a new one-step photographic process

Journal of the

OPTICALOf

SOCIETYAMERICA

VOLUME 37, NUMBER 2 FEBRUARY, 1947

A New One-Step Photographic Process

EDWIN H. LAND

Research Laboratory, Polaroid Corporation, Cambridge, Massachusetts

(Received January 6, 1947)

This paper describes a process which produces finished poisitive pictures, directly fromthe camera, in about one minute after the exposure. The camera is described as containing astrip of paper in addition to the negative material and as feeding this strip of paper, in contactwith the exposed negative, through simple pressure rollers and thence out of the camera box.The process to be used with the camera is described as having but one step, instead of themany steps of conventional photography; and it is shown that this can be achieved by asso-ciating with the strips a reagent which, when spread between the two strips by the pressureof the rollers, is capable of developing the silver halide of the negative and forming the positiveimage at one and the same time. Several classes of processes are discussed, their characteristicsexplored and certain principles established for obtaining satisfactory picture quality, sta-bility, speed of operation, etc. In one of these processes, the reagent spread between thenegative and the positive strip consists of a small amount of viscous liquid containing, inrather high concentration, the necessary constituents for developing the negative image, forforming at the same time a silver complex with the unexposed grains in the negative, for trans-ferring the soluble complex to the positive sheet, and there creating and stabilizing the positivesilver image. This process runs to completion in about one minute. When the two strips arepeeled apart, both are essentially dry. One strip is the finished positive picture. The processoperates at temperatures from less than 30'F to over 100'F. The paper discusses the controlof the rates of reduction, silver ion complex formation, and ion diffusion in the several reactionfronts; principles of stabilization of the positive picture; control of the relative rates of growthof density in negative and positive; conversion of the silver ions to particles of silver of adequatesize and the control of the color of the image as a function of particle size; and various char-acteristic curves which have been obtained with different negative materials. These factorsare then interpreted with relation to the photographic usefulness of the process.

I. INTRODUCTION

SEVERAL years ago the writer undertook theproblem of devising a camera and a photo-

graphic process that would produce a finishedpositive print, directly from the camera, immedi-ately after exposure. From the point of view ofthe user, the camera was to look essentially likean ordinary camera, the process was to be dry,the film was to be loaded in one of the usual

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ways, the positive print was to look essentiallylike a conventional paper print, and this printwas to be completed within a minute or twoafter the picture was taken. This paper describesthe development of processes of this kind and acamera to be used with them.

In photographic literature there appear to beno references to such a system of photography,even as an ideal. Recently, however, there has

EDWIN H. LAND

been one suggestion of the need for a dry andapproximately instantaneous camera. 1

While there seems to be little record of activityleading to a completely simplified camera, muchhas been done to reduce the number of steps inordinary photographic processing. A brief reviewof standard processing, and simplifications of it,follows:

a. Conventional Processing 2

Expose, develop the negative, rinse, fix, wash,dry, expose the positive through the negative,develop, rinse, fix, wash, dry.

b. Reversal Processing,3 as Used for 16-Milli-meter Home Movies

Expose, develop, rinse, dissolve out developedsilver, wash, expose with a predetermined flash(to give over-all exposure to remaining grains asa function of average initial exposure), develop,rinse, fix, dry.

c. Tintype or Ferrotype4

Expose a special emulsion (coated on a blacksupport, historically in collodion rather than ingelatin), develop to grayish rather than blacksilver, rinse, fix, wash, dry. This process dependson using a black background, and obtains thewhites by using the silver grains not as absorbersto provide blacks, but as diffusers and reflectors.The images seem dull by modern standards,although frequently attractive because of thedelicacy of tone rendition.

d. Reversal by Solarization5

Expose (on an emulsion so prepared, by pre-exposure to light or by chemical treatment, thata further exposure leads to a decrease in density

1V. Bush, Atlantic Mo. 176, 103 (1945) states, "Willthere be dry photography? . . . When Brady made hisCivil War pictures, the plate had to be wet at the time ofexposure. Now it has to be wet during developmentinstead. In the future perhaps it need not be wetted at all.. . .Often it would be advantageous to be able to snapthe camera and to look at the picture immediately."

2 L. P. Clerc, Photography Theory and Practice (PitmanPublishing Company, New York, 1937), Chaps. XXVI,XXVIII, XXIX, XXX, XXXI, XXXVIII.

3 Reference 2. Chap. XXXIII.4J. M. Eder, History of Photography (Columbia Uni-

versity Press, New York, 1945), fourth edition, ChapterXLVI.

5 C. E. K. Mees, The Theory of the Photographic Process(The Macmillan Company, New York, 1942), Chap. VII.

from maximum upon development), develop,rinse, fix, wash. This process is much too slowfor general use. It occasionally is useful for ob-taining direct positive copies in a short time,and in the studio or darkroom.

II. PRINCIPLES FOR A ONE-STEP PROCESS

In formulating the principles to be followed indeveloping a one-step process, certain premiseswere adopted concerning the sensitivity, andsimplicity of the materials and mechanisms tobe used.

a. The Photo-Sensitive Material

Since the ordinary silver halide negative emul-sion requires development of the exposed grainsto a negative image in black silver, if the maxi-mum sensitivity of the emulsion is to be obtained,and since this implies an additional step or groupof steps to obtain a positive image, it is temptingto consider using a photo-sensitive material otherthan a silver halide emulsion. However, exami-nation of other photo-sensitive materials atpresent available proves completely disappoint-ing, since the sensitivity of these other materialsis of the order of 1/100,000th of that of ordinaryphotographic materials used in amateur cameras.Indeed, for tungsten light the fraction of sensi-tivity may be much smaller. Therefore, in spiteof the fact that some of these materials, such asthe diazo dye-couplers, can be processed readily,as by ammonia vapor for example, they cannotbe considered except for specialized work.

As contrasted with all other photo-sensitivematerials, the silver halide emulsion of conven-tional photography has the enormous advantageof forming a latent image in terms of a catalyst.Even in the formation of this catalyst, thereappears to occur a remarkable collecting phe-nomenon. When the whole silver halide grain isstruck by the photons of the incident light, allthe photoelectrons generated throughout thegrain are collected by a few sensitivity speckswithin it. The mass of the catalyst is multipliedby about 105 times when the silver halide is con-verted to silver by action of the developer.Thus, there are two important multiplicationphenomena involved, in the operation of theemulsion. Therefore, if one were tempted, evenafter discarding the ammonia-developed diazo

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PHOTOGRAPHIC PROCESS

FILM

POSITIVE

KNIFE BLADES S W/- REAGENT CONTAINER (POD)

PRESSURE ROLL OF POSITIVE IMAGEROLLS SUPPORT SHEET

FIG. 1. Schematic diagram of camera producing one-steppositive prints.

dye-coupling processes mentioned above, to ven-ture into a research program for a new way ofmaking an image in terms of a catalyst, it wouldbe wise to hesitate because of the advantages ofthe two-stage multiplication that occurs inmodern silver halide emulsions.

Finally, it is incorrect to consider photo-graphic emulsions without also considering thereducing agents that have been found to besuitable for use with them. The modern emul-sions have been designed to be used with thesereducing agents, which fall in a rather limitedchemical class. It is only the reducing agents inthis class that can discriminate between theunexposed grains and the grains so minutelyaltered by exposure. Moreover, it is only whenthe crystals are prepared carefully and embeddedin just the right kind of medium that a highdegree of discrimination can be maintained bythe grains, for a given developer, between thestate of being reducible and the state of beingnon-reducible as a function only of exposure tolight.

It was, therefore, taken as a ground rule forthis investigation 6 that, in spite of the facts thatstandard emulsions yield a negative rather than

6 The camera mechanisms and techniques of providingdeveloper that will be described are useful, however, withother photo-sensitive surfaces, in those special fields wherestrong actinic illumination can be provided artificially.

a positive, and that the formation of this negativerequires the use of a reducing agent, presumablyassociated with some amount of liquid, a waynevertheless had to be found of designing the"dry" camera to use a silver halide emulsion orsome equivalent crystalline suspension.

b. The Camera

The ideal simple camera might be one inwhich the sensitized surface, after exposure, issubjected to some simple mechanical action, suchas being passed through a pair of small steelrolls at the edge of the exposure aperture. Thus,after exposure, the film would simply be ad-vanced through the rolls, where the pressurewould develop the exposed grains to white andthe unexposed grains to black. While this kindof direct development of a silver halide emulsionis, of course, fanciful, the concept of the cameraitself serves as an excellent approach to theactual problems. If, for the person who used it,the camera as finally developed seemed to havethe simplicity of this imaginary one, then for allpractical purposes it would be as satisfactory.This imaginary camera was, therefore, taken asa symbol for the mechanics of processing.

On the two premises, then, of the use of asimple dry camera and the use of fast silverhalide emulsions, the operation of the systemwas conceived as follows:

The camera, as shown in Fig. 1, has the usuallens, bellows, and film cartridge for the negative.An additional sheet is provided which can be ina roll in about the position where the film in anordinary camera is wound after exposure. Thissecond sheet feeds out of the camera, along withthe exposed negative, through a pair of pressurerolls. A reagent, in the form of a minimumamount of liquid, is fed between the two sheetsjust before they enter the pressure rolls, so thatit is spread as an extremely thin layer betweenthe two sheets, temporarily bonding them to-gether. The sheets have outer surfaces opaqueto actinic light to protect the negative frombeing fogged. The thin reagent layer developsthe negative, and forms the positive on one ofthe inner surfaces. The small amount of liquidin the reagent is imbibed into one-or both of thesurfaces. After a reasonable processing time,about a minute, the two sheets are peeled apart.

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EDWIN H. LAND

Both are found to be essentially dry. On one ofthem is the finished positive image. This may beeither on top of the negative or on the othersheet. The latter position has the advantage ofautomatically removing the geometric reversalthat occurs when the positive image lies overthe negative.

To meet the requirement that for the user thecamera be "dry," it is desirable that no tanks,tubes, or injection systems be built into it(although they might be feasible for specialmodels for industrial applications). Therefore,it is desirable to associate the reagent with thenegative or the positive sheet. Three ways ofdoing this are being investigated: (1) The liquidis contained in minute, frangible, sealed cells ina layer inside the negative or the positive sheet.(2) The liquid is in a viscous plastic coating onthe surface of the positive sheet which is rolledup in such a way as to seal the liquid within theroll. (3) The reagent is contained in small podsthat are as wide as the sheet but only a smallfraction of its length.7

The camera and film and reagent-spreadingtechniques described make it possible for theoperator to expose the picture and immediatelyto advance from the camera the exposed frame,covered by the associated sheet. The opaquesandwich is cut or torn off and, when strippedapart a minute later, has on one of the innersurfaces the finished positive image.

Obviously, a heavy burden of requirements is

7 Since this third way seems most promising, some detailsfollow: The pod is a water- and oxygen-opaque sheet,folded, sealed at the ends, filled with reagent, and bondedalong the long edge. This edge-bond is so made that whenthe pod passes through the rolls, the hydraulic pressure ofthe liquid within bursts this edge and the small quantityof liquid is then transported ahead of the rolls and betweenthe two sheets. The quantity of liquid is so metered thatthere is negligible excess over that required for making thevery thin layer. Provision is made for the small excess sothat its existence is not apparent to the operator. Thepods may be fastened either to the roll of negative materialor to the roll of positive material so that they are auto-matically carried into place as the two sheets feed throughthe rolls. One extremely important detail is that if thereagent were to have the viscosity of ordinary developingsolutions, rather complicated devices would be required tospread it uniformly. But as the reagent is made moreviscous, the spreading problem becomes much simpler.An interesting consequence of the use of viscous reagent isthat after the small amount of water in the reagent hasbeen imbibed in the adjacent layers, there is left on oneof them the very thin film of the material that was usedto render the reagent viscous. In some of the processes tobe discussed, the positive image is formed within this thinlayer.

being thrown on the composition of the reagentthat is spread between the sheets and perhaps,also, on the dry reagents that may be includedin the outer sheets. Moreover, there is anothervery important requirement that has not yetbeen discussed: reliability of operation over awide range of temperatures. Since ordinarilychemical reactions double the velocity with every100 C rise in temperature, the reagent must notonly develop the negative and form the fixedpositive, but it must also accomplish these ob-jectives with acceptable uniformity over a serv-iceable range of temperatures, perhaps from 300to 1000 F.

Finally, before discussing the classification ofprocesses in which the reagent may be calledupon to perform these tasks, it should be notedthat if the whole process is to be completedwithin a minute, it is economical of time to havethe positive and negative processes occur con-comitantly. Furthermore, it will be shown laterthat this economy of time is important inobtaining high resolving power.

III. CLASSIFICATION OF PROCESSES

So many requirements are to be made,of thethin layer of reagent that it seems desirable toconsider these requirements systematically. Thechemical detail of the processes will be discussedin Section VI.

The general question that will now be exam-ined is: In what ways might a reagent act toform a negative and a positive at the same time?

Class A-Exhausted Developer Processes

Assume that the reagent that is spread overthe exposed silver halide emulsion layer in thenegative has the characteristics of an ordinarydeveloper, except that it is concentrated enoughto have within its very small volume the mole-cules needed to complete the development of thenegative and alkaline enough to effect promptdevelopment. Shortly after the reagent is spread,the negative image will have developed, and inso doing, it will have formed in the reagent layeran image in terms of oxidized developing agent.The exposed silver halide grains will, as theydevelop, oxidize the reagent responsible for theirdevelopment, and this oxidized reagent will atleast for a short time remain in the neighborhood

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of the exposed grains. If the positive layer is anordinary photographic emulsion that has beenpre-exposed uniformly, then it, too, will develop;but the amount of development at any pointwill be limited by the amount of unoxidizeddeveloping agent available at that point. Therewill thus be formed on this ordinary photographicpaper a positive image. This image will beunsatisfactory for our purposes in at least twoof three ways: (1) it will not be fixed; (2) eitherthe highlights will not be white, because develop-ment is allowed to start in them immediatelyupon the spreading of the reagent and beforethe development of the negative has proceededfar enough to exhaust the developing agentappreciably; or (3) the definition of the positiveimage will be impaired because development ofthe positive is delayed (to preserve contrast) sothat lateral diffusion of the developing agentoccurs during the time of development of thenegative.

Thus, this simplest of approaches involvingthe simultaneous development of negative andpositive would at first appear not to be suitable.(Actually, in the primary process to be discussedin Class C, the phenomenon of developer ex-haustion is made use of, not so much for forma-tion of the image, as for assistance in stabiliza-tion of the image.) Some devices can be employedto improve developer exhaustion techniques.(1) A barrier layer of plastic, coated over thepositive sheet, can be used to delay developmentof the positive until development of the negativeis complete. (2) The developing agent, in an inertstate, may be incorporated in the emulsion layer,so that development of the positive is not ini-tiated until exhaustion of the developer is accom-plished.

Class B-Oxidized Developer Processes

These processes, which have been exploredwith some success, call for forming from theoxidized developing agent either a bleach for ablack dye, or a white pigment to be precipitatedon a black surface. In spite of lack of experiencewith the questions of photographic quality in-volved in forming a white image on a blackbackground, this class of processes was examinedseriously because of the simple quantitative rela-tionship that can exist between the number of

grains developed, the number of molecules ofoxidized developing agent formed, and the num-ber of molecules of white pigment resulting. Thepigment is produced by the reaction of theoxidized developing agent with additional rea-gent located either in the viscous layer or in oneof the outer sheets of the sandwich.

Loss of contrast does not occur in Class Bbecause the positive cannot start forming untila reaction has occurred in the negative. Lossof definition can be minimized because it ispossible to utilize the reactions of developmentas fast as they occur to provide the material forthe positive image. This class thus satisfies anecessary condition for high resolving power.

Class C-Soluble Silver Complex Processes

It is with the process now to be described thatmost of the remainder of this paper (after ClassD) will be concerned. It is characterized by thesimultaneous development of positive and nega-tive images in adjacent layers, the positive imagebeing formed from the silver halide of the unex-posed grains of the negative. In the ordinaryreversal process used for amateur motion picturefilm described above, it will be recalled that thepositive is made by first developing the exposedgrains, removing the silver, then fogging anddeveloping the remaining grains. That is, statedgenerally, the positive is made from the unex-posed grains in the negative. In this generalsense, the processes of Class C bear a resemblanceto the ordinary reversal process. In the Class Cprocesses, the reagent dissolves the unexposedgrains while it is developing the exposed grainsand precipitates the solution of unexposed grainsas a metallic silver positive in a layer adjacentto the negative. The reagent is spread betweenthe negative and positive sheets, diffuses intothe negative layer, develops it, and at the sametime removes from it the unexposed grains whichit then reduces to metallic silver, either in thethin layer of reagent or on the surface of thepositive sheet, so that when the negative isstripped away the positive image remains. Thepositive image thus formed is similar to that ofan ordinary print in that the highlights are theregions where no silver is precipitated and theshadows are the regions where varying amountsof silver have been precipitated.

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EDWIN H. LAND

These processes were developed in response tothe fourfold question: Is it possible to form areagent which will (1) develop to silver thosegrains in the emulsion that would be developedin an ordinary tray or tank developer, and (2) atthe same time dissolve the other grains and yet(3) allow this solution of unexposed grains todiffuse out of the negative layer, and also (4)cause the metal ion in the solution to precipitateas the positive image in the layer adjacent to thephoto-sensitive emulsion? In view of the prob-lems previously discussed, some additional ques-tions must be asked about a reagent for Class C:Can this reagent be so made that no image canform in the positive until the negative image hasbegun to be developed? Can the image in dis-solved halide be utilized as fast as it is formedso that time for lateral diffusion is minimized?Can the whole process of simultaneous formationof negative and positive proceed rapidly enoughto complete the picture within a minute? Finally,can the relative rates of dissolving, diffusing, andprecipitating on the one hand, and developingon the other hand, be kept sufficiently invariantwith temperature so that pictures of substantiallythe same quality will be formed over a widerange of temperatures?

There is some limited experimental support forthe feasibility of a reagent which will combinethe functions of developing the exposed grainsand dissolving the unexposed grains.8 In theseexperiments, it was found possible to make uppreparations of developers containing hypo whichwould, for certain emulsions and at certaintemperatures, develop and fix a negative in onebath over a period of about twenty minutes.9

For one-step processes, the problems to be solvedare far more difficult. It is required not onlythat the process work satisfactorily over a rela-tively wide temperature range, but also that theprocessing time be reduced to a minute, and thata usefully sharp and dense image be formed fromthe solution of unexposed grains.

Now, if there existed a solvent that discrimi-nated directly and effectively between unexposedand exposed grains in favor of the former, thetransport of the silver ions would be greatly

8 W. D. Richmond, "Concurrent development andfixation," Brit. J. Phot. 36, 827-8 (1889).

9 A. Lumiere, L. Lumiere, and A. Seyewetz, "Combineddevelopment and fixation," Brit. J. Phot. 72, 44-45 (1925).

simplified. All that would be required would beto subject the negative to the solvent action, andto precipitate the silver in a layer outside thenegative. However, this kind of solvent wouldbe a radically new kind of developer since, for it,the latent image would be. a catalyst inhibitingsolution. Development would mean that thereagent would ignore the exposed grains anddissolve the unexposed. Ordinary hypo and othersilver halide solvents do discriminate veryslightly between exposed and unexposed grainsin a silver halide emulsion, the unexposed grainshaving a slightly greater solubility than theexposed. The discriminating effect, however, isso trivial that it cannot be utilized practically.

It is apparent, then, that since there is nosatisfactory reagent for dissolving unexposedgrains without dissolving exposed grains, animage-forming technique that is to utilize unex-posed grains in solution must depend on removingexposed grains from the realm of solubility. Thissuggests that as the proposed reagent diffusesinto the negative layer, the exposed grains mustbe developed into silver at least as rapidly as theunexposed grains go into solution. This depend-ency implies that the silver grains are essentiallyinsoluble in the reagent that is chosen as solventfor the unexposed silver halide grains. It alsoimplies that the unexposed grains are adequatelyinsensitive to the reducing action of the reagent.One can thus visualize, as the reagent permeatesstratum after stratum of the negative layer, anadvancing reaction front in which exposed grainsare not dissolving but are developing to silverand unexposed grains are not developing but aredissolving.

These are stringent demands indeed, becausethe requirement that the process be complete inone minute means that the developing potentialof the reagent must be high. Furthermore, therequirement that the total quantity of reagentfor development of negative and positive becontained in the very thin viscous layer, that isspread between the two sheets, means that theconcentration of reducing agent must be ex-tremely high. Thus the solution of silver ionsmust exist unreduced in the negative in a vigor-ous reducing environment. Similarly, the exposedbut undeveloped grains must not dissolve in avigorous dissolving environment.

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There is a happy combination of requirements,however, in the need (1) for having the reactionproceed rapidly, and (2) for utilizing the imagein dissolved grains as rapidly as it forms. It isdesirable to complete the process rapidly, notonly for convenience but also because it wouldbe extremely difficult to maintain the dissolvedions unreduced in the negative layer for a longtime. Thus rapid processing makes more silveravailable to the positive and, incidentally, helpsto limit the time of diffusion.

The processes of Class C, then, call for sym-metrically advancing fronts of reaction in nega-tive and positive, the front in the negativecomprising developing and dissolving, the frontin the positive comprising developing the dis-solved silver ions.

One way of looking at Class C is that itprovides a mechanism for developing both theexposed and the unexposed grains at the sametime but in different strata. Section V(e) of thepaper will discuss in detail the relative rate ofgrowth of these two images, which are formedat about the same time.

Class D-Coupler Processes

Modern three-color photography depends onreacting the oxidized developer in place withcouplers to give dyes. In some of these processes,a soluble coupler is used in the reagent, and inothers the coupler is incorporated in the emul-sion, either in insoluble form or associated withsome insoluble organic material. Couplers canbe used in one-step processes in ways analogousto Classes A, B, and C above.

Coupler Analog of Class A

If the coupler is included with the reagent sothat a dye forms where the silver particles de-velop, the residual unoxidized developer with thecoupler dissolved in it can be made to couple byan oxidizing agent at the surface of the positivesheet. This process has the same limitations asClass A (developer exhaustion) and can be simi-larly improved by the use of a barrier layer ofplastic over the oxidizing agent, although againat a sacrifice of definition.

Coupler Analog of Class B

In the ordinary photographic color processes,the oxidized developer becomes one of the partic-

ipants in the coupler reaction to form the color.These color processes yield a negative and henceare usually used in the second development stageof a reversal process. In Class B processes, thepositive is obtained directly rather than afterreversal by causing the coupler product to be ahighlight material rather than a shadow ma-terial. A dye cannot be used but a bleach orpigment can be used in this way.

Coupler Analog of Class C

The coupler analog of Class C requires anequilibrium for the solubility of the couplersimilar to the equilibrium that has been achievedin Class C for the relative solubility of exposedand unexposed grains. A coupler is present inthe negative layer and its molecules have mod-erate mobility. As the developing agent diffusesinto the negative layer, the exposed grains oxidizethe developer in their neighborhood, and theoxidized developer couples with the coupler togive a dye of lower mobility. In the region of theunexposed grains, the coupler dissolves in thedeveloper and diffuses out of the negative intothe adjacent layer. If in the positive layer thereis present an oxidizing agent of low mobility,then when the coupler diffusing from the unex-posed portion of the negative reaches the oxidiz-ing region, it will couple with the oxidized de-veloper in that region. This will produce a posi-tive image, and the color of the image will dependon the combination of developing agent andcoupler that is chosen. These combinations have,of course, been thoroughly studied in color pho-tography techniques and need not be given here.Not so much.has been done to find products thatwill couple to give a black directly, but mixturesof couplers or developers can be used. The re-quirements with relation to solubility are similarto those for the silver ions in Class C. Develop-ment should just precede the dissolving reaction,and the reaction in the positive layer with thetransferred solution should be rapid and shouldutilize the solution from each stratum as fast asthe development of the negative in that stratumoccurs.

IV. THE PROBLEM OF STABILIZATION

Obviously, one of the characteristics of anyprocess which we select must be that it yields a

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EDWIN H. LAND

print which requires no washing. Since anyincompleted reactions are likely to go to comple-tion, and since reagents subject to aerial oxida-tion are likely to oxidize with time, particularlyif the oxidation is accelerated by radiation, anunwashed print in general can be expected tobecome stained. Furthermore the density of theimage-forming material may be destroyed bybleaching reactions if the development residuesare left at random in the print.

The stabilization of the one-step positive printmust thus be made an integral part of theprocess. In general, as much of the reagent aspossible, insofar as it is not part of the imageitself, should be left in the negative layer, whichis stripped away. The reagents that must remainin association with the image should be left in anenvironment in which they do not discolor.Precautions must be taken to keep out of thefinal image reagents that might react to bleachthe image-forming material. Particularly impor-tant among the techniques that have been usedto obtain satisfactory stabilization is the use inthe positive layer of components that diffuseinto the reagent during the latter part of theperiod of image formation, to decrease the reduc-tion potential by automatic neutralization oracidification of the reagent layer after the imageis formed. Other techniques involve the use ofanti-oxidants, the concentration of the excessreagent and its oxidation products in a layerother than that of the positive image, and theuse of developing agents that do not form coloredoxidation products.

V. DETAILED DESCRIPTION OF ACLASS C PROCESS

a. Resum6 of Theories of the Nature of theLatent Image in Silver Halide Emulsions

Our experiments have led to the developmentof a Class C process which permits the directproduction of positive images of satisfactoryphotographic quality. In this method, both thenegative and positive images are formed frommaterials initially present in the negative layer;consequently, a consideration of the structureand composition of the emulsion, and the natureof the latent image was very important in thedevelopment of the process.

The silver halide emulsions for a fast negativematerial comprises a suspension in gelatin ofcrystals ranging in size from 1 to 3 microns.The crystals are essentially silver bromide andmay have some iodide ions in the lattice. Theobserved increase in sensitivity when iodideions are present is possibly associated with thefact that silver iodide is not isomorphous withsilver bromide. In a given emulsion a group oflarge grains is more sensitive than a group ofsmall grains.

The role of the gelatin is vital and complicated(it will be seen that it takes on in the Class Cprocess duties similar to but beyond those thatit has in the usual process). First, the gelatinprovides for the crystals a mechanical suspensionthrough which the components of the developingreagent, including water, can migrate readilywithout seriously altering the mechanical struc-ture. Gelatin is one of the best protective colloids,and in this capacity it plays a part in the initialpreference of the developer for reacting with themore exposed grains. Prior to the isolation andidentification of the organic sulphur compoundsin gelatin, this protein was the only reliablesource of the sulfur whose presence is necessaryfor the preparation of sensitive negative material.Furthermore, gelatin must accept bromine atomsreleased in exposure, and in the course of develop-ment, it must allow bromide ions of the crystalto go into the solution while the silver ions arereduced to silver at the site of the original halidegrain. Insofar as grains with low exposure tendto dissolve in the developer, the gelatin mustinhibit the reduction of their silver ions untilthese ions can be washed out.

Each crystal possesses certain active centersor sensitivity specks. These specks are believedto consist of minute aggregates of silver sulfidewhose sulfur is derived from the gelatin medi-um, or may be added artificially. For optimumsensitivity, there should be a limited but suffi-cient number of sensitivity specks in each crystal.In ordinary development, the sensitivity specksat the surface of the grain appear to be theimportant ones. During exposure photons strikethe whole presented area of a grain and those

10 C. E. K. Mees, The Theory of the Photographic Process(The Macmillan Company, New York, 1942), Part I.W. F. Berg, Ann. Rep. Chem. Soc. London 39, 49 (1942).

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that are absorbed produce within the crystalphotoelectrons which are free to diffuse becausetheir energy lies in the conductivity bands.Because of the lower level of the conductivityband of the sensitivity specks, the electronsenter them and charge them negatively. Thisnegative charge precipitates on the sensitivityspeck, as silver, a number of silver ions that arefree in the crystal lattice. The resulting minutedeposit of silver at the surface of the crystalconstitutes a break in the ring of charged ionssurrounding the whole crystal. During develop-ment a reducing agent delivers at this point astream of electrons. Each of the latter serves forthe discharge of a further silver ion of the crystal.The process results in the extrusion of micro-filamentary silver and continues until the reduc-tion of the silver halide crystal is complete,provided that sufficient developing reagent isavailable. The formation of the catalytic silverspeck seems to occur with a quantum efficiencyof one. About 200 quanta seem to be required toexpose a grain.

b. The Reagent

It will be recalled that in Class C processes, areagent is necessary which will develop the moreexposed grains while at the same time it dissolvesthe less exposed grains. With the above pictureof the nature of negative naterial and of thelatent image in mind, let us examine in detail therequirements for such a reagent. (1) It must bea reducing agent which will develop the exposedgrains very rapidly while dissolving the less ex-posed grains. (2) It must not readily precipitatesilver ions in solution as silver in the negative, inspite of the congestion of the gelatin with freshlygrowing grains of silver derived from the moreexposed silver halide crystals. (3) It should ulti-mately reduce these ions to silver either in thereagent layer or on the surface of the positivesheet.

We have found that a combination of fivesubstances" satisfies these conditions: hydro-quinone in the presence of sodium hydroxideserves as the reducing agent, sodium thiosulfate

11 It will be clear from the discussion in Section III thatmany other combinations are possible; a large numberhas been investigated, but the discussion in this paper isconfined to the combination which has been most thor-oughly studied.

as the silver solvent. These reagents are dissolvedin a solution of a high molecular weight materialsuch as sodium carboxymethyl cellulose or hy-droxyethyl cellulose. Sodium sulfite is also added.These reagents, except for the high molecularweight material, are familiar in ordinary photo-graphic practice; the uses to which the combina-tion is put in this process, however, are novel.

Both the actual operation of this process andthe theory of the system are probably bestintroduced by a consideration of the function ofthe high molecular weight material. This plasticserves a variety of important purposes. Theviscosity that the plastic imparts to the developeris valuable in the mechanics of bursting the pod,in spreading the reagent neatly between the twolayers, and in insuring that they are immediatelyand uniformly wetted. It serves as a spacerbetween the two outer sheets, holding them farenough apart to provide an adequately largereservoir and sink for the ions and moleculesthat must enter and leave it, and yet it keeps thesurfaces near enough together and is thin enoughso that the image particles precipitated in it aremade up from dissolved components that havearrived along a very short radius of diffusion.In some cases, this plastic interlayer serves as asupport for the opaque particles of the positive.The two sheets as they emerge from the cameraare bonded together just tightly enough so thatthey do not separate spontaneously during thetime of processing and yet, when they haveimbibed most of the water from the developerin the course of the time that they are bonded,they can be peeled apart. Either of them, de-pending upon design, may carry with it theextremely thin film of plastic.

The plastic molecules also exert an interest-ing protective colloid action. Several differentmethods for the production of positive imageswith this reagent have been developed. In one ofthem the plastic acts as a poor protective colloidcompared to the gelatin of the negative, whilein a second, the protective colloid action issufficient to prevent precipitation until the ionsreach the locus of the positive image.

When this viscous developing agent is spreadbetween the negative emulsion and the positivesupporting layers, the excellent wetting prop-erties of the reagent combined with its high

69

EDWIN H. LAND

30°F 70Fl

.6

. NEGATIVE MAXIMUM NEGATIVE MAXIMUM

.8

.4POSITIVE MINIMUM POSITIVE MINIMUM

0

LG .

POSITIVE MAXIMUM;.2 _ POSITIVE MAXIMUM

I W NEGATIVE MINIMUM

.4V4 # NEGATIVE MINIMUM

0 40 SO 120 160 200 240 40 80 120 160 200 240

DEVELOPMENT TIME IN SECONDS

development potential lead to immediate reduc-tion of the more exposed grains. Considerablenegative density is apparent almost immediately.(Cf. Figs. 2 and 3.*) A consideration of the prop-erties of emulsions leads us to believe that thelargest grains of silver halide in the surface layerof the emulsion are attacked first by the hydro-quinone, while the smallest grains in the samelayer are attacked by the thiosulfate ion with theformation of a soluble silver-thiosulfate complexion. As the reaction front moves into the negativethe reaction in the surface layer continues, thehydroquinone attacking successively smaller andsmaller grains while the hypo continues the solu-tion process, attacking larger and larger grains.In the end, all of these surface grains have beenconverted, some to silver and some to silver-thiosulfate complex ion. During the process thereaction boundary moves deeper and deeper intothe negative layer, and in its wake follows thetrain of reactions described in detail for thesurface layer.

It is clear that the ratio between the rate offormation of silver and the rate of formation ofthiosulfate complexes is an important determi-nant of the character of the positive image. If

* It should be noted that all positive curves in this paperare curves representing the entire process, from exposureof negative to development of positive, whereas the nega-tive curves are of the usual type. Some of the negativecurves represent tank development, others the new process,as indicated in the legends. All densities are measured byreflection except for the negative in Fig. 9.

FIG. 2. Plot of growth with develop-ment time of negative and positivedensities (both maximum and mini-mum) at 300F and at 70'F.

the thiosulfate reaction is very rapid, as com-pared with the deposition of negative silver,only a weak negative is formed. On the otherhand, when the silver forming reaction in thenegative is too rapid with relation to the forma-tion of the positive, a fogged negative and a weakpositive are formed, since then the reagent issuch that once it has developed the more exposedgrains it may continue developing, encroachingon the grains which in the usual photographicprocedure would remain undeveloped. This mayhappen for three reasons: (1) the reagent has ahigh development potential, (2) ordinary re-straining agents such as sodium bromide may beomitted since they may restrain the developmentof the positive, and (3) there is a limit to thedemand that can be made on the gelatin as aprotective colloid. Therefore, it may be just asimportant that the development boundary in thenegative not precede the solution boundary con-siderably, as it is that the solution boundary notprecede the development.

There is a striking difference between the waysin which the hydroquinone and the hypo areused. As the reaction front advances into thenegative layer the hydroquinone is taken to ahigher oxidation state by each grain of silverhalide which it develops. In this process no useis made of this oxidation product. The thiosulfateion, on the other hand, which migrates in gelatinalmost as rapidly as in plain water, is converted

70

I

I

I.-


L-

FIG. 3. Characteristic curves for one- Zstep positive prints and their negatives aat the end of development times of from2 to 150 seconds.

to silver thiosulfate complex, which diffusesrapidly into the viscous layer where it is liber-ated, while the silver which it carries is deposited.Thus a small concentration of thiosulfate ionscan be used in an essentially cyclic process.*

c. Mechanics of Precipitation

In the ordinary photographic negative or printthe opaque grain is formed as described above bythe reduction of the silver ions in the crystal insitu. In our process the exposed silver halide inthe negative is presumably developed in the con-ventional way although electron microscopicstudies would be desirable to determine whetherthe thiosulfate ions attack appreciably the grainswhich are to develop.

In the Class C process it is necessary to dis-solve silver halide grains that are not properlymembers of the negative image, to transport thesilver ions from these grains to an adjacent layer,and to convert these silver ions to silver in par-ticles that are large enough to have the desiredcolor and which are located near the normalprojection (from the plane of the negative image)of their parent silver halide grains. Since we have

* This whole process offers singular opportunity forstudying some of the mechanisms of development. Theactive part of the latent image in an ordinary non-solventdeveloper appears to be at the surface of the grain. Thecolor sensitizing dyes are adsorbed at the surface of thegrain and it seems worth investigating the local of thelatent image formed through the medium of dye. Sincethis process uses such vigorous solvent action along withthe developing agent it may be that latent image compo-nents on the inside of the grain are significant for it andthat perhaps a shift in color sensitivity might be found.Some preliminary evidence has been found that there issuch a shift in color sensitivity. This investigation will bereported in a later paper.

-2.0 -1.0 0.0LOG EXPOSURE

discussed the technique of choosing the rightcrystal for solution and the transport of theions, there remains then to be considered themethod of forming silver atoms from the ionsand the methods of aggregation of these atoms.

The silver ions are released by breaking up thesilver thiosulfate complex. Three ways of doingthis are: (a) by building up a concentration ofcomplex ions in the reagent layer. When theconcentration in this reducing agent becomeshigh enough, silver atoms are apparently formedwhich then condense together to form silver par-ticles; (b) by reacting with the thiosulfate por-tion of the complex an ion' of a metal whichforms an insoluble thiosulfate; (c) by reactingwith the silver ion a sulfide, selenide, or similarion to form a speck of the silver salt. Around thisspeck silver particles seem to form, taking theirelectrons from the developing agent.** In prac-tice processes (a), (b), and (c) can occur con-currently and are often so employed, the em-phasis being shifted from one to another depend-ing on the particular photographic problem.

(a) The concentration and condensation process. Thishas the advantage of directly producing silver aggregateslarge enough to give blue-black images. The precipitationdoes not occur until enough silver atoms are available andis presumably initiated by a relatively few accidentalcenters. These conditions are suitable for forming largeprecipitated particles. This process is particularly adapted

** An analogy to the theory of the development of thelatent image is possible. It can be argued that the develop-ing agent provides electrons to the silver sulfide speck,just as it is proposed that it provides electrons to thesilver speck of the latent image, and that the arrivingcomplex silver ions are discharged with precipitation oftheir silver on the charged silver sulfide specks.

71

EDWIN H. LAND

1.00

.80

za

J

w-

.60

.40

.20

400 500 600

WAVELENGTH IN MILLIMICRONS

to those emulsions in which there are marked differencesbetween the exposed and~unexposed grains, and in whichthe contrast and gamma are high. With such emulsionsthe condensation process can give extremely high gammaand good blacks and whites.

In the condensation process precipitation is not relatedin a simple way to the availability of silver halide fromthe negative because precipitation in the positive waitsupon some specified degree of saturation which is a functionof the environment and then occurs vigorously. If the con-centration is lower than this specified amount, precipita-tion may not occur. This is why this process is particularlysuitable for those problems requiring all-or-none effects,although it can nevertheless be used for producing con-tinuous and graded scales with the right choice of negativematerial.

(b) The use of metallic ions to form insoluble thio-sulfates. This variant of process (a) is useful for buildingup the density and for making it possible to use smalleramounts of solvent. Since the precipitation of the silveragain depends on the accumulation of silver atoms on arelatively few accidental centers, blue-black tones areproduced.

(c) The use of anions such as the sulfide or selenide ionsto produce an initial insoluble grain. These ions break upthe complex by reacting directly with the silver ion. If thesulfide ions are widely distributed, as they would be if insolution, an enormous number of centers of growth ispresent and the silver formed on these centers is in ahighly dispersed state. Images made under these condi-tions will be yellow or orange. Since precipitation is ini-tiated by the sulfide centers rather than by high concen-tration, a relatively small amount of solvent can be used.In processes (a) and (c) when the silver is precipitated,the thiosulfate portion of the complex is left free to beused over again and this is particularly important inprocess (c) since the large concentration of silver (andsolvent) is not required to initiate grain growth. Thetechniques of process (c) have proved to be particularlydesirable for forming scales in the positive that have anexcellent potographiic relationship to the scale in thenegative. For this reason it is important to consider how

FIG. 4. Spectral reflectance curves ofa range of colors of one-step positive

-- images.

700

to obtain dark brown, brown-black, or blue-black imagesby this type of process.

As indicated above, an excessive number of small par-ticles comprising the image presents a serious problem.

If the particles are only slightly enlarged bythe addition of silver, the image is bright yellow.If on the other hand sufficient silver is presentedfor the growth of these particles to a size largeenough to give a dark brown or a black image,then the density is too high in the middle tonesand highlights: the resulting image is dark andhas very low contrast. This difficulty cannotbe surmounted by the use of a smaller numberof sulfide ions. We have computed that if blackimages of good contrast are to be obtained,only 1011 ions per cubic centimeter of reagentare permissible. This is equivalent to a concen-tration of one part in ten billion, which is ofcourse absurdly low.

The next step was to put on the surface of thepositive support sheet, microscopic crystals ofa metallic sulfide in very low concentration. Thecomposition of these crystals was so chosen thatthey would dissolve slowly in the viscous reagent.It was found possible to prepare crystals whoserate of solution (or the mobility of whose dis-solved ions) was low enough so that sulfide ionsremained in the environment of the minute crys-tals until silver ions from the negative reachedthis location. Consequently the cluster of sulfideions precipitated a cluster of silver sulfide par-ticles, close enough together so that additionalsilver precipitated on them aggregated to massessubstantially larger than those produced by dis-solved sulfide ions as described above.

I I

-._ _ , ,~~~~--.:.---~~~~~~~~~~~~~~.......

72


0.8

0.7

0.6

0.5

FIG. 5. Distribution on color triangleof a range of colors of one-step positiveimages (numbered circles correspond tocurves on spectral reflectance chart).

0 0.4

03

0.2

0.I*

Another fundamental advance made it possibleto obtain still larger masses of silver. Techniqueswhich will be described in a subsequent paperwere developed for forming low density galaxiesof extremely small sulfide crystals loosely clus-tered together. Although these galaxies were insome cases made of sulfides with a high opticalabsorption coefficient, the mass density of thegalaxy was so low that little or no optical densitywas apparent when the positive sheet was cov-ered with these groups of microscopic crystals.In this variant of the process, when the viscousreagent was spread over the sheet covered withthese galaxies, very low density clouds of sulfideions were available, the diameter of which waswas of the order of the diameter desired for thesilver particles which composed the final image.When the silver complex ions arrived at thesegalaxies, silver sulfide was formed at each of thesulfide ions and was covered immediately by amass of metallic silver reduced from the otherions in solution. In this way dark brown, brown-black, and blue-black images were obtained.

d. Color of the Image

It is an elementary fact of colloidal chemistrythat metallic sols are colored. The range of size

700

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7VALUES OF X

for particles that can stay in suspension in theordinary sol is of a lower order than that involvedin making good neutral photographs. The imagesdescribed in the preceding section, made by initi-ating precipitation with a trace of sodium sulfidedissolved in the viscous reagent, are yellow andare much the same color as a classical silver sol.

A careful study of the change in color of silverimages was made by Chapman Jones 12 by depos-iting known quantities of mercury on small part-icles in the silver image. His approach was prag-matic and accurate, and produced a whole rangeof colors in photographic images. Nevertheless,his problem was different from that of this paper.He used in his experiments an already existingsilver image, and deposited mercury on the part-icles that composed the image. The problem pre-sented by the new process is to grow particles inthe correct place directly from solution and togrow them, as nearly as possible, all to the samesize. The importance of the ion galaxy techniquesdiscussed in the preceding section is that withthose techniques the particle size of the silverseems to be determined by the diameter of thegalaxy. The silver aggregates act as if there weremetallic conduction between the particles early

12 C. Jones, Phot. J. (RPS) 51, 159-174 (1911).

73

EDWIN H. LAND

FIG. 6. Characteristic curvesof a variety of tank-developednegatives and corresponding one-step positive prints.

in the course of the deposition of the silver on thegalaxy, so from the start, the particle acts like alarge particle. Its absorption coefficient increasesas deposition proceeds, but its color seems to re-main nearly constant with a slight change frombrown towards black. It is scientifically interest-ing and useful in practice to be able to makeimages over the wide range of colors that can behad by developing the particles in the positiveimage to different sizes in different pictures.

Figure 4 shows the spectral reflectance curvesof a range of color that can be obtained with thisprocess, and Fig. 5 shows the position of thecolors of the same samples on the color triangle.The samples were chosen to have a reflection den-sity of 0.5 approximately in the middle of thevisible spectrum. The trend from the side to themiddle of the triangle shows the progression ofimage color toward a neutral gray as the sulfideions are provided, first, in solution, second, assmall crystals uniformly distributed on the pos-itive image sheet, and then, as galaxies of verysmall crystals. 3

e. Review of the Simultaneous Growth ofNegative and Positive

Now that the fate of the individual part-icles has been described from the time of spread-ing of the reagent to the formation of the com-plete negative and positive images, the over-all

13 One precaution should be observed in relating thesecolors to those of silver sols or of lantern slides made toshow the color of different size silver particles. These arereflectance curves and may be complicated by the relationof transmitted, reflected, and back-scattered light. Studiesare being started now on the transmission color alone ofthese suspensions and will be reported later.

growth of the positive and negative photographicimages will be discussed.

Figure 3 shows a group of negative curves andthe corresponding group of positive curves, thepairs being identical except for the length of thedevelopment period. For example, the right-handnumber 2 points to the characteristic curve forthe negative that goes with the left-hand number2, which points to the positive characteristiccurve. The value 2 indicates that the sheets werepeeled apart and that image growth was stoppedat the end of two seconds.

It will be noted that density has formed in thenegative rapidly, that it leads the growth of den-sity in the positive slightly, that after 30 secondsthe maximum density of the positive is startingto exceed the maximum density of the negative,and that finally, at the end of the process themaximum density of the positive is considerablyhigher than that of the negative. This latter re-markable phenomenon might be thought to be aresult of incomplete development of the negativein this process. As can be seen in Fig. 6, this is notthe reason. In Fig. 6, the characteristic curves ofa variety of negative materials developed to theirmaximum contrast by tank development areshown plotted with the characteristic curves forthe positives made by the new process from nega-tive material of the same type.

In most of the examples of Fig. 6 the maximumpositive density is greater than the maximumnegative density obtained by tank development.(The exceptions are usually caused by somespecial condition in the negative such as delib-erate fogging.) The higher density of the positivesmeans that the material of the positive has a

l.0

5,: c

I8 A f - I

D _ _ _ I I

'.5

I0

0.5

LOG EXPOSURE

To 11 fin � � n A n ae ^ _ A A A .

74

I

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E I* cu -LU 1 . u.


higher photometric coefficient than the ordinarydeveloped silver halide grain.

Figure 2 shows negative maxima and positiveminima and negative minima and positive max-ima as a function of time at two differenttemperatures. This figure demonstrates that asatisfactory ratio obtains in the advancing re-action front between the rate of development andthe rate of solution, and that this ratio is sub-stantially maintained over a wide temperaturerange. The lower right-hand curves, as comparedwith the lower left-hand curves, indicate that therise in temperature causes more of the less sensi-tive grains to develop in the negative, eitherdirectly or after dissolving. The density attribut-able to this silver precipitated in the negative isseen to be missing, to some extent, from the posi-tive maximum curve just above it, as comparedwith the curves in the lower left.

-2.0 -1.0 0.0LOG EXPOSURE

FIG. 7. Characteristic curves of one-step positive prints,made with identical negative emulsions, but with variationsin treatment of the positive sheet.

With a given negative material a wide rangeof characteristic curves and contrasts can beobtained, depending on the relative rate ofgrowth of the negative and positive, and on thecovering power of the particles. Figure 7 showsa variety of characteristic curves obtained witha single negative material by changing the typeof anion clusters in the positive supporting sheet.There is, however, a limit to the variation thatcan be produced by the variation in the proces-sing of the negative material, and a much widerrange of characteristic curves is obtained bychoice of a variety of negative emulsions withwidely different characteristics. Figure 6 is anexample of a set of rather similar negative mate-rials with their corresponding positive curves.Figure 8 is a set of characteristic curves for theover-all process using a variety of very differentnegative emulsions. The striking differences inspread and curve shape in the latter case areobvious.

2.0

1.5PRINT

' a LO \ /-N~~~-EGATIVE

0

.5

-2.0 -1.0 0.0

LOG EXPOSURE

FIG. 9. Characteristic curves for a tank-developedVerichrome negative, and a one-step positive print madewith an identical negative emulsion.

C14 1.0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-1.0

- -0 -O.o -2.0 -I., -2.0 -0.0 0.0 0.5 O L2 [.LOG EXPOSURE 0_5 0.5

FIG. 8. Characteristic curves of one-step positive printsfrom a variety of negative emulsions processed against 0 ,0identical positive sheets. -2.0 -1.0 0.0

LOG EXPOSURE

14 C. E. K. Mees, The Theory of the Photographic Process, FIG. 10. Characteristic curve and its derivative curve forp. 231. a one-step positive print.

75

EDWIN H. LAND

FIG. I I. A halftone made from a print made by the process.

VI. PHOTOGRAPHIC USEFULNESS OF THEONE-STEP PROCESS

Some of the primary factors in determining thephotographic usefulness of a process are the sen-sitivity, latitude, resolving power, shape of thecharacteristic curve, the range of densities, min-imum density, color, and stability. Since the lastfour factors have already been discussed thissection will deal with the first four.

The process described in this paper can be usedwith a range of emulsions of varied sensitivityfrom contact paper to highly sensitive negativematerials. Frequently the process appears to in-crease speed of the negative material relative tothe rated speed. This effective increase in speed

can be understood from Fig. 9, in which thecharacteristic curve for Verichrome emulsiondeveloped in a tank is contrasted with a positiveprint made from the same type of emulsion. Itwill be noted that the characteristic curve forthe positive lies in the low exposure end of thenegative curve, and that the positive process hashad the effect of giving a greatly increased slopeto the curve for the new process for a regionof the negative curve in which the slope is small.In conventional practice, with this particularnegative material, it would have been possible toexpose so as to use nearly the same portion of thenegative curve that the new process does, and agood print could have been made by selecting the

76


right paper. But the recommended speed for thenegative material would have led to a greater ex-posure to enable the operator to take advantageof the latitude of this negative material. Anterror" in exposure made by the operator wouldsimply have shifted the significant portion of thepicture along the long sloping curve of the nega-tive response. In making the print, this "error"would have been compensated by selecting aproper paper and exposure. In the new processthe operator would be required to use the smallerexposure and to treat the film as if it had onlythe higher sensitivity.

The latitude of any one-step process will obvi-ously be less than for the combination of thenegative material and a choice of positive papers.In this respect the new process is like most of thecolor photography processes. It is necessary forthe best results to choose the correct exposurefor the particular characteristic curve. A correctexposure lies within a range of about two stops.This limitation is perhaps offset by the fact thatthe operator can see the picture as taken andcorrect for any error at once in a new exposure.

The resolving power of the process has beenfound to vary between 30 lines per mm and 10lines per mm, depending on the type of reagent,emulsion, or positive sheet that is chosen. Theresolving power requirement varies with theplan for use of the image. If a large hand-heldprint is produced directly from the camera sothat enlargement is not necessary, then imagesappear reasonably satisfactory with a resolvingpower of 10 lines per mm. If images are to be pro-jected, a resolving power of at least 30 is desir-able.

As Fig. 8 shows, a wide variety of character-

istic curves can be obtained by varying the char-acteristics of the emulsion. This figure containsthe curves obtained by using a range of commer-cial emulsions. Figure. 7 shows how the charac-teristic curve was changed by using the sameemulsion but varying the composition of thereagent in the positive sheet. It is also possibleto produce a somewhat similar range of curves byvarying the composition of the developing re-agent. Figure 10 shows a typical "good" char-acteristic curve obtained by using an emulsionespecially prepared for the process. The curve forthe derivative is plotted for the convenience ofthose who wish to use Jones"5 criteria for esti-mating the relation of characteristic curve to thequality of the print. In general, it would appearthat since a wide variety of curves can be madeand since the characteristic curve of the nega-tive can be related to the characteristic curveof the positive, any desired curve can be plannedon in defining the process. A great many photo-graphs have been made, one of which is repro-duced here (Fig. 11).

ACKNOWLEDGMENT

The writer wishes to thank all of those mem-bers of the Polaroid Research Department whohave participated in this investigation for theirenthusiastic and effective assistance, and to noteespecially the contribution of his first assistanton this program, Mrs. Eudoxia Muller Wood-ward. The writer also wishes to thank the East-man Kodak Company for its cooperation insupplying a variety of special emulsions for thisinvestigation.

1" Reference 14, p. 824.

77

a new one-step photographic process

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