5 minutes. The exposed sheet is developed by dipping into a saturated solution of silver nitrate in methanol and then in a solution of 5 g. of phenidone and 40 g. of citric acid monohydrate on one liter of methanol. A visible negative image of the positive exposure image is obtained. The visible image-bearin g print is then immersed in an aqueous solution of sodium thiosulfate and finally washed with running water.

EXAMPLE 6 The procedure of Example 5 is repeated using light of wavelength 6900 A. at an intensity of 316 ,uwatts/cm. in the image formation step with similar results.

The following examples illustrate the effectiveness of the present process in sensitizing photoconductors to visible light in contrast with media comprising photoconductors sensitized with dyes to visible light.

EXAMPLE 7 A titanium dioxide paper, prepared as in Example 1, is exposed to a mercury zenon lamp for 5 minutes at an intensity in the ultraviolet of approximately 20,000 ,uwatts/cm.

The thus-sensitized paper is stored for seven days and then samples are exposed to visible light for 5 sec. at all wavelengths longer than 4100 A. at the intensity given. Similarly a dyed paper containing titanium dioxide (D- 96 is similarly exposed to the same light at the same intensities with the following results:

1 2-p-dimethylarnlnostyrylt-methylthiazole methoehloride.

The procedure of Example 7 is repeated with the exception that the wait time after sensitizing the photoconductor is eight days and the exposure to visible light is for 15 seconds to all wavelengths longer than 4650 A. at the intensity given in the following results:

Density above fog Sensitized Dyed undyed paper paper Integgity (nwatts/cmfi):

EXAMPLE 9 The procedure of Example 8 is repeated employing all wavelengths of light longer than 4950 A. for 5 seconds at the intensity given in the following results:

Density above fog Sensitizetl Dyed undyed paper paper EXAMPLE 10 The procedure of Example 8 is repeated with the exception that the wait time after sensitizing is 16 days with almost identical results as illustrated in the following results:

Density above fog Sensitized Dyed undyed paper paper The sensitizing effect is thus illustrated to be a longlasting effect, which is quite stable.

The developer solution employed in the foregoing examples contains silver ion which is preferred. In practice, the developer may include any metallic ion which is at least as strong an oxidizing agent as ionic copper, e.g. gold, mercury, platinum, lead and copper.

In lieu of developing with redox system one may employ resins which are affected by the light activated areas of the photoconductor medium to produce relief images. For example, a resin coating comprised of 15 parts acrylamide to 1 part methylene-bis-acrylamide will be rendered insoluble to water and the soluble resin may be removed by water-washing the medium leaving the background of the medium free of resin while the image areas retain the insolubilized resin, resulting in both a visible and a relief image.

In general, any image-producing agent may be employed to correct the latent image into a visible image. For example, solid toners may be employed as described in British specification No. 935,621. In addition, visible images may be produced using charged particles as employed in xerographic developing. As is appreciated by those skilled in the art, the selection of suitable imageproducing agents is predicated on the activation of the photoconductor. I

If desired, the image-producing agent may be applied to the medium after decay of the initial sensitization, and prior to image exposure as mentioned hereinbefore.

The present process also permits use of dyes to sensitize the photoconductor to additional ranges of electromagnetic radiation. Such dyes are well known to the art and include, for example, cyanine dyes, dicarbocyanine dyes, the carbocyanine dyes, and hemicyanine dyesr After the sensitization of the photoconductor, image formation and development, the dye may be removed by dissolving the dye out of the substrate or by contacting with a suitable oxidizing agent. Preferably, the dye may be removed by contacting the medium with a solution of a thionate, e.g. sulfites and/or bisulfites, preferably in'the form of salts with alkali or alkaline earth metals. The preferred method of bleaching with thionates is described in commonly-owned copending application U.S. Ser. No. 641,126 filed May 25, 1967, the disclosure of which is incorporated herein byreference.

In the step of sensitizing-the medium comprising a photoconductor, the use of bandgap light for this purpose has been described as a preferred method. In addition to bandgap light the medium comprising a photoconductor may also be sensitized by use of gamma rays or X-rays, neutrons and/ or heat in lieu of bandgap light. The medium after sensitizing by these additional methods of activation is then allowed to decay and is useful in the same manner in producing images as herein described.

In the image-forming step, customary methods such as projection or contact printing may be utilized. A master is either projected on the medium or printed by contact with the medium using art-recognized procedures.

Since the present invention permits recording of images on recording media by use of ordinary light, i.e. visible light, it provides a relatively simple and economical method. A further advantage of the present process resides in the ease of erasure for correction, e.g. by exposure of selected areas of the latent image to high intensity of bandgap light of the photoconductor or by any of the art recognized procedures such as application of heat to the areas selected for erasure. Over printing may then be accomplished for effecting correction of the stored images. In actual use, the recording medium would be in the form of a roll of tape or film.

The latent images produced in accordance with the present invention, or development, yield negative visible images corresponding to the original pattern of light. Such negative visible images may be used in forming positive images used standard techniques known in the photographic art.

The latent images may also be used to produce positive visible images without being developed first as negatives. As previously mentioned, the latent images tend to decay as a function of time. The decay of the latent image may be facilitated by heat treatment using temperatures up to about 250 F. The decayed latent image, on flooding of the photoconductor medium with visible light preferably light of the same wavelength as that used in the latent image formation becomes a positive latent image, which on development e.g. with liquid redox systems, gives a visible positive image corresponding to the latent negative image. This procedure is described in commonly owned application U.S. Ser. No. 653,148, concurrently filed herewith.

What is claimed is:

1. A method of producing a latent image comprising the steps of:

(a) exposing to activating radiation a medium comprising a photoconductor to thereby activate exposed portions to form a latent image capable of reducing metal ions thereon corresponding to the exposed portion of the medium;

(b) decaying the latent image so formed so that the photoconductor is in a state intermediate between the active and inactive states in that the photoconductor on contact with a solution of metal ions in the absence of activating radiation is substantially non-reducing; and

(c) exposing the previously activated medium to light of wavelength longer than bandgap light for a time and intensity suflicient to activate the photoconductor so exposed such that said exposed photoconductor on contact with a solution of metal ions in the absence of activating radiation is substantially reducing.

2. A method as in claim 1 wherein the exposure of step (a) is a substantially uniform exposure and wherein the exposure of step (c) is an image-wise exposure.

3. A method according to claim 1 including the further step of producing a visible image by contacting the medium with an image-producing agent.

4. A method according to claim 3 wherein the imageproducing agent comprises one which undergoes an oxidation/ reduction type reaction upon contact with an activated photoconductor.

5. A method according to claim 4 wherein the medium is contacted with the image producing agent prior to exposure to said light of longer wavelength.

6. A method according to claim 4 wherein the medium 1s contacted with the image producing agent subsequent to exposure to said light of longer wavelength.

7. A method according to claim 1 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

8. A method according to claim 7 wherein the photoconductor is selected from the group consisting of titanium dioxide, zinc oxide, zirconium dioxide, alumnum oxide, chromium oxide, magnesium oxide, indium trioxide and cerium dioxide.

9. A method according to claim 1 wherein the photoconductor is titanium dioxide.

10. A method according to claim 2 wherein the light of wavelength longer than said bandgap light is of a wavelength which provides optimum density difierence bet-ween the visible image and the background of the medium.

11. A method according to claim 2 wherein the initial exposure is to the bandgap light of the photoconductor, or gamma or X-ray or neutron irradiation. A method of producing a latent image comprising: nltlally exposing and decaying so that the photoconductor 1s 1n a state intermediate between the active and inactive states in that conductance band electrons of the photoconductor are in secondary energy levels a copy medium comprrsmg titanium dioxide to thereby produce a decayed copy medium sensitive to light of Wavelength longer than the bandgap light of the titanium dioxide and then exposmg the decayed copy medium to a pattern of light of wavelength longer than the bandgap light of titanium dioxide to form a latent image thereon corresponding to said pat tern.

13. A method according to claim 12 including the further step of producing a visible image by contacting the medium with an image producing agent.

14. A method according to claim 13 wherein the imageproducing agent comprises a solution of metallic ions.

15. A method according to claim 14 wherein the medium is contacted with the image-producing agent prior to exposure to said light of longer wavelength.

16. A method according to claim 14 wherein the medium is contacted with the image-producing agent subsequent to exposure to said light of longer wavelength.

17. A method according to claim 12 wherein the light of longer wavelength is of a wavelength which provides optimum density difference between the visible image and the background of the medium.

18. A method according to claim 12 wherein the light of longer wavelength is of a wavelength ranging from about 4200 A. to about 7000 A.

19. A method according to claim 17 wherein the light of longer wavelength is of a wavelength ranging from about 4500 A. to about 6000 A.

20. A method according to claim 14 wherein the metallic ions are at least as strong an oxidizing agent as ionic copper.

21. A method according to claim 20 wherein the metallic ions are silver ions.

22. A method according to claim 12 wherein the initial exposure is to the bandgap light of titanium dioxide, gamma or X-ray or neutron irradiation.

23. A method according to claim 2 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

24. A method according to claim 1 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

25. A method according to claim 1 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

26. A method of producing a photosensitive medium comprising titanium dioxide which comprises the steps of exposing said medium to the bandgap light of titanium dioxide wherein the exposure energy is at least about 10 millijoules/cm. of surface of said medium and decaying the activation so produced so that the photoconductor is in a state intermediate between the active and inactive states in that conductance band electrons of the photoconductor are in secondary energy levels, and subsequently exposing said previously activated medium to light of a wavelength longer than bandgap light.

27. A method according to claim 26 wherein the exposure energy ranges up to about 20,000 milliwatts per square centimeter of surface of said medium.

References Cited UNITED STATES PATENTS 3,152,903 10/1964 Shepard et al. 9664 3,380,823 4/1968 Gold 9627 3,414,410 12/1968 Bartlett et al 961X FOREIGN PATENTS 1,043,250 9/ 1966 Great Britain 961 OTHER REFERENCES Shattuck et a1., 'Postexposure of Latent Electrostatic Images, IBM Tech. Discl., vol. 8, No. 4, September 1965, p. 529.

GEORGE F. LESMES, Primary Examiner 'R. E. MARTIN, Assistant Examiner US. Cl. X.R. 961; 25065.1

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