US3551147A - Process for producing photographic images with photosensitive material - Google Patents

Description

G. M. FLETCHER PROCESS FOR PRODUCING PHOTOGRAPHIC IMAGES Dec. 29, 1970 WITH PHOTOSENSITIVE MATERIAL Filed July 15, 1967 90:! EMOSV ALISNEG GERALD FLETCHER AGENT.

United States Patent Office 3,551,147 Patented Dec. 29, 1970 PROCESS FOR PRODUCING PHOTO-GRAPHIC IMAGES WITH PHOTOSENSITDE MATERIAL Gerald Matthew Fletcher, Arlington, Mass., assignor to Itek Corporation, Lexington, Mass., a corporation of Delaware Filed July 13, 1967, Ser. No. 653,147 Int. Cl. G03c 5/04 U.S. Cl. 96-27 34 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of producing photographic images by reactivating a decayed reversible latent image on a photosensitive substrate. The photosensitive substrate is comprised of a photosensitive material which becomes reversibly activated upon exposure to activating radiation but preferably remains unchanged after such exposure and is hereinafter referred to as a photoconductor. The decayed latent image is formed by exposure to a pattern of light to which the photoconductor is sensitive to form a reversible latent image which decays as a function of time. The decayed latent image is reactivated by exposure of the image to light of wavelength longer than that employed in the original image formation after which the image is rendered irreversible and visible as desired, by development by known methods, e.g. using image-producing agents. The invention also relates to an improved process of forming the original reversible latent image by use of activating means and after decay, partial or complete, of the latent image, exposing the image to light of wavelength longer than the band-gap light of the photoconductor. The quality of the visible image obtained by reactivation is improved by selection of light of specific wavelength to ensure optimum density difierences 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 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 systems 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.

The latent reversible image formed on the media are of sufiicient duration to permit storage of such images over protracted periods of time. The duration of the latent images is a function of the photoconductor of the media, the intensity of light utilized in image formation and other factors known to those skilled in the art. In some cases, it is found that the latent image will remain for periods of hours and even days, whereas in others, the latent image may decay rather rapidly particularly when the intensity of light used in the image formation step is not high. The decay of the latent image is evident in the quality of the visible negative image produced on development of the latent image, the degree thereof being determinate of the image intensity and quality. When complete decay occurs, the usual developing process will not produce a visible image.

SUMMARY OF THE INVENTION It has now been unexpectedly found that partially or completely decayed latent images on media comprising a photoconductor may be reactivated by exposure to light of wavelength longer than the activating light used in the original latent image formation. The reactivated reversible latent image may then be converted to an irreversible, visible image by known development methods. Production of the visible image is preferably substantially immediately after reactivation 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 developer may be applied to the medium prior to reactivation in which case the visible image is obtained on reactivation of the medium.

Optimum results are obtained when the original latent image on the photoconductor medium is formed with light of wavelength corresponding to bandgap light of the photoconductor. The wavelength of light utilized in re-- activating the latent image is preferably that which provides maximum density difference 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 sensitization of the medium with activating light, i.e. bandgap light, 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 compounds of a metal and a nonmetallic element of group VI-A 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, thorium oxide, cerium oxide, hydrated potassium aluminum silicate (K Al Si O -2HO), 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) and tin disulfide (SnS- metal selenides such as cadmium selenide (CdSe). Metal oxides are especially preferred photoconductors of this group.

Periodic Table from Langes Handbook of Chemistry, 9th edition, pp. 56-57, 1956.

Titanium dioxide is a preferred metal oxide because of its unexpectedly good results.

Also useful in this invention as photoconductors are certain fluorescent 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 teraarylazacyclooctatetraenes, and thiazines, such as 1,3-diphenyl-4,5-bis(p-methoxyphenyl)imidazolidinone-2; 4,5 bis(para methoxyphenyl)imidazolidinone-2; 4,5 bis(paramethoxyphenyl)imidazolidenthione-2; 3,4,7,8 tetraphenyl 1,2,5,6-tetraazacyclooctatetraene-2,4,6,8; and methylene blue.

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

While the exact mechanism of the present process is not known, it is believed 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 whereby 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 materials have a photoconductor effect 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.

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 more 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 or 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 latent image 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 latent image on the photoconductor medium is exposed to 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 suflicient to raise electrons from the valence band to the conductance band. This results in reactivation of the latent image on the medium, and when the medium is brought into contact with an electron acceptor, electron transfer occurs. Accordingly, if the medium after reactivation is contacted with a liquid redox 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 negative of the decayed latent image.

The foregoing theoretical explanation is offered to enable at 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.

The exposure of the decayed latent image on the photoconductor medium may be conveniently accomplished by flooding the entire medium with light of the selected wavelength. 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 even 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 need not be developed immediately but of course should be protected from unintentional light activation. As desired, the visible image may be produced before appreciable decay 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 heating the areas to be erased or by corona discharge. By exposure of the photoconductor in the active state to deactivating radiation such as infrared radiation the activated photoconductor can be erased so that the electrons from the conduction band are substantially moved into the valence band so that photoconductor is in the inactive state.

The time interval between reactivation and production of the visible image is preferably of short duration. Immediate development, While not necessary, minimizes the eflFect of 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 reactivation step and the visible image will form on reactivation. 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 reactivation, exposure of the medium to the solvent will result in image formation.

As indicated herein, the developer may be applied immediately after the reactivation step, 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 wavelenth of exposure energy =l5.3 m. joules/cm? and the density above fog measured for each example 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.

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 difierences between the image and the background of the medium to otbain a negative 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 Wavelengths as is evidenced by curves A and B which are determined in identical manner as curves A and B with the exception that the exposure energy of visible light used is 73.2 m. joules/cm. At higher exposure energy, the density differences 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 back ground of the medium. Curve A corresponds to the area activated by flooding with visible light, i.e. the image while Curve B corresponds to the area unafiected by exposure to visible light in the reactivation step.

In the initial sensitizing of the photoconductor bandgap 6 light is employed. Exemplary photoconductors with corresponding hand gap 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 admixture 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 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 I 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 tothe following examples, given by way of illustration of the methods of carrying out the invention.

EXAMPLES l-6 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.

Sheets of the coated paper are exposed to light images using 3660 A. light for five minutes at an intensity of 36.6 ,uwatts/cmfi. After a varied Wait period, i.e. decay time, the sheets are flooded with visible light of wavelength 5000 A. for one minute at an intensity of 300 ,awatts/cmfi. The sheets are then developed by dipping into a saturated solution of silver nitrate in methanol followed by dipping into a solution comprising 5 g. phenidone and 40 g. citric acid monohydrate in one liter of methanol.

The decay time of the original latent image is varied from 1 to 210 hours with the following results:

Original Image restored Decay image, by flooding time, density with light, hours above fog density above log EXAMPLES 7-9 Using the same procedures as in Examples l6, the sheets are exposed to image patterns with light of wave length 3660 A. for 30 seconds at an intensity of 36.6 awatts/cm. After decay, the sheets are flooded with visible light of wavelength 5000 A. for one minute at an intensity of 300 ,uwatts/cm. and the sheets developed in the same manner with the following results:

Original Image restored Decay image, by flooding I time, density with light, hours above tog density above fog EXAMPLE The procedures of Examples 16 are repeated using sheets prepared with an acrylate binder in lieu of polyvinyl alcohol with siniilar results.

EXAMPLE 1 1 The procedures of the preceding examples are repeated with zinc oxide as the photoconductor in lieu of titanium dioxide.

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 parts acrylamide to 1 part methylene-bis-acrylamide will be rendered insoluble to water and the soluble resin may be removed by water-washing to medium leaving the image areas of the medium free of resin while the background retains the insolubilized resin, resulting in both a visible and a negative relief image.

In general, any image-producing agent may be employed to convert the latent image into a visible image. For example, solid toners may be employed as described in British specification 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.

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 invention may be also employed in connection with positive image production as described in British specification 1,043,250. The positive image is pro duced by unformly exposing a medium comprising a photoconductor to activating radiation and exposing the activated medium to a pattern of deactivating radiation to form a latent image corresponding to the pattern. The latent image need not be developed immediately and, in many instances, it is desirable to avoid development until some time has elapsed, e.g. in data-storage systems. The activation remaining on the medium after image formation by deactivating tends to decay as a function of time but is reactivated by exposure to light of Wavelength longer than the activating radiation initially employed, after which development will provide visible images of excellent clarity as observed in the preceding examples. Such visible images are positive images where developers such as silver salts and reducing agents are employed. As previously mentioned, any image-producing agent may be used to render the latent image visible but the developed image will dilfer from that obtained in the preceding working examples by virtue of the fact that the latent image is inactive while the remaining areas of the medium are activated.

In the original uniform sensitization of the medium comprising the photoconductor, the sensitizer is prefera'bly the bandgap light of the photoconductor, X-ray or neutron irradiation or heat.

The light employed in the restoration of the latent image should preferably be of a wavelength which provides optimum density difference between the visible image and background of the medium as previously described.

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 discarbocyanine dyes, the carbocyanine dyes, and hemicyanine dyes. 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 by reference.

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

The present process provides a substantial improvement in the process of recording data on media comprising photoconductors, e.g. as described in the aforementioned British Patent 1,043,250, by enabling not only restoration of decayed latent images but also by increasing the intensity of latent images, as is evident from the results obtained in the examples.

Since this improved process substantially improves the efficiency of latent image storage, it is of particular importance in the area of data storage using media comprising photoconductors. A further advantage of the present process resides in the ease of erasure for correction, e.g. 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 such use, the recording medium would be in the form of a roll of tape or film. Of special significance, is the use of the present process for recording latent images of random events the occurrence of which is not predictable. When it is desired, the latent images of selected events may be rendered visible by developing, or, if none of the random events depict occurrences for which a permanent record is desired, it is possible to erase the developed images and reuse the medium. To illustrate, the present process may be used in conjunction with a motion picture type camera with intermittent film advance or with a still camera or a camera rising either roll or sheet film. The type of camera employed will be determined by the type of events or actions to be recorded and the particular photoconductor medium desired.

In this use of recording media, the present invention is of considerable importance in view of the restoration of decayed latent images in such media.

Commonly owned copending application Ser. No. 360,006, filed Apr. 15, 1964, now US. Pat. 3,390,620, describes suitable apparatus and processes for this use and the disclosure thereof is incorporated herein by reference.

What is claimed is:

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

(a) imagewise exposing to activating radiation 2. medium comprising a photoconductor to thereby activate exposed portions to form a latent image capable of reducing metal ions thereon corresponding to said imagewise exposure;

(b) decaying the latent image so formed 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

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

2. A method according to claim 1 wherein the medium is contacted with an image-producing agent to form a visible image after step (c).

3. A method according to claim 2 wherein the image producing agent comprises a solution of metal ions.

4. A method according to claim 3 wherein the medium is contacted with the image producing agent prior to exposure to said light of longer Wavelength.

5. A method according to claim 3 wherein the medium is contacted with the image producing agent subsequent to the exposure to said ilght of longer wavelength.

6. 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 and wherein the exposure of step (c) is for a time and intensity at least equivalent to one minute at an intensity of 300 ,uwatts/cm.

7. A method according to claim 6 wherein the photoconductor is selected from the group consisting of titanium dioxide, zinc oxide, zirconium dioxide, aluminum oxide, chromium oxide, magnesium oxide, thorium oxide and cerium dioxide.

8. A method according to claim 1 wherein said pattern of activation is produced using the bandgap light of the photoconductor.

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

10. A method according to claim 1 wherein the light of wavelength longer than said activating light is of a wavelength which provides optimum density diiference between the visible image and the background of the medium.

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

(a) exposing a medium comprising titanium dioxide to a pattern of light of wavelength corresponding to the bandgap light of titanium dioxide to form a latent image thereon corresponding to said pattern;

(b) decaying the latent image, so formed, until the photoconductor on contact with a solution of silver ions in the absence of activating radiation is substantially non-reducing; and

(c) exposing the medium to light consisting essentially of light of wavelength ranging from about 4200 A. to about 7000 A.

12. A method according to claim 11 wherein the medium is contacting with an image producing agent which undergoes an oxidation/reduction reaction to form a permanent image after step (c).

13. A method according to claim 12 wherein the image-producing agent comprises a solution of metallic 14. A method according to claim 11 wherein the light employed in step (c) is of a wavelength which provides a maximum density difference between the visible image and the background of the medium.

15. A method according to claim 12 wherein the medium is contacted with the image-producing agent prior to the exposure to said light of longer wavelength.

16. A method according to claim 12 wherein the medium is contacted with the image-producing agent subsequent to the exposure of said light of longer wavelength.

17. A method according to claim 13 wherein the metallic ions are silver ions.

18. In the method of exposing a medium comprising a photoconductor to an image pattern of activating light to form a latent image corresponding to the pattern and subsequently rendering the latent image visible by contacting the medium with an image-producing agent, the improvement which comprises the additional step subsequent to the exposure step of decaying the medium 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 exposing the medium to light of wavelength longer than said activating light prior to contacting with said agent.

19. A method according to claim 1 wherein the photoconductor is dye-sensitized.

20. A method according to claim 1 comprising the additional step of at least partially erasing the latent image formed in step (a) prior to the exposure step of step (c).

21. A method of producing an image comprising the steps of:

(a) uniformly exposing a medium comprising a photoconductor to activating light;

( b) exposing the so-activated medium to a pattern of deactivating radiation to erase the activation of the photoconductor so that the electrons from the conduction band are substantiallv moved into energy levels near the valence band and the photoconductor is in the inactive state to form a latent image thereon corresponding to said pattern;

(c) decaying the activated medium 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 nonreducing; and

(d) exposing the medium to light of wavelength longer than bandgap light.

22. A method according to claim 21 wherein the deactivating radiation is infrared radiation.

23. A method according to claim 21 wherein the medium is contacted with an image-producing agent which undergoes oxidation/reduction reaction to form a permanent image after step (c).

24. A method according to claim 23 wherein the image-producing agent comprises a solution of metallic ions.

25. A method according to claim 24 wherein the medium is contacted with the image-producing agent prior to the exposure to said light of longer wavelength.

26. A method according to claim 24 wherein the medium is contacted with the image producing agent subsequent to the exposure to said light of longer wavelength.

27. A method according to claim 21 wherein the photoconductor is a compound of a metal with a non-metallic element of Group VI-A of the Periodic Table.

28. A method according to claim 27 wherein the photoconductor is selected from the group consisting of titanium dioxide, zinc oxide, zirconium dioxide, aluminum oxide, chromium oxide, magnesium oxide, thorium oxide and cerium dioxide.

29. A method according to claim 21 wherein said pattern of activation is produced using the bandgap light of the photoconductor.

30. A method according to claim 21 wherein the photoconductor is titanium dioxide.

31. A method according to claim 21 wherein the light of wavelength longer than bandgap light is of a Wavelength which provides optimum density diiference between the visible image and the background of the medium.

32. A method according to claim 24 wherein the metallic ions are silver ions.

33. A method as in claim 1 wherein the decaying of step (b) is for a time period of at least 4 hours.

34. A method as in claim 11 wherein the decaying of step (b) is for a time period of at least 22 hours.

References Cited UNITED STATES PATENTS 3,152,903 10/1964 Shepard et a1. 96-64 3,380,823 4/1968 Gold 96-27 3,414,410 12/1968 Bartlett et al. 961X FOREIGN PATENTS 1,043,250 9/1966 Great Britain 961 OTHER REFERENCES Shattuck et al., Postexposure of Latent Electrostatic Images, IBM Tech. Discl., vol. 8, No. 4, Sept. 4, 1965, p. 529.

GEORGE. F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner

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