US3661587A

US3661587A - Photographic media

Info

Publication number: US3661587A
Application number: US717976A
Authority: US
Inventors: Howard Adrian
Original assignee: Itek Corp
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1968-04-01
Filing date: 1968-04-01
Publication date: 1972-05-09
Anticipated expiration: 1989-05-09

Abstract

In photographic elements comprising a reversibly activated photoconductor and incorporating a photographic reducing agent, increased shelf life results from using an N-(carboxy-substituted alkyl) aniline compound containing an amino or hydroxy substituent on the aromatic nucleus as the reducing agent.

Description

O United States Patent [151 3,661,587

Adrian 1 May 9, 1972 54] PHOTOGRAPHIC MEDIA OTHER PUBLICATIONS [72] Inventor: Howard Adrian, Sudbury, Mass. llford Manual of Photography, 5th Ed., (1958) p. 399 [73] Assignee: Itek Corporation, Lexington, Mass. 1322:31 1 ndbok of Photography, McGrawnHm, [22] Filed; APB 1, 9 Clerc. Photography, Theory & Practice, 2nd Ed., Henry [52] U.S. Cl. ..96/76, 96/48 PD, 96/66 [51] Int. Cl ..G03c 1/48 [58] Field of Search ..96/29, 48, 64, 66, 76

[56] References Cited UNITED STATES PATENTS 2,163,166 6/1939 Wilmanns et a1. ..96/66 3,152,903 10/1964 Shepard et al. ..96/64 3,424,582 1/1969 Berman et al ..96/48 Greenwood & Co., Ltd, London 1937) p. 243

Primary E.\'aminer--Norman G. Torchin Assistant E.\aminer.lohn Winkelman Att0rney-1-lomer 0. Blair, Robert L. Nathans and W. Gary Goodson [57] ABSTRACT In photographic elements comprising a reversibly activated photoconductor and incorporating a photographic reducing agent, increased shelf life results from using an N-(carboxysubstituted alkyl) aniline compound containing an amino or hydroxy substituent on the aromatic nucleus as the reducing agent.

13 Claims, No Drawings PHOTOGRAPHIC MEDIA BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved photographic medium containing a photographic reducing agent in the medium.

2. Description of Prior Art Data or image storage media comprising radiation-sensitive materials such as titanium dioxide are described in detail in U.S. Pat. Nos. 3,152,903; 3,052,541; French Pat. Nos. 345,206 and 1,245,215 and in commonly owned co-pending U.S. application, Ser. No. 199,211, filed May 14, 1962 in the name of Elliot Berrnan et al, now abandoned. In the aforementioned US. patent application, a radiation-sensitive material, such as titanium dioxide, functions as a photosensitive component of the media and exposure of said media to activating means such as radiant energy, electron beams or the like results in the storage of a reversible latent image pattern therein. The reversible latent image pattern exists for a finite time during which said pattern can be converted to an irreversible form and read out visually by contacting said pattern with a suitable image forming material, such as a chemical redox system. In the aforesaid U.S. and French patents, the radiation-sensitive material is combined with at least one component of an image-forming material prior to exposure to activating means. For example, U.S. Pat. No. 3,052,541 describes photosensitive copy media comprising a photosensitive material such as titanium dioxide in combination with a reducible metal ion such as silver nitrate. This copy media is exposed to activating means and then contacted with a reducing agent to produce a visible image. On the other hand, U.S. Pat. No. 3,152,903 discloses a system wherein the photosensitive material is used in combination with both oxidizing agent such as silver nitrate and a reducing agent such as hydroquinone. Upon exposure to suitable activating means, a visible image is formed, One of the limitations of the above-mentioned data or image storage systems is that those photo graphic systems containing the reducing agent, e.g., hydroquinone, in or in contact with the photosensitive medium rapidly undergo a loss of potency of the reducing agent, presumably due to degradation of the reducing agent while the medium containing same is stored, that is for significant periods of time prior to photoexposure and development. This is a well-known drawback in the use of the common photographic reducing agents, hydroquinone and metol. When used, the reducing agents must be stabilized, e.g., with oligosaccharides, and even when stabilized the maximum permitted shelf-life is only about 6 months.

The incorporation of the reducing agent in the photosensitive medium is highly desirable since it obviates the need of one of the steps of conventional photographic development, which is obviously quite desirable particularly in automated processing. Thus, there is a need for photographic, i.e., photosensitive, media in which the reducing agent is present, with the media retaining a substantial amount of the reducing potential of the reducing agent over prolonged periods of storage. Previous attempts at providing such media have, as yet, been unsuccessful SUMMARY OF THE INVENTION It has now been unexpectedly found that certain photographic reducing agents can be incorporated into photosensitive media, especially those previously described, comprising at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern and remain substantially stable over prolonged periods of storage. The reducing agents are N-(carboxy-substituted alkyl)anilines with an amino or hydroxy substituent on the aromatic nucleus. Additionally the photosensitive medium may comprise at least one image-forming oxidizing agent.

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 VIA of the periodic table (*Periodic table from Langes HANDBOOK OF CHEMIS- TRY, 9th Edition, pp. 56-57, 1956) such as metal oxides, such as zinc oxide, titanium dioxide, antimony trioxide, aluminum oxide, zirconium dioxide, germanium dioxide, indium trioxide, hydrated potassium aluminum silicate (K Al Si O H O), tin oxide (SnO bismuth oxide (Bi O lead oxide (P- bO), 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 selendides such as cadium selenide (CdSe). Metal oxides are especially preferred metal oxide because of its unexpectedly good results. Titanium dioxide having an average particle size of about 250 millimicrons or less is a preferred photoconductor for this invention.

Also useful in this invention as photoconductors are certain fluorescent materials. Such materials include, for example, compounds such as silver activated zinc sulfide and zinc activated zinc oxide Organic photoconductors suitable for use in this invention are, for example, the imidazolidinones, the imidazolidinethiones, the tetraarylazacyclooctatetraenes, and thiazines, such as l,3-diphenyl-4,5-bis( p-methoxyphenyl)imidazolidinone-2; 4,5-( bis(para-methoxyphenyl)imidazolidinone-2; 4-phenyl-5-(para-dimethylaminophenyl)imidazolidinone-2; 4,5-bis(para-methoxyphenyl)imidazolidenthione-Z; 3,4,7,'8-tetraphenyll ,2,5,6-tetraazacylco-octatetraene-2,4,6,8; and methylene blue.

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

While the exact mechanism of the present process is not known it is believed that the sensitization, 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 theabsence 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.

The reducing agents contemplated are N-(carboxy-substituted alkyl)anilines with anamino or hydroxy substituent on the aromatic nucleus. Such compounds include phenylene diamine compounds in which one of the amino groups contains the said substituent group and aminophenol compounds with the required N-substituent. The benzene ring of these compounds can contain further substituents such as alkyl, e. g., methyl, ethyl, butyl, n-octyl; halogen, e.g., chloro or bromo; alkoxy, e.g., methoxy or propoxy; or amino, e.g., NH NI-ICl-I N(Cl-l NHEt etc. In addition, the carboxy-substituted alkyl group can contain additional substituents, e.g., hydrocarbon radicals, such as methyl, ethyl, cyclohexyl, propyl, phenyl, benzyl, tolyl and the like, with substituents such as the carboxy, alkyl, alkoxy, halo and amino.

The particularly preferred reducing agents are those of the formula:

in which R is hydroxy or amino; R is hydrogen or lower alkyl; and Z is an alkylene radical containing up to five carbon atoms in the principal chain between the two valences of the radical; or an acid addition salt thereof.

Of these compounds, the preferred are those in which substituent R is para to the amino substituent. Exemplary of the preferred compounds are:

N-(p-hydroxyphenyl)glycine N-( p-aminophenyl)glycine N-(p-hydroxyphenyl)-a-alanine N-(p-hydroxyphenyl)-B-alanine which are readily available or easily prepared. Additional compounds falling with the ambit of the foregoing description include:

N-(o-aminophenyl)glycine N-(p-aminophenyl)glutamic acid N-(o-hydroxyphenyl )aspartic acid N-( p-aminotolyl)-a-aminobutyric acid N-(p-amino-o-methoxyphenyl)glycine N-(p-hydroxyphenyD-lysine N-( p-hydroxyphenyl)a,a-diaminoppimelic acid N-(p-hydroxyphenyl)-B-phenylalanine Of the foregoing compounds, the preferred is N-(p-hydroxyphenyl)glycine since it is readily available and economical.

The reducing agent may be incorporated into the photosensitive medium by any of the art-recognized methods. The reducing agent can be in the emulsion containing the photosensitive material, e.g., the photoconductor, or in a suitable undercoating or overcoating for the photosensitive layer. The reducing agent and the photosensitive material, e.g., the photoconductor, may be premixed and then applied to a suitable substrate, e.g., film, paper, metal foil, fabric, glass and the like, using suitable binding materials. Alternatively, the photosensitive medium may be sprayed with or immersed into a solution of the reducing agent and the solvent then removed by drying. In all instances, it is preferred to obtain a substantially uniform distribution of the developing agent for maximum efficiency. For improved shelf-life, it is preferred to maintain some water in the emulsion layers on the substrate. Accordingly, the drying of the coating on the substrate can be readily controlled to permit retention of any desired level of water content. Preferably, the water content is controlled at or near the maximum content that can be tolerated by the coating without sacrificing mechanical properties the are required.

The concentration of the reducing agent on the photosensitive medium is not critical and can be determined with a minimum of routine experimentation. As will be appreciated by those skilled in the art, the amount of reducing agent will be determined by the sensitivity requirements of the developing process used, as well as by the requirements of image density, the degree of photoactivation of the photosensitive material of the medium and similar such considerations. For most purposes, a concentration of at least about 0.1 percent by weight based on the photosensitive material is usually desirable although it is generally preferred to use slightly higher concentrations, particularly where long shelf-life of the photographic media is contemplated, e.g., concentrations of at least about 0.5 percent by weight and higher. Of course, should the quantity of reducing agent prove less than necessary, it is always possible to augment the reducing potency by addition of reducing agent during processing. However, to realize the full benefit of the present invention, it is preferred to utilize sufficient reducing agent in the photographic medium to meet any eventuality, particularly long shelf-life. This is accomplished by using excess reducing agent in the formulation of the present photographic media. The maximum amount of reducing agent is dictated by the tolerance of the emulsion therefor since excessive amounts may render the emulsions too difficult to handle in processing. In general, the amount of the reducing agent is dictated by practicality of handling and the usual dictates of reason with which the skilled artisan is more than familiar. For best overall results, concentrations in the range of 3 percent to [0 percent by weight based on the photosensitive material will suffice to meet any eventuality and at the same time be economically feasible.

The reducing agents of the present invention may also be used in conjunction with auxiliary developing agents which can be incorporated into the photographic medium. Since these agents are not the primary reducing agent, they may be used in smaller concentration than the principal reducing agent. Thus, it is possible to utilize small concentrations of conventional developing agents such as hydroquinone and Metol [p-(N-methylamino)phenol] or alternatively pyrazolidone derivatives such as l-phenyl-B-pyrazolidone, lp-tolyl-3-pyrazolidone and other known pyrazolidone derivatives. If desired, these auxiliary reducing agents may be utilized in the developing process rather than incorporating them in the photosensitive medium.

The inert carrier sheet of the present new media comprises any suitable backing of sufficient strength and durability to satisfactorily serve as a reproduction carrier. The carrier sheet may be in any form such as, for example, sheets, ribbons, rolls, etc. The sheet may be made of any suitable material such as wood, rag content paper, pulp paper, plastics such as polyesters, e.g., polyethylene terephthalate, and cellulose acetate, cloth, metallic foil and glass. The preferred form is a thin sheet which is flexible and durable.

It is also useful to use a binder agent to bind the photosensitive material and reducing agent to the carrier sheet. In general, these binders are translucent or transparent so as not to interfere with transmission of light therethrough. Preferred binder materials are organic materials suchas resins, e.g., butadiene-styrene copolymers, poly(alkyl-acrylates) such as poly(methylacrylates), polyamides, polyvinylacetate, polyvinyl alcohol and polyvinylpyrrolidone. When the photographic media of this invention are used to record information, e.g., images, the photoconductor should be deactivated, conveniently by conditioning in the dark before photoexposure. Such conditioning is generally conducted from I to 24 hours. After deactivation, the photoconductor is not exposed to activating light prior to its exposure for recording an image pattern.

The period of exposure will depend on the intensity of the light source, the particular imaging material, the photoconductor, the type and amount of catalyst, if any, and like factors known to the art. In general, however, the exposure may vary from about 0.001 second to several minutes.

The photographic media of this invention can be used to produce positive or negative prints as described in the aforesaid copending application, Ser. No. 199,21 1.

The photographic media of this invention are especially sensitive to ultraviolet light but the spectral sensitivity may be altered by art-recognized procedures. For example, the spectral sensitivity is extended into the visible region of the spectrum by inclusion of dyes, such as cyanine and hemicyanine dyes as described in commonly assigned copending application, Ser. No. 633,689 filed Apr. 26, l967, or by doping of the photoconductor with metal ions, such as chromium, tungsten and molybdenum.

After the photographic media of this invention are photoexposed, a visible image is obtained by contacting the exposed media with image-forming materials such as described in the aforesaid copending application, Ser. No. 199,2ll and in US. Pat. 3,152,903. These image-forming materials include oxidizing agents and are, in general, the oxidizing component of what are generally referred to in the art as physical developers. The oxidizing agent is generally the image-forming component of the image-forming material. Either organic or inorganic oxidizing agents may be employed and preferably include the reducible metal ions having at least the oxidizing power of cupric ion such as silver, mercuric, plumbic, auric, platinum, nickelous, stannous, plumbous, cuprous and cupric ions. Other suitable oxidizing agents are permanganate ion, various leuco dye materials such as disclosed in commonly assigned copending application, Ser. No. 623,534, and the like. Organic oxidizing agents include tetrazolium salts, such as tetrazolium blue and red diphenyl carbazone and genarcyl red 68 (methine dye). As previously mentioned, where desired the exposed media may be contacted with additional reducing agents which supplement the reducing action built into the media of this invention, either together with the oxidizing agent, or in a subsequent treatment.

The image-forming materials or oxidizing agent may contain organic acids which can react with metal ions to form complex metal anions. Further, these agents may contain other complexing agents and'the like to improve image formation and other properties found to be desirable in this art.

Additional stabilizing and fixing steps such as known to the art may also be added to the processes of this invention in order to increase the life and permanence of the final print.

The following examples are given to illustrate this invention.

EXAMPLE 1 This example illustrates the stability of the reducing agent in the photographic media of this invention.

Two grams of N-(phydroxyphenyl)glycine is added to 238 g. of a slurry of titanium dioxide and polyvinyl alcohol (in a ratio of 4:l) in water 19 percent solids). The resulting mixture is hand coated on paper sheets (Sample A) and the sheets are air-dried. A second series of sheets (Sample B) is prepared in identical manner incorporating glycerine along with the reducing agent. Samples of each of the Sample A and Sample B variations are periodically exposed to light using a Mervap light source of seconds exposure and then processed to obtain a visible image, the density of which is measured on a standard densitometer. The processing solutions are:

Sensitizer: 55 mg. of silver nitrate/liter of distilled water. Fixer: benzotriazole (4 g.) sodium sulfite (2 g.) methanol (100 ml.) distilled water (550 ml,)

The exposed media containing the reducing agent are immersed in the sensitizer for l0 seconds, in fixer for 20 seconds and a water wash for seconds. Fresh solutions are used for each of the periodic determinations.

The results are summarized in the Table l which follows:

Control samples of the paper without developer but processed identically showed a density of zero.

The overall relative speeds of the tested samples decrease with time but the reducing potency does remain significant particularly in the samples which lacked glycerine. The D max for similar tested samples prepared by controlled automated coating on paper followed by controlled heating to dry the coated sheets are superior to those obtained by hand preparation.

The reducing potency of similar sheets containing Metol (pmethylaminophenol) tested under identical conditions is zero after 1 week as evidenced by failure to obtain a visible image on sensitizing with aqueous silver nitrate.

EXAMPLE 2 The procedure of Example 1 is repeated except the amount of reducing agent is 6 grams instead of 2 grams with similar results.

EXAMPLE 3 EXAMPLE 4 Double-weight baryta paper is coated with an emulsion of the following constituents:

10 g. titanium dioxide 4 g. N-(p-hydroxyphenyl)glycine 0.3 g. commercial wetting agent 10.0 g. gelatin 1.3 g. glycerine 2.0 g. 3.7% formalin (pH of the emulsion 7.8)

Samples of the coated paper are periodically exposed for 30 seconds in a Rexo contact printer, then sensitized by dipping into aqueous silver nitrate for 0.5 second, and fixed by immersion in aqueous sodium thiosulfate (40 g./l.) followed by water washing. The image densities are then determined. The results are shown in Table 2.

TABLE 2 Time (weeks) Dmax. l 1.01 2 0.93 3 0.93 4 0.89

EXAMPLE 5 An accelerated test to simulate periods of storage commonly employed with silver halide emulsion films is used to determine the stability of the present media. Coated paper samples'prepared by the procedure of Example 4 are heated for 3 days at F. and 62 percent "relative humidity (simulating one year storage'for silver halide emulsion film). The samples are then exposed, sensitized, fixed and washed as in Example 4 and the image densities determined. These samples are compared with identical paper samples not subjected to the accelerated test but otherwise identically exposed, sensitized, fixed and washed. The results show that there is no appreciable difference in the Dmax of the developed images of the respective samples.

Baryta paper coated with an emulsion containing 4.2 of N- (p-hydroxyphenyl)glycine and 10 g. of titanium dioxide (emulsion weight 200 g.) is subjected to the same accelerated test and the reducing potency retained is adequate by photographic standards.

EXAMPLE 6 The procedure of Example 4 is repeated using Lee Schoeller paper in lieu of the double-weight baryta paper with similar results.

For example, the paper is coated with an emulsion containing 2.0 g. N-(p-hydroxyphenyl)glycine and 10 g. of titanium dioxide (total emulsion weight 200 g.). After 1 week, the coated paper is exposed, sensitized, fixed and washed as in Example 4 and the Dmax of the visible image is 1.36.

The image-forming materials can also be incorporated into the media of the present invention prior tophotoexposure and the visible image obtained after exposure by immersion in water, or by applying water to the medium by art-recognized techniques, e.g., by spraying. On photoexposure, a medium containing the image-forming material and the reducing agent will become at least partially developed since the oxidationreduction reaction can take place in the medium itself prior to water-wetting.

The methods for incorporating the image-forming materials into the present media are generally known to those skilled in the art and have been described in the literature, e.g., in, U.S. Pat. No. 3,152,903. For example using silver ion as an example, silver nitrate in solution can be applied to the present media in the same general manner as the reducing agent of this invention, preferably in the binder prior to coating the substrate as hereinbefore described.

EXAMPLE 7 Double weight baryta paper is coated with the emulsion described in Example 4 to which is added 4.0 g of silver nitrate.

Photoexposure of the coated paper followed by water spraying gives a visible image of good density.

What is claimed is:

1. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; and

b. as photographic reducing agent, an N-[carboxy-substituted alkyllaniline with an amino or hydroxy substituent on the aromatic nucleus.

2. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; and;

b. a photographic reducing agent of the formula in which R is hydroxy or amino;

R is hydrogen or lower alkyl;

and,

Z is an alkylene radical containing up to five carbon atoms in the principal chain between the two valences of the radical;

or an acid addition salt thereof.

3. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern;

b. a photographic reducing agent of the formula in which R is hydroxy or amino; R is hydrogen or lower alkyl; and,

Z is an alk]ylene radical containing up to five carbon atoms in t eprmcipal chain between the two valences of the radical; or an acid addition salt thereof.

4. Medium as in claim 3 wherein the reducing agent is N-(phydroxyphenyl )glycin'e.

5. Medium as in claim 3 wherein the reducing agent is N-( phydroxyphenyl )-a-alanine.

6. Medium as in claim 1 wherein the photoconductor is an inorganic compound formed between a metal and a nonmetallic element of Group VlA of the Periodic Table.

7. Medium as in claim 3 wherein the photoconductor is a semiconductive metal oxide or metal sulfide.

8. Medium as in claim 3 wherein the photoconductor is titanium dioxide.

9. Medium as in claim 3 wherein the photoconductor is titanium dioxide of an average particle size of about 250 millimicrons or less.

10. Medium as in claim 3 wherein the photoconductor is titanium dioxide of an average particle size of about 250 millimicrons or less and the reducing agent is N-(p-hydroxyphenyl)glycine.

l l. A photographic medium comprising at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; at least one image,- forming oxidizing agent; and a reducing agent therefor, characterized in that the reducing agent is an Nicarboxy-substituted alkyl) aniline with an amino or hydroxy substituent on the aromatic nucleus.

12. Medium as in claim 11 wherein the reducing agent is N- (p-hydroxyphenyl)glycine.

13. Medium as in claim 12 wherein the image-forming agent is silver ion.

Claims

2. A photographic medium comprising: a. at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; and; b. a photographic reducing agent of the formula in which R is hydroxy or amino; R1 is hydrogen or lower alkyl; and, Z is an alkylene radical containing up to five carbon atoms in the principal chain between the two valences of the radical; or an acid addition salt thereof.
3. A photographic medium comprising: a. at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; b. a photographic reducing agent of the formula in which R is hydroxy or amino; R1 is hydrogen or lower alkyl; and, Z is an alkylene radical containing up to five carbon atoms in the principal chain between the two valences of the radical; or an acid addition salt thereof.
4. Medium as in claim 3 wherein the reducing agent is N-(p-hydroxyphenyl)glycine.
5. Medium as in claim 3 wherein the reducing agent is N-(p-hydroxyphenyl)- Alpha -alanine.
6. Medium as in claim 1 wherein the photoconductor is an inorganic compound formed between a metal and a non-metallic element of Group VIA of the Periodic Table.
7. Medium as in claim 3 wherein the photoconductor is a semiconductive metal oxide or metal sulfide.
8. Medium as in claim 3 wherein the photoconductor is titanium dioxide.
9. Medium as in claim 3 wherein the photoconductor is titanium dioxide of an average particle size of about 250 millimicrons Or less.
10. Medium as in claim 3 wherein the photoconductor is titanium dioxide of an average particle size of about 250 millimicrons or less and the reducing agent is N-(p-hydroxyphenyl)glycine.
11. A photographic medium comprising at least one photoconductor which becomes reversibly activated upon exposure to an image pattern of activating radiation and thereby capable of causing chemical reaction at portions of said medium corresponding to said image pattern; at least one image-forming oxidizing agent; and a reducing agent therefor, characterized in that the reducing agent is an N-(carboxy-substituted alkyl) aniline with an amino or hydroxy substituent on the aromatic nucleus.
12. Medium as in claim 11 wherein the reducing agent is N-(p-hydroxyphenyl)glycine.
13. Medium as in claim 12 wherein the image-forming agent is silver ion.