US 2692830 A
Description (OCR text may contain errors)
Oct. 26, 1954 E. H. LAND 2,692,830
PHOTOGRAPHIC SILVER HALIDE ,TRANSFER PROCESS' Original Filed Nov. 3, 194A FIG. 4
Patented Oct. 26, 1954 PHOTOGRAPHIC SILVER HALIDE TRANSFER PROCESS Edwin VH. Land, Cambridge, Mass., assigner to Polaroid Corporation, poration of `lieierware Cambridge, Mass., a corvOriginal application November 3, 1944, Serial No. 561,696,'now Patent No. 2,500,421, dated March 14, 1950. Divided and this application February 2, 1950, Serial No. 141,906
(o1. .e-ss) 5 Claims. 1
This invention relates to photography and more particularly to methods for forming fixed, stable, positive photographic prints.
This application is a division of my copending application Serial No. 561,696, filed November 3, 1944 for Photographic Silver I-Ialide Transfer Process (now Patent No. 2,500,421, issued March 14, 1950).
Objects of this invention are to provide a positive image by transfer from a latent negative image in a photosensitive silver halide layer by a process wherein the positive image is provided at a surface which is contrasting in visual appearance to developed masses of silver adapted to provide the positive image `and especially at a surface of this character which is substantially White, and to provide a transfer process wherein soluble silver complex is transferred from a photosensitive layer through a water-permeable opaque layer and is deposited at an outer surface of sa'id opaque layer whereby a positive image formed from said transferred complex is viewable against the opaque layer as a background and wherein said opaque layer has a visual appearance in contrast to developed masses of silver.
Further objects of the invention reside in the process of forming a positive image wherein a liquid processing composition is permeated into a water-permeable opaque layer of a visual appearance, contrasting in nature to the visual appearance of developed masses of silver, and wherein liquid processing material permeated into said water-permeable opaque layer is absorbed into a photosensitive silver halide layer in superposed contacting relation thereto for the development of a latent image in said photosensitive layer and the formation therein of a transferable imagewise distribution of soluble silver complex, which complex is transferred through `said waterpermeable layer to the surface thereof furthest removed from the photosensitve layer where said complex is deposited `and reacted to provide a positive image in silver which is viewable against the water-permeable opaque layer without observation of the negative image in the photosensitive layer and without removal of the Lphotosensitive layer from said superposed contacting relation; and to processes of the character described wherein said water-permeable opaque layer is substantially white and more especially comprises a water-permeable material having a white pigment substantially uniformly dispersed throughout said layer.
lOther objects vof the invention will in .part be obvious and will in part appear hereinafter.
The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplied in the following detailed disclosure and the scope of the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description `taken in connection with the accompanying drawings wherein:
Figure 1 is an enlarged, diagrammatic, fragmentary, sectional view of sheet materials for carrying out one method for forming a xed positive image in accordance with the present invention;
Fig. 2 is a view similar to Fig. 1 of the fixed positive print obtained `by the method and materials illustrated in Fig. l;
Fig. 3 is a view similar to Fig. 1 of film means comprising a photosensitive layer and a receiving layer whereby a modification `of the process of the invention may be carried out;
Fig. 4 is an enlarged, diagrammatic, transverse sectional View, with the thicknesses of the materials greatly exaggerated, of a liquid-containing sheet which may be used to provide the liquid reagent for carrying out the novel process of the invention; and
Fig, 5 is a View similar to Fig. 3 of a modification of the lm means of Fig. 3.
To obtain a positive image in accordance with one method comprehended by the present invention, a photosensitive layer comprising a photosensitive material of the type which is rendered developable Yby exposure to light, e. g., a silver halide, a mixture of silver halides, or a silver mixed halide, is Adifferentially exposed, for example, in a camera, to predetermined subject matter so as to form therein a latent image of said subject matter. The exposed layer is then permeated with a liquid reagent which comprises a developer for reducing the exposed silver salts to silver and a compound for reacting with the undeveloped silver salts to form a silver complex soluble in the reagent. The complex-forming reaction takes place at a more rapid rate than the rate at which the developer will reduce the unexposed silver salt whereby a predetermined portion of the unexposed silver `salt is transformed to the soluble complex. The positive image of the subject matter to which the photosensitive layer is exposed is obtained from the soluble silver complex by transporting said complex to an adjacent receiving layer of water-permeable material, and reducing said complex in the latter layer to provide an image-forming component comprising silver.
After the formation'therein of the image, the receiving layer may be separated from the photosensitive layer and the image viewed from the side which Was adjacent the photosensitive layer. It is also possible to permanently secure the receiving layer to the photosensitive layer and to form said receiving layer of a relatively waterpermeable, nontransparent and preferably opaque, white material sufficiently thin so that the complex silver ions may permeate through said layer to the surface thereof remote from the photosensitive layer to form an image comprising silver visible from the latter surface of said layer.
To remove the liquid reagent from the receiving layer after the formation therein of the positive image, and thereby to dry said receiving` layer,
.it is preferable to select a photosensitive layer which is substantially thicker than the receiving layer, and to select as the carrier for the photosensitive layer a material which is more liquidabsorbent than the material `of the receiving layer. For example, the carrier material for the photosensitive layer may be a relatively unhardened gelatin, while the receiving layer may be formed of a substantially more hardened gelatin.
It is also preferable to have the photosensitive layer remove from the receiving layer any developer which remains in said layer after the positive image is formed therein. To effect this result, the quantity of reagent used for the development of a predetermined area of the photosensitive layer is controlled to provide less developer than is required to develop all of the silver halide in Said area. In addition, the reagent is preferably introduced into the photosensitive layer after the exposure of the latter from the side of said layer Which is adjacent to the receiving layer. This may be accomplished, for example, by spreading the reagent between the surfaces of the photosensitive and receiving layers as the latter are brought into face-to-face contact, or by using the receiving layer as the means for transporting the reagent to the photosensitive layer. This concentrates the reagent at the interface of the photosensitive and receiving layers as the image-forming reaction begins. As a result, the developer acts first to develop the exposed silver halide in the portion of the photosensitive layer adjoining the interface and then to develop the silver halide complex which is transported to the receiving layer. After enough time has elapsed totransport the silver complex to the receiving layer and to form from said complex the positive silver image, the photosensitive layer is preferably uniformly exposed to light such as daylight or ordinary roo-m lighting to render the excess silver halide therein developable. As a result, all the remaining developer tends to migrate to the latter layer and to be exhausted therein in the development of the exposed silver halide.
The photosensitive layer may also be employed as the medium for removing any excess of the substance for forming the soluble silver complex with the silver halide. This is accomplished by employing substantially less of said substance than can form a soluble complex rwith the unexposed silver halide in the photosensitive layer and by selecting as said substance a compound such as sodium thiosulfate, which, when in the presence of an excess of silver halide, forms an insoluble complex with said halide. The reagent is caused to permeate the photosensitive layer from the side of the latter layer adjoining the receiving layer,` and as a result there is an excess of sodium thiosulfate at the surface portions of the photosensitive layer. Thus, at the beginning of the reaction, because of the excess of sodium thiosulfate, there is formed a soluble silver complex which is transported to the receiving layer, Where it is reduced to silver. However, as the remaining sodium thiosulfate permeates the photosensitive layer, it encounters an excess of silver halide and forms an insoluble complex with the latter. This tends to concentrate any of the complex-forming substance, in excess of that necessary to form a positive image, in the photosensitive layer.
The photosensitive layer is preferably a silver halide emulsion comprising a silver halide, or a mixture of silver halides, or a silver mixed halide in a suitable colloidal carrier, preferably gelatin. Other nlm-forming colloids may be used as carriers, for example other proteins or their derivatives such as glue and albumen, or Water-permeable organic plastics such as collodion, cellophane, polyhydroxyalkane, polyvinyl alcohol, and certain of the ethers of cellulose. The emulsion may contain any of the various modifying agents ordinarily incorporated in commercial emulsions such, for example, as dyes, antihalation layers, overooatings and sensitizers, and, in fact, suitable materials for providing the photosensitive layers for practicing the invention are the commercially available photographic films and'papers. It is to be understood that the lm base may be any material which provides a satisfactory support for the photosensitive layer, although where the photosensitive layer and the receiving layer are elements of a unitary film structure, it is preferable to mount the photosensitive layer on a. base which is transparent so that the latter layer can be exposed through said base.
The image-receiving layer is preferably formed of a water-permeable material, for example, regenerated cellulose, polyvinyl alcohol, polyhydroxyalkane, sodium alginate, cellulose ethers such as methyl cellulose, or their derivatives such as sodium carboxymethyl cellulose or hydroxyethyl cellulose, papers, proteins such as glue or gelatin, carbohydrates such as gums and starch, and mixtures of these materials where the latter are compatible. To form a nontransparent and preferably substantially opaque receiving layer from the relatively transparent materials, it is possible to incorporate in the latter a suitable pigment, such, for example, as titanium dioxide, against Which an image comprising silver is visible. Where the receiving layer is to be separated from the photosensitive layer after the formation of the image therein, it is possible to employ relatively thin, transparent layers of the above materials as the receiving layers and to mount said layers on a nontransparent base, for example, a paper, or a plastic impregnated With a suitable pigment. Where the image is to be formed so as to be visible from the side of the receiving layer remote from the photosensitive layer, a relatively thin, White, nontransparent, permeable film, preferably coated on the remote side with a transparent, permeable film, may be employed as the receiving layer.
If gelatin serves as the receiving layer, it is preferable to at least partially harden the same as by a suitable hardening agent as, for example,
5 potassium alum, potassium Achrome-alum or formaldehyde.
As pointed out above, the novel reagent for permeating the photosensitive layer in accordance with the methods of the invention comprises a substance for developing the exposed silver halide `and a substance which reacts with the silver halide to form a silver complex soluble in the reagent, the complex-forming reaction taking place at a less rapid rate than the `development of the exposed silver halide. Substantialy all of the known photographic developers may be employed as such in the reagent, provided they do not react with the complex-forming substance.
Examples of preferred developers are hydroquinone, monomethyl-p-aminophenol sulfate, a mixture of hydroquinone and monomethyl-paminophenol sulfate, p-aminophenol hydrochloride, p-hydroxyphenylaminoacetic acid, p-phenylenediamine, pyrocatechin, and diaminophenob hydrochloride. Other suitable developers are pyrogallol, chlorohydroquinone, o-phenylene diamine, 2-cxymethyl-p-aminophenol, and bromohydroquinone. The reagent preferably comprises an aqueous solution of the developer.
Examples of materials which may be used in the reagent for the purpose of forming a soluble silver complex with the undeveloped silver halide of the photosensitive layer are sodium thiosulfate, ammonium thiosulfate, and ammonia. For obvious reasons, it is preferable that the complexforming substance be one which does not desensitize the silver halide emulsion and one which is not toxic. For example, a compound such as sodium cyanide which will give satisfactory photographic results when used in the reagent in accordance with the methods of the invention is generally not desirable because of its toxic character.
The liquid reagent may also contain an accelerator, preferably an alkali such as sodium hydroxide, potassium hydroxide, sodium carbonate, borax, sodium metaborate, paraformaldehyde, or trisodium phosphate, a restrainer such as potassium bromide, potassium iodide, or sodium chloride, and a preservative such as sodium sulte.
In the event that the developer and/or the substance for forming the soluble silver complex is a solid at ambient temperatures, it is possible to incorporate either or both of said substances in solid form in the photosensitive layer and/or the receiving layer. With this arrangement, in order to provide the liquid reagent, the layer or layers containing said solid substances are permeated with a solvent, for example, water, for said substances. This may be accomplished by immcrsing the layer or layers in a water bath or by bringing into engagement with one of said layers, while the two are juxtaposed,` a liquidcontaining sheet, such as a blotter, saturated with water.
Referring to Fig. l, there are shown diagrammatically materials for carrying out one modication of the method or the present invention, and, as shown, said materials comprise a sheet of photographic film or paper I which preferably has as its photosensitive layer I2 a silver halide-gelatin emulsion, and a positive print member le which embodies an image-receiving layer I5 preferably mounted on a suitable base I8.
In one preferred embodiment of member I4, image-receiving layer I6 is formed of gelatin which is hardened as by immersion in a 10% 6. aqueous solution of formaldehyde for approximately ve to fifteen minutes, and base I8 is a cellulosic plastic sheet. To provide a relatively white, opaque background for the silver image, at least the surface layer of the cellulosic base on the side remote from layer I6 is impregnated or coated with a suitable White pigment 20 such, for example, as titanium dioxide. Layer I5y is preferably substantially thinner than layer I2.
To practice the invention, lm I0 is exposed to predetermined subject matter to have a latent image of said subject matter formed in layer I2. Thereafter, layer I`6 of positive print member I4 is immersed in a solution consisting of:
G. Sodium sulfite (anhydrous) 9.0 Hydroquinone 4.5 Sodium hydroxide 3.75 Potassium bromide 3.0 Sodium thiosulfate 10.0
Water to make cc.
for approximately fifteen seconds. Immediately after the immersion, layer I6 is pressed into faceto-face contact with photosensitive layer I 2 in a dark room as, for example, .by passing the two sheet materials, i. e., lm I Il and positive print member I4, between a pair of squeegee rollers. The films are held in contact for approximately one minute in the dark and thereafter film I0, while still in Contact with member It, is exposed uniformly to light for .a brief time as, for` example, for fteen seconds in a room illuminated by ordinary incandescent lighting. After the eX- posure, member Il! is stripped from film Ill and contains av xed stable positive of the subject matter of the latent image. The image-forming component Z2 (Fig. 2) comprises silver and is the insoluble product of the reaction of the soluble silver complex and the developer, `and differs from the silver of .conventional `photographic images wherein the silver image is obtained by a direct reaction between .the .exposed silver halide and the developer. It .is noted that the imagereceiving layer is brought into face-to-*face contact with the photosensitive layer, and as a result the complex which is transported by the reagent from the photosensitive layer to the receiving layer travels such a short distance that no substantial lateral diffusion thereof takes place, a clear and accurate positive being obtained. Although it is preferable to bring the receiving layer into face-to-face contact with the photosensitive layer, the process of the invention can be carried out with a layer of relatively permeable material, for example, an antihalation layer, interposed between the photosensitive layer and the receiving layer, provided that said intermediate layer is sufficiently thin so that no substantial lateral diffusion of the soluble complex in its travel from the photosensitive ,layer to the receiving layer will take place.
In another embodiment of member I4, base I8 is a cellulose derivative, for example a cellulose ester or mixed ester such as cellulose acetate, cellulose acetate butyrate or cellulose acetate propionate, and layer IE is formed by regenerating one surface of said derivative to cellulose. A positive print member of this type may be formed by converting to cellulose the surface layer of the nonpigmented side of cellulose acetate, or by converting to cellulose one of vthe Surface layers of a sheet of a mixed cellulose ester impregnated with a white pigment. Thelconversion `may be effected `by immersing the side ofthe sheet in which the cellulose is to be formed in a solution consisting of:
Sodium hydroxide g 50 Methanol cc Water cc 100 After immersion, the sheet is preferably dipped in a 4% solution of sulfuric acid and Washed in water.
It is also possible to use a sheet of regenerated cellulose, i. e., cellophane, as receiving layer I6, and said sheet may be laminated to a suitable white, nontransparent base such as a sheet of a mixed cellulose ester impregnated with a White pigment whereby to provide positive print member I4.
Examples of other suitable positive print members are gelatin coated papers such as imbibition papers, for instance those used in color photography; gelatin coated plastic or paper films as, for example, bromide papers or process lrns as for instance moderately slow films employed for copying purposes, with the silver halide dissolved out and with the gelatin preferably at least partially hardened; relatively pure alpha papers Without any coating layer; and the latter papers coated with gelatin either in hardened or unhardened condition, the gelatin being cast or rolled thereon.
Examples of other novel reagents for forming positive images in accordance with the invention are given below, but it is to be expressly understood that these examples are illustrative only and that the materials as well as the proportions may be varied to a substantial degree, as will now be apparent to those skilled in the art, Without departing from the scope of the invention.
Example 3 The developing solution of Lumiere and Seyewetz, reported in the British Journal of Photography, January 23, 1925, pages 44 and 45, comprising the following ingredients:
Sodium sulfite (anhydrous) g-- 40 Diamidophenol hydrochloride g 5 Tribasic sodium phosphate g Hypo (crystals) g Water cc 1000 In a modification of the lm means of Figs. 1 and 2, film I0 and positive print member I4 may be parts of a composite film. Layer I6 of member I4 is preferably permeated with the liquid reagent prior to the assembling of the composite film and is separated from the photosensitive layer I2 by a water-vapor impervious liquid-retaining sheet. The liquid reagent is retained in layer I6 by a suitable water-Vapor impervious casing Which includes said liquidretaining sheet. The latter is constructed so as to be readily removed from between layers I2 and I6 so that after exposure of layer I2 said retaining sheet may be removed and layer I0 caused to engage layer I2, permeating the latter layer With` image-receiving layer consists of a sheet of gelatin, cellophane or other relatively Water-permeable, transparent, lm-forming material without a backing. The sheet is immersed in the liquid reagent and is then superimposed on the photosensitive layer after the exposure of the latter. After the positive image is formed in said imagereceiving sheet, the latter is separated from the photosensitive layer and is transferred to a suitable backing sheet, for example of paper or pigmented plastic.
If a positive transparency is desired, the method of Fig. 1 may be carried out with a transparent image-receiving layer mounted on a transparent base.
Another form of nlm means whereby the novel method of the present invention may be carried out is shown in Fig. 3 and comprises a photosensitive layer 24, supported by a transparent film base. 26. -Any conventional negative film is satisfactory for this purpose. There is provided on the surface of photosensitive layer 24 a relatively water-permeable, white and substantially nontransparent layer 28 which is relatively thin, having a thickness, for example, of the order of .0001 to .0005 inch.
Layer 2B preferably comprises a Water-permeable, fllm-forming material as, for example, polyhydroxy alkane, sodium alginate, gelatin, cellulose, methyl cellulose, sodium carboxymethyl cellulose, or polyvinyl alcohol, containing a substantial amount of a relatively white pigment such as titanium dioxide. The film-forming material is preferably present only in a sufficient amount to bind the pigment into a lm whereby a maximum pigment content for a predetermined film thickness may be obtained. A permeable, relatively opaque layer formed by casting a sheet from a mixture of thirty parts by weight of sodium alginate and twenty parts by weight of titanium dioxide from a Water solution comprising approximately one-hundred seventy-five parts by weight of water and thereafter adhered to the photosensitive layer, for example with water, gives good results.
Examples of other nlm-forming mixtures which may be used are: gelatin and titanium dioxide, polyhydroxy alkane and titanium dioxide, hydroxyethyl cellulose and titanium dioxide, methyl cellulose and titanium dioxide, and sodium carboxymethyl cellulose and titanium dioxide. These mixtures may be cast from solution directly onto the photosensitive layer, or films of the mixtures may be first formed and then adhered to the photosensitive layer.
Various other methods for providing these relatively thin, white, nontransparent, permeable films on the photosensitive layer will now be apparent to those skilled in the art, and it is to be expressly understood that films formed by any such methods are within the scope of the present invention. In some instances it may be desirable to provide as an outer surface for nontransparent layer 28 a permeable, transparent film, or to have the outer surface portions `of said layer non-pigmented and transparent.
To carry out the novel method of the invention with this lm, photosenstive layer 24 is first exposed through transparent base 26 and thereafter layers 24 and 2B are permeated by the liquid reagent as by bringing into face-to-face contact with layer 23 a sheet 3Q which has been immersed in the liquid reagent. The liquid reagent on the surface of sheet 39' is transferred from said sheet to layer 26 and permeates the latter layer and layer '24. The developer in the reagent reduces the exposed silver halide, and the unexposed silver halide reacts With the complex-forming substance to provide the soluble silver complex. The latter permeates through layer 28 to the outer surface thereof, and When reduced in said layer forms a positive image comprising silver visible from said outer surface.
Layer 28, in addition to providing a White background `for the positive silver images, acts as an opaque screen to render invisible the photosensitive layer and the developed silver image in the latter layer. When the positive image hasformed in layer 28, sheet 3b is separated from the film.
Good results are obtained by using a sheet of relatively nonpermeable plastic such as cellulose acetate as the liquid-transferring sheet 3D, provided'that said sheet is brought into contact with layer 28 immediately after the immersion in the liquid reagent. The cellulose acetate, although relatively nonabsorbent, retains enough reagent on the surface thereof to e'ect the desired reaction. i
It is also possible to provide the reagent for permeating layer 24 and layer 28 of Fig. 3 by means of a liquid-containing sheet comprising a liquid-retaining member such as a blotter or other relatively porous material encased Within a vapor-impervious film, for example of wax, Waxcoated paper, or metal foil, which normally retains the liquid within the sheet. 'I'he sheet material maybe constructed so that the application of mechanical stress thereto Will release the liquid reagent therefrom. For example, the sheet may be equipped with a liquid-retaining Wall or membrane formed from a relatively brittle plastic, such as styrene, which may be fractured by the application of tensile stress or compression to the sheet. In another form, the liquid-retaining member may be provided with a liquid-retaining Wall of Water-vapor impervious material which may be readily removed from the liquid-containing member when release of the reagent over a predetermined area is desired. The liquid-containing sheet may be a part of the film, mounted adjacent layer 28 with its frangible liquid-retaining Wall or its removable Water-vapor impervious Wall separating said layer 28 from the liquidcontaining member. After layer 24 has been exposed, the nlm is subjected to suflicient tension and/or compression to cause the fracture of the liquid-retaining member, or the liquid-retaining film is removed opposite the exposed area Whereby the liquid reagent is caused to permeate layers 24 and 28, and causes the desired positive image to be formed in layer 28. The portion of the liquid-containing sheet from which the liquid has been released is then stripped from the remainder of the lrn. The liquid-containing sheet may be separate from the remainder of the nlm and brought into face-to-face contact with layer 28 of the nlm. after the latent image is formed in the lm. In either event, the release of the liquid reagent may be effected within the camera Wherein the photosensitivelayer is exposed as, for example, by providing the camera with the necessary mechanical means for fracturing or removing the liquid-retaining Wall as the nlm is metered through the camera after exposure. If the liquid-retaining sheet is normally separated from the lm and is only brought into face-to-face contact with the lm after. the latter has been exposed, a separate roll or pack thereof is mounted Within the camera.
An example of a liquid-containing sheet material of this character is illustrated diagrammatically in Fig. 4 as comprising a liquid-containing sheet member `32 such as a l'olotter encased Within a vapor-impervious nlm, such as a film 34 of metal-coated, Wax-impregnated paper. A liquid-retaining Wall 36 is provided adjacent one surface of liquid-containing member 32.` Wall 36 may be arelatively brittle plastic, for example styrene, which can be readily fractured by the application of mechanical stress, or it may be a readily removable nlm of Water-vapor impervious material such as a metal-coated, Waximpregnated paper.
In a modification of the novel method of the invention, `the film means in Fig. 3 is used in conjunction with a sheet material (Fig. 5) comprising a transparent base 40, for example, of `cellulose acetate or any other relatively transparent carrier sheet, coated with a layer 42 formed of a transparent Water-permeable material of the type which may be used as a receiving layer. A preferred material for this purpose is partially hardened gelatin. A latent image is formed in layer 24 by exposing said layer through lm base 26 in a suitable camera apparatus. Thereafter, layer 42 of sheet40, 42 is brought into face-to-face Contact with layer 2B, layer 42 being saturated With the liquid reagent prior to its engagement with layer 28. This step may be carried out in a camera or after the film comprising layer 24 has been removed from the camera in which said layer is exposed. Layer 42 may be permeated with the liquid reagent, for example by immersing sheet 4t, 42 in said reagent. When layers 42 and 28 are brought into face-toface contact, the liquid reagent contained in layer 42 permeates through the permeable unit formed by layers 24, 28 and 42. The soluble silver cornplex formed by and dissolved in said reagent is reduced partly in layer 23 and principally in gelatin layer 4?.. Accordingly, after the positive image is formed, sheet 40, 42 is not removed from the remainder of the film, but remains a part thereof. The image which is formed in part in layer 42 and in part on the outer surface of layer '.28 is visible through transparent base 4B, said base serving as a protective coating for said image.
Where the receivingr layer serves as the medium for transporting the liquid reagent to the photosensitive layer and is adapted to contain the reagent for prolonged periods, it is important to employ as a receiving layer a material which is not soluble in the liquid reagent, and which is not hydrolyzed by the liquid reagent. A suitable material for this purpose is regenerated cellulose. As a base for said layer, it is preferable to employ a material which is not soluble in, is not permeable to, and is not hydrolyzed by the liquid reagent. An example of a suitable material for this purpose is a sheet of ethyl cellulose.
Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
l. In a photographic process for forming by transfer a positive image in a water-permeable opaque layer comprising image-receptive material of a nature having a visual appearance in contrast to developed masses of silver and wherein said opaque layer has a rst surface in contact with a face of a photosensitive element which includes a photographically responsive silver halide layer, containing a latent negative image, adjacent said face and wherein said opaque layer has a second surface in superposed and spaced relation to said first surface and forming the side of said opaque layer furthest from said photosensitive element and from which said transfer image is viewed, the steps which comprise distributing a liquid composition comprising a photographic silver halide developer and a silver halide solvent which forms a soluble and reducible silver complex with silver halide substantially uniformly over said second surface of said opaque layer to uniformly and exclusively permeate said opaque layer with said liquid and transporting liquid from said opaque layer into said photographically responsive silver halide layer, said distribution being under such conditions that said liquid composition wets said photographically responsive silver halide layer only through said opaque layer, by absorbing liquid from said opaque layer through said face of said photosensitive element in contact with said opaque layer; developing said latent image to silver with developer absorbed into said photosensitive layer in conjunction with said liquid composition and providing in said photosensitive layer, in solution in said liquid composition, an imagewise distribution of silver complex which is transferable by diffusion in said liquid composition from said lphotosensitive element; transferring from said photosensitive layer at least a part of said imagewise distribution of said silver complex by diffusing liquid composition which contains said complex in solution through said face of said photosensitive element and the first surface of said opaque layer and into said opaque layer to the viewingsurface thereof without appreciably disturbing the imagewise distribution of said silver complex in solution and depositing said complex within said opaque layer and including the deposit of said complex substantially at said viewing surface of said opaque layer; developing with said developer the silver of said complex deposited in said opaque layer and including complex deposited substantially at said viewing surface whereby to provide from said latent image a positive image in silver; and retaining said opaque layer in contact with said photosensitive layer, said opaque layer having an opacity of suicient density so as to substantially conceal the developed photosensitive layer from view when observed from said second surface of said opaque layer.
2. The process for forming a positive image as set forth in claim 1 wherein said opaque water-permeable layer is substantially white.
3. The process for forming a positive image as set forth in claim 1 wherein said opaque layer comprises a water-permeable material having a white pigment substantially uniformly dispersed therein.
4. The process for forming a positive image as set forth in claim 1 wherein said opaque layer comprises a water-permeable material having titanium dioxide substantially uniformly dispersed throughout said layer.
5. The process of forming a positive image as set forth in claim 1 wherein said liquid composition is substantially uniformly coated onto` said viewing surface of said opaque layer and wherein said opaque layer is substantially white.
References Cited in the le of this patent UNITED STATES PATENTS