US2440106A - Light-polarizing image and methods for forming the same - Google Patents

Description

prii 20, 1948.. E. H. LAND ET AL 2,440,106

LIGHT-POLARIZING IMAGE AND METHODS FOR FORMING THE SAME Filed Apfil 26 1946 FIG. 3 FIG. 4

l VENTORS M,

Patented Apr. 20, 1948 LIGHT-POLARIZING IMAGE AND METHODS FOR FORMING THE SAIME Edwin H. Land, Cambridge, Joseph Mahler,

Brookline, and Mark Hyman, Jr., Cambridge,- Mass., assignors to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware Application April-26, 1946, Serial No. 665.084

9 Claims.

This invention relates to novel methods for forming light-polarizing materials and particularly has reference to forming light-polarizing images in transparent, hydrophilic, molecularly oriented, linear polymeric plastics.

This application is in part a continuation of our copending application Serial No. 434,568, filed March 13, 1942, for Light polarizers and methods and materials for forming the same.

Important objects of the invention are to provide methods for forming light-polarizers and light-polarizing images by predeterminedly incorporating an oxidizing compound or agent in a linear polymeric plastic element or sheet and treating the sheet with a material or agent which is oxidized in the presence of the oxidizing compound to produce a dichroic stain in any portion of the sheet containing the oxidizing agent, the molecules of the sheet in any portion thereof containing the oxidizing agent being substantially oriented at some stage of the method; and to provide methods of the foregoing character wherein the oxidizable material is an iodide or a mixture of iodides from which iodine is released by an oxidation reaction with the oxidizing agent, and wherein the oxidizing reaction is brought about in the presence of a solvent for the oxidizing agent as for example an acid and particularly by reacting the oxidizing agent with an acid solution of an iodide or mixture of iodides.

Other objects of the invention are to provide processes of the character described which are particularly adapted for the reproduction from photographic negatives of light-polarizing or dichroic images in molecularly oriented plastic materials and particularly for the formationof dichroic images in such materials and their combination as superposed rightand left-eye stereoscopic images. g

Still further and yet more specific objects are to provide a new process for forming light-polarizing material through the employment of lightsensitive materials, particularly to utilize the direct action of light on plastics which are capable of having their molecules oriented and which have been rendered light-sensitive as a step in the formation of light-polarizing images therein; and to provide a process wherein suitable sheet plastic is rendered light-sensitive, differentially exposed to light and treated with an agent adapted to react. with the exposed material to produce a dichroic stain such as iodine in the sheet, at least the portions of said sheet penetrated by the dichroic stain being molecularly oriented at some stage of the process; as well as to provide a (cits-6) process wherein the differential exposure of the light-sensitized plastic takes place through a photographic or other negative and the resulting exposed portions are developed to a dichroic stain which, will constitute a, positive reproduction of the negative. 1 j I Yet other more specific objects of the invention are to provide a process for forming light-polarizing material by introducing an oxidizing compound or agent into a linear polymeric plastic in predetermined portions of the plastic whereby to provide an image formed of the oxidizing agent, and then utilizing the oxidizing agent to form a dichroic stain in the portions of the plastic in which it is incorporated, at least thoseportions of the plastic containing the dichroic stain having their molecules substantially oriented at some stage of the process; as well as to provide a process for the reproduction of a photographic negative image as a light polarizing positive image by subjecting a suitable plastic, sheet to differential imbibition of a solution of an oxidizing compound or agent to form a positive image in the sheet in terms of the oiddizing'a'gent and then treating the sheet witha material oxidized by said oxidizing agent to providea dichroic stain, in the portions of said sheet containing said oxidizing compound, and especially to carryout such practices by providing a positive image in the form of a wash-"off gelatin relief, saturating the relief with an oxidizing-agent and. printing on the plastic sheet with the treated wash-01f relief whereby to provide an image in the sheet comprising the oxidizing agent.

Other objects and advantages will in part appear and in part be pointed out in the course of the following descriptions ofv the invention, which are given as non-limiting examples in connection with the accompanying drawings, in

which: v Fig. 1 is a diagrammatieview illustrating a convenient arrangement for carrying out the process of the invention wherein light-polarizing images are formed, in lightesensitive plastic material and shows the exposure'of a sheetlof a suitably sensitized plastic through. a" photographic negative;

Fig. 2 is a diagrammatic view representing a step in ,the process of the invention'utilizing a wash-off relief inf the formation of a light-polarizingima'ge;

' Fig. 3 is a diagrammatic perspctiveview illustrating an image-carrying elementhaving a lightpolarizing image formed'ther'ein in accordance with either of the practices described in connection'with Fig. 1 0 1 2; and

in transparent, hydrophilic, linear polymerid'" 4 conveniently by imbibing the sheet in a solution containing the desired light-sensitive agent. Among the materials found suitable for this purpose, the preferred materials comprise dichro- 5 mates, such for example asarnmoniuln dichromate; potassium dichroinateFand sodium, dichromate; with "ammonium "dichr'omate "being preferred for the reason that it is the most soluble. The just-described dichromates may be used plastics. All of these ractidsigener icauyinvolve" 1o ly 1, a x fi of two or more of them may the introduction or incorporation of lii'bxi'dizing compound or agent within cially sheet material, and. th oxidizer to the end of stainingthe'plast dichroic staii'l. v

A preferred process and one which is admirably suited for explaining the invefitibii'andespecially the fundamentals thereof is illustrated in con nection with Fig. 1 and comprises three main steps-treatment of a sheet of the desired ma- 20 trial with; a suitableflightfsensitiv. material, xposu'ie of the sensitized sheet to'li and the foimatizinpr sh iniagej [the s et comprising-an oxidizing agent, renew d by'ltr at l t of the sheet with an agent, which when oxidized; pro:

duees a dichroicstain in the exposed areas of tlfesheet. T Infidditidfitd: steps, it 11 laybe esirable ii i senje 'casesto add a fourth step 60m: pri'sihg'tratm'"enlibf'jfiif staine sheet man tensififer barn? Ptiitiemai'noveaynesin'nr fact so r es mayconvenientlyflief'abinplis edfbyfsoft en 50 n' it e seti s f tex i e wa tr f heat; andtlien stretching dexte until"suitableorientation of me m eculesfhas been obtained, In thecase of polyvi 'yl'al'cohol,

diti'oii for the fon s non 6r diehroieimag'es-tnere; inby extending't'o'fro awe-to four arrive-times J gthI"v swear specific" examples and formulae, relate ing to thevarious practices of the inventioiijiwill 'if' By wayyof, lllustration the invention ill'b a sheaf i QQ 15in .solve be employe'cl as the light-sensitive agent. Dichromates are particularly useful as light-sensitive agents incarrying out the present invention since thesmaterials are good oxidizers.

if r; the light-sensitive agent may be simply watei, but this is not wholly satisfactory 'foi' the rea-soh that it tends to penetrate the sheet too rapidly and thus tends to soften it It is preferable, therefore, to add to the solution a liquid which is miscible therewith but is agnon 'solvent jfof the plastic as" well" "as non.- e'abtiv th. t e othe mie'a s t r e 'ixa d hi ni-a j g m m enfl lQWP PJ mi Oi i' -i the"sheet"and thusjaid infniaintainingits stiff ness. Suitable examplesdncilude acetone; niejth 1 1 and l h l i fi n? lume h r used =inay vary anywhre up to approximately 99% of-the totai voluine'bf solventpresent.-

The proportion of sensitizing" agent? to solvent 7 a widerange from=as was 1 to 2 1 'to-as-- high as 40% "based on the-"v0 urn'e rovided the var-iatibn is compensated for by va ation *ofthe-t-i-i'rie of'trea-trnent approxi mately inversely 7 to the pro-portion of sensitizing agent. Saidtime f treatment may-vary 'frm a few seamlesupward but {should-preferably continue' erily long enough 'tb-pfermit 'thesolution to---'pen etrat e'- the-surface of the sheet "uniformly for a short" distaiice; and in no case=sh0uldY-th sheet be allowed to -soak-so-long that it softens unduly. It has-end been round desirable to add a-wtting agem suehpror eXainple,-as soap-"or di'hiiffl soliillin slllffisllccilflt in order bdth -t O accelerate the sensitizin treatment andt'o pro mote -uniform-sensitizationi A satisfactorysensitizing 'bath has the iioil l l'llla immediately given 2 Formula Water cc" Acetone 1 cc; 25 Ammoniumdichromatei; g nz; '1'6 Wetting "agent; gm; 1

fshouldbethroug'h'the negative-*to berep Diluted,

l Fl fl wh e n e "e fi h i re r se ted: mounted wide a "photographic ne'gative-Fl'zfand sensitized, as pointed out above. If, for example, an R2 Photofiood bulb mounted approximately six inches from the surface of sheet in be used as the light source, a sheet sensitized as described above for two to three minutes should be suiliciently exposed in from forty to sixty seconds.

It should be noted that during the exposure, and particularly in the case of exposure for a long period, it may be advisable to shield sheet l from the infra red rays emanating from the light source, as by means of a suitable filter I5. If it is desired to print by means of the ultra-violet rays emanating from source it, element l5 should also include filtering material for visible light, and the exposure time should be increased to about five minutes, owing to the uniform absorption of the ultra-violet light by negative l2.

The effect of the exposureof the sensitized sheet of the invention to light is to harden the surface of the sheet approximately proportionally to the degree of exposure. As will be seen from Fig. 1, th degree of exposure varies according to the variations of density in negative l2, 1. e.,

' more light is transmitted by the lighter areas of the negative than by the darker areas. The result is that the surface of sheet ii! is hardened to a varying degree corresponding inversely to the density of negative [2. In other words, where the negative is lightest, sheet If] is most exposed and most hardened, and where the negative is darkest, sheet It is relatively unexposed and remains relatively soft. Thus the dichromat in the soft or relatively unexposed portions of the sheet may be readily washed out of the sheet whereas the dichromate in the hardened or heavily exposed portions of the sheet is removed with dimculty. When the sheet is washed, fol-' lowing exposure, the efiect produced is to leave an image therein in terms of an oxidizing agent, that is in terms of the dichromate or mixture of dichromates used for sensitizing purposes.

As intimated, the next step in this embodiment of the process is to eliminate unexposed lightsensitive material from sheet ill. This may be done in the present example by a simple washing with Water to remove the dichromate from the unhardened areas of the sheet, and the time of Washing varies more or less according to the degree of sensitizing and the time of exposure, the important factors being that all unexposed dichromate should be removed but that the washing should not continue so long that the highlights of the picture are washed out or the exposed dichromate aifected. It is for this reason that the sensitizing treatment is preferably confined to the surface of the sheet; the unexposed dichromate may then be removed inas short a time as possible. For a sheet sensitized and exposed as described above, the washing should be completed in approximately two minutes, but it must be emphasized that this time is by no means critical and may be varied considerably without appreciably altering the result.

The next step in the process is to treat the exposed and washed sheet with an agent or com-- position in solution form which is somewhat analogous in its action to a photographic developer in that it reacts with the exposed material in 6 solution. The dichroic stain thus formed s'ub stantially replaces the oxidizing agent, that is to say, portions of the sheet in which an oxidizing agent was present prior to the oxidation treatment just described contain substantially only a dichroic stain following such treatment.

The oxidation treatment is readily carried out with a material which is oxidized by the oxidizing agent in the sheet to give a dichroic stain and which is dissolved in a solvent material mutual to said oxidizing agent. An iodide or mixture of several iodides provides an oxidizable material of the character desired and when the same is reacted with the oxidizing agent in the sheet in the presence of a solvent also common to the oxidizing agent, for example an acid, iodine is released from the iodide or mixture of iodides to form a dichroic stain, the released iodine replacing the oxidizing agent. The iodide or iodides should be water soluble and are preferably contain'ed in a solution which is rendered acidic.

These characteristics in the developer may conveniently be secured either by using an alkali iodide, particularly ammonium iodide or an alkali metal iodide such, for example, as potassium iodide or sodium iodide, and adding one or more acids to the solution, preferred acids being hydrochloric and hydrobromic acids. Alternatively, other acids such, for example, as sulphuric, nitric, phosphoric, or acetic acid may be used. Other iodides may be employed, for example, ferrous iodide or cerous iodide, or an iodide solution which is naturally acidic such, for example, as

hydrio-dic acid, may be employed.

It has already been pointed out that the result of the exposure step illustrated in Fig. 1 is to harden the surface of the sensitized sheet difl'erentially, depending upon the optical density of the photographic negative at any given point. It follows, therefore, that after the exposed sheet has been Washed, the amount of dichromate remaining therein will similarly vary differentially over its entire area. Then when the sheet is subjected to the oxidation treatment, the iodine stain absorbed in any particular area will depend upon the amount of dichromate present in that area. That is to say, the stain will be relatively intense in the areas where there is considerable dichromate, namely the areas corresponding to light areas in the original negative. Conversely, there will be relatively little dichromate in the areas corresponding to the dark areas in the negative, and the amount of stain absorbed there- In other in will therefore be substantially less. Words, the final product of the oxidation step of the operation will be a positive reproduction in light-polarizing areas or the original negative image represented by element l 2 in Fig. 1,

As is the case with the other agents and steps of this invention, the composition of the developing solution which contains the oxidizable material may vary to a considerable extent, depending both on the results desired and on the way in which the preceding steps were carried out. For example, if the sheet was sensitized in a concentrated solution and exposed for a relatively long period, a concentrated developing solution may be used, but if the sheet was sensitized in a dilute solution and exposed for a short time, the developing solution and time of treatment therewith must be correspondingly modified. Similarly, the particular acid used affects the time of the oxidation reaction, use of a strong acid like hydrochloric requiring a shorter treating time than one containing a tive 12 produced by conventional photographic methods. Conversely, if analyzer 30 Were rotated through ninety degrees with respect to print 29, it would then act to absorb the remainder of the component differentially absorbed by the print, with the result that the image would disappear and the combined elements would appear substantially uniformly transparent.

The process of the invention is well adapted to the production of dichroic stereoscopic images. For example, the image shown in Figs. 3 and 4 may be assumed to be one image of a stereoscopic pair. In that event the other image should be reproduced on a plastic sheet having its molecules oriented substantially at right angles to the direction of molecular orientation in sheet 20. The two sheets may then be superimposed with the images thereon in relative stereoscopic registry, and the resulting composite image will have a three-dimensional effect when viewed through proper polarizing viewing means, in accordance, for example, with Patent No. 2,203,687.

The terms dichroisrn and dichroic as used herein and in the claims are intended to refer to the property of polarizing transmitted light by differential absorption according to vibration direction. It will be noted that in its simplest form the process of the invention may be practiced to produce a uniform dichroic light-polarizing sheet. This may be done, for example, by sensitizing a sheet of the desired molecularly oriented plastic, exposing the sheet uniformly to light, and then treating as described above to produce a uniform dichroic stain therein. It is to be understood that this and similar modifications of the process of the invention are to be construed as coming within the scope thereof.

It should be pointed out that it is not atall essential to the practice of this invention that the various steps of the preferred process thereof be carried 'out continuously. For example, the sensitizing treatment may take place at an entirely different time from the remainder of the process. It should also be noted that these sheets are relatively low in light sensitivity, and that only moderate care need be taken to protect them from exposure to extraneous light while carrying out the other steps of the process of the invention.

The foregoing practices involving the use of a light-sensitive oxidizing agent provide a relatively simple process by which dichroic images may be easily and readily formed. However, it is to be kept in mind that the invention has for a concept the use of any oxidizing agent which will release a dichroic stain from a solution of a suitable oxidizable material and the incorporation of the oxidizing agent in predetermined portions of a suitable plastic by practices other than those described as depending upon the ability of an exposed light-sensitive oxidizer to substantially remain in the plastic while the unexposed oxidizer is removed by washing. Thus, the inventive concept comprehends in addition to oxidizing agents which are sensitive to light, the use of relatively light-insensitive oxidizing agents incorporated or introduced into a sheet of a suitable plastic. Many oxidizing agents in addition to the dichromates and including inorganic and organic materials are known and all are included within the scope of this invention. As a few illustrative examples of suitable inorganic materials of this character, mention may be made of metallic salts such as ferric salts, for example ferric chloride,

and cupric salts, for example cupric sulfate, cupric chloride and cupric nitrate, as well'as sodium salts of oxidizing. acids such as sodium perchlorate and sodium perborate. Examples of organic oxidizing agents suitable for use with this concept of the invention may be found in pbenzoquinone and otherquinones. The oxidizing agents disclosed may be employed singly or a mixture of several of the same may be used.

Preferably the oxidizing agent should be water soluble and all of those just named are of this character. An oxidizing agent may be predeterminedly introduced into a suitable plastic sheet by imbibition from a water solution. On treatment of a sheet in which one or more oxidizing agents have been imbibed with any of the treatin or developing solutions heretofore described in connection with the preferred practice, a dichroic stain in the form'of iodine will replace the oxidizing agent.

In dichroic image'formation, a water solution of an oxidizing material or mixture of such materials preferentially imbibed into the sheet from asaturated wash-off gelatin relief to provide a desired image comprising an oxidizing agent. Following introduction of the oxidizing agent into the plastic sheet, the sheet is treated in any of the developing solutions heretofore mentioned. A water solution of from 2% to 5% concentration of any of the materials just mentioned, is suitable for use in carrying out. these practices,

A gelatin wash-01f relief which carries a positive reproduction of a photographic negative image may be prepared in accordance with conventional practices. For example, such a washoif relief may be formed in the manner set forth in Patent No. 2,315,373, issued March 30, 1943,

to Edwin H. Land, for Process for forming lightpolarizing images. Briefly, the relief is made by printing through a photographic negative 'onto suitable wash-Joli? relief film. In this regard, the rinting step illustrated in connection with Fig. 1 for forming an imagecomprising an oxidizing agent in light-sensitive plastic sheet is similar to that employed for exposing wash-01f relief film if wash-off relief film is considered as substituted for the sheet l0.

Following development of the wash-01f relief,

1 which is carried out by standard procedure, the

relief is saturated in a water solution of the desired oxidizing agent or agents. Usually, one or two minutes will be suflicient to suitably saturate the wash-01f relief.

The developed wash-off relief, which is illustrated in Fig. 2 by the reference numeral 40, followingits saturation with the oxidizing agent is placed in face to face contact with the surface of the plastic sheet in which the dichroic image is to be formed whereby to print such image on the sheet. The relief and the plastic sheet are pressed into contact, a convenient method for carrying out such a step being illustrated in Fig. 2 wherein the developed and saturated relief 40 is shown superposed with the relief side thereof upon the oriented surface of 'a sheet of polyvinyl alcohol 42. Fig. 2 as a matter of convenience shows the two elements 40 and 42 supported on a flat surface such as the surface 44 and any device such as a rubber roller 45 or squeegee pressing the two sheets evenly together. The relief and the plastic sheet should be allowed to remain in contact for several minutes during which time the oxidizing agent will transfer from the relief to the plastic sheet i'n-well-understood manner.

Since the amount of the oxidizing agent held by the relief variesin accordance with the thickness of the gelatin layer at each particular point,

l l the oxidizing agent:will?be difierentially imbibed into-theplastio sheet proportifinally -totheness of *the -='reli'ef ;"and ibecause theL-rvariatio the thickness of the relif corresponds -to the variation in density of theprigiiial photographic negative image sfrom which tl'ie -='relief waspro duced; a positive: reproduction of: the' negative image f in :te'rm's 0i the oxidizing agent -willi' be formed inthe sheet 42.

Having formedan image of 'an oisidizinggagent,

dizahle material, for examplesinamacid solutionof an iodide or mixture of iodides. Following di chroic image --fo'rmati'o'n whieh taks place'- ina mann'er'lik'ethat-alreadyset=fort heshe'etmay be washed and dried-: The efict-of='=this process is to provide a dichroic image'inoriented'plaistic sheet similar to th'at' disclosed in connection with Figsw2 and 3. -A --pair ot imagesproducediwthis' manner and-forming a' le-i't -eye-ima id@ ht;

eye image mayr'be superposed ppomeachfother'to provideastereoscopic image'which'mayi'beviewed through suitable 'viewingnevi'cesz" As stated" in com:ecti'o'irwith"the process"em": ploying a li'ght sensitive'oxidizingxagentfthe in.-. vention may- 'be practiced to; producee uniform dichroic' light-polarizing sheet byjmbibiiig olur tion ofa" relatively li ht-i-insen'sitive' oxidizing agent throughout "thesheetgandxthenitreatng the sheet, and imhib ed oxidizingwa entcto creatdiafli; chroic stain; V e K v V v 7 When formin ,liehtepolarizi g-im ee bypn erential imbibitionrof anoxidizingagentiriplastic' sheet, it is also to'Joe, notdthat it'dsnot. essential torthe. pra'ctieejofgjthe; invention that. the yario'us stepsof the processdescribed-i'ri .connectio'nithere with. .be carried "out. .contin uously';, For-example; the imbibiti'on of the oxidizingagentintheplastid sheet may take place at "an entirely di-firenttime from the remainder ofnthatprocess V The, term ffoxidiz'ingiagentias used ir'i-the specieiioation and,claimsiisiowbeluirdeiistoodsassmearie mg a singleoxidizingt naterialora mixture ofisev eral oxidizing material-s1.

It should be understood thatmanyil s of im-, ages other. than r\photographicsnegatives may'r be reproduced/by. thisprocess;v such (for, examplesasa negative ;or= positive of @anysother: partially-alight: transmitting,.-type orian Lopaque obi'iect ofiwhioh it is. desired ,to make-.asi-lhouettei Therinycntion is particularly. adapted, vlfornexampler,to theireproductio xofi'stencils drawings; E'halfetonesa-Pr-inted mattersand the. like; Negative l Zin Fig:- 1 -:isloo'uld be considered,therefore,asrepresentativeefany such similar medium,whichmvill difierentiallwab sorb .the light incident onstl'ie rsensitized sheet andthe'reproductions therefromrmadeeby the sprocess of this inventiona will 1 be positivei repmduotions; that is tosay; the slight values-theremi-will barreversed withre spe'ct to said negative As has been. previously stated the ioregoingidea scription'ismerelyjilustrative, .v onemt more ops verative'rfor'ms. pfxth'e; processes iofethe'present in= vention; It must: again7be'emphasizedithatseach 12 degree thereofain. one iorsmore steps; cambe compensated'for in the other steps:

Siiice'certain changes :mayhe made in carrying out the above-process without departing iromthe scope; of theiinventiomit is? intended that all matter contaiiied-sin :th'eeabove description shall be interpreted-as illustrative' and not in" a limiting sense.

Whatisclaimed .is:;;

1.: The.:processwhichccompriscs treating a carrier sheet-withsa dichromatesolution.to'render it light-sensitive;exposing :said sensitized sheet to lightirthrou-gh; :a; differentially; light-transmitting medium; removing the unexposed dichromate therefrom; and :treating: saidjsheet with an acid solutionaofuan iodiderswhichreacts with; said dichromate to 'giveaaflichroic image as an-oxidation product;

2.?3I'he process whichpomprisestreating a sheet fsaim'olecularlyroriented' transparent,- hydrophilic;linearpolymeric plastic with a dichromate solution to: render; it light-sensitive, exposing said senSitized-:,Sheetf tO light throughi-ia differentially lightl-transmittingtmedium; removing :the unex-- posed Lidichmmate etherefrom, and. treating 1 said sheet' withi anzacidic" iodide solutionfwhich reacts with; the di'chromate ;,ther.ein. to produce -a dichroic stain:

3;;The aiprocess which comprises i imbibing a sheet of.arsubstantiallyloriented, transparent, hydrophilicplinearpolymeric; plastic -in'-a dichromate solutionrforriiastimezsuch ithat only a relatively thimlayer onithe'esurface-of said-sheet is penetrated:bysaid sclution, exposing said sheet to light through: a differentially -light-transmitting: me-

dium; removinglthe unexposed dichromate therefrom, and then developing the exposed, dichromate f in :an acidic iiodide solution which. reacts with the dichromate to producea dichroie stain in the exposedareasofsaid sheet;

4:: The process-which comprises treating a sheet OfJI-"PI'EdQtBIIHi-DBGIY molecularly oriented polyvi-nyl'alcohol with'a dichromate solution to render it=v light-sensitive:exposing :saidsensitized sheet toirlightithi'oughraa differentially light-transmitting 1 medium,.: removing the unexposed dichromateytherefrom-,.and treating said sheet with an iodiderinz-anz acid: medium whereby the -iodide is oxidized byjthe ,-exp.osedzdichromate to iodine to form an image-tinziodine in; said'zcarrier'.

5: :The process which comprises treating-a sheet of i'predetermined1y.-: mole'cularly oriented po1yvinyl alcohol with-adichromate solution to render it 1light-sensitive, exposing said sensitized sheet to?rlightathroughi :a' differentially light transmittirig.:medium;removing the unexposed dichromate therefrom;- andthemtreating-said sheet with an acidic iodidesisolution .WhiChf reacts :with the exposed dichitomate therein toproducea dichroic stainwinttheiexposed areas thereof;

'6:2-Thaprocesstwhich:comprises treating a sheet ofr ipredeterminedly"-mo1ecularly oriented polyvinyl:alcolmlyvitl'i a .d-ic hromate solution to render itslightesensitivee'exposing said:fsensitized .sheet to *lighttthroughzta:"difierentially light transmitting mediumgrremoving the unexposed dichromate therefrom; andz-then: treating said I sheet with *a solution: containingzan: acid and at least: one -waten-soluble iodide salt ada'ptedito react with said exposed'dichromate 'to produce a dichroic stain initherexposedrareas of said sheet.

7. Thesprocess:which' comprises treating a sheet ofipredeterminedly -rmolecu1a'rly oriented polyvinyralcohol with a -dichromate. solution to render it light sen'sitivei-exposing said-sensitizedsheet to light through a differentially light-transmitting medium, removing the unexposed dichromate therefrom, and then treating said sheet with a solution containing an acid and at least one water-soluble iodide salt adapted to react with said exposed dichromate to produce a dichroic stain in the exposed areas of said sheet, said solution containing also a relatively weak alkali in sufiicient quantity to maintain substantially constant the pH value thereof.

8. The process which comprises treating a sheet of predeterminedly molecularly oriented polyvinyl alcohol with a dichromate solution to render it light-sensitive, exposing said sensitized sheet to light through a difierentially light-transmitting medium, removing the unexposed dichromate therefrom, and then treating said sheet with a solution containing an acid and at least one water-soluble iodide salt adapted to react with said exposed dichromate to produce a dichroic stain in the exposed areas of said sheet, said solution containing also a reducing agent.

9. The process which comprises treating a sheet of predeterminedly molecularly oriented polyvinyl alcohol with a dichromate solution to render it light-sensitive, exposing said sensitized sheet to light through a differentially light-transmitting medium, removing the unexposed dichromate therefrom, and then treating said sheet with an acidic iodide solution which reacts with the exposed dichromate therein to produce a dichroic stain in the exposed areas thereof, said solution also containing an agent which is adapted to slow the penetration into the sheet of said solution whereby to restrain softening of the sheet, said agent being substantially unreactive with the ingredients of said solution and said dichromate and also being a nonsolvent of polyvinyl alcohol.

EDWIN H. LAND. JOSEPH MAHLER. MARK HYMAN, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 175,009 Banks Mar. 21, 1876 217,404 Quartley July 8, 1879 1,214,940 Miller Feb. 6, 1917 2,185,018 Sauer Dec. 26, 1939 2,204,604 Land June 18, 1940 2,409,923 Barnes Oct. 22, 1946 2,409,959 Ryan Oct. 22, 1946 FOREIGN PATENTS Number Country Date 526,780 Great Britain Sept. 25, 1940 OTHER REFERENCES Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans Green and Co., New York, 1922, vol. II, page 608 cited. (Copy in Div. 59.)

Babor, "General Chemistry, Crowell Co., New York, 1929, page 798 cited.

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