The chemistry of color photography - Journal of Chemical Education

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Wayne C. Guida and Douglas J. Raber Universitv of south Florida Tampa, 33620

II

The Chemistry of Color Photography

Photography is a widespread activity which appears to he undergoing a substantial increase in popularity, especially among college students ( I ) . The chemical reactions involved in photography therefore rank among the most common carried out bv scientist and lavman alike. Nevertheless, many students of chemistry lack an understanding of these reactions. Although the suhiect of hlack and white photography has been treated in a number of articles and textbooks (2), the chemistry of color photography is infrequently discussed in the undergraduate curriculum (3). Yet the latter represents a fascinating and elegant combination of photochemistry (energy transfer), organic chemistry (dye formation), physics (nature,of light), psychology (color perception) and engineering (isolation of different chemical processes within various layers of the film). I t is the intent of this article to provide a brief introduction to the chemical reactions involved in color photography and to the physical principles which permit these reactions to reproduce colored images (4).

Figure 1. a. The additive primary colwr. Projection of overlapping circles ol blue, green. and red light onto a viewing screen. Superimposition of all three colored lights (in me proper intensities) resuns in the perception of white (center). b, The subbactlve primary colors. Three overlapping filters (yelow. magenta. and cyan) are siluated behxeen the viewer and a source of white lluhl. Each finer removes one of the adddive orimarv - .colors Where the three fehers (01 m e proper dansdes) overlap (center) all mree adddlve pnmary cob OIS are removed, and me voswsr psrcewes omack Both a and D show me comDlemantary relalansnnps between b ~e and y e l b w green and magsnta. and red and cyan

The Perceotlon of Color The human visual system is capable of distinguishing among a vast number of different colors (5).and the color percel'ved is dependent upon a subtle comhination of effects involving the areas of physics, psychology, and physiology. From a chemist's point of view it might initially seem a hopeless task to devise a method for reproducing these different colors in a photographic process which relies on the selective absorption of light by different chemical species. However, as recognized nearly 200 years ago by Young and later discussed by Helmholz (6),color vision is basically a trichromatic phenomenon (i.e., a wide range of color effects can be produced by using various combinations of three differently colored light beams). Consequent1y;a photographic system for color reproduction can also be based on only three colors. The retina of the eye actually contains several distinct types of color receptors which are sensitive to particular regions of the visible spectrum (7): the interpretation by the central nervous svstem of a stimulus or comhination of stimuli then results in the perception of a particular color. Current theories 1.5. 8, of human color oerception are hoth complex and contr&rsial, and they cinnotbe considered in detail in this article. The first demonstration of photographic color reproduction (9) was carried out by Maxwell in 1861 in an effort to demonstrate the trichromatic theory of color vision. Using Maxwell's approach, hlack and white photographs of some scene are taken through three different colored filters (blue, green, red). The intent of such an experiment is t o split the light coming from the subject into three components, and to record the scene separately in terms of each of these component colors. Each photographic record is made as a hlack and white transparency or slide, and the three resulting slides are then projected simultaneously onto a screen using colored light sources. The light source for each slide corresponds to the filter through which the photograph was made; thus the slide which recorded the image in terms of red light (by photographing through a red filter) is then projected using red light. When the three slides are projected simultaneously, the resulting single

image on the screen reproduces the colors of the original scene (9). This techniaue for nhotoeraohic - reoroduction of color is called an additive process since the viewer perceives the results of superimposition of three colored images on the viewing screen, and historically the additive process has olaved an imoortant role in the develooment of color ohoto*aphy (9): However, the additive process is limited t o ~roiectedimaees (color television being one of the few examples of its current use), and modern-photographic technology utilizes the more versatile subtractiue method of colo;>eproduction. In contrast to the additive method in which light from several monochromatic sources combines t o produce the sensation of a particular color, the subtractive method uses a single source of "white" light from which certain wavelengths of light are removed or filtered out. The net result for the two methods is equivalent, and in both cases essentially the same combinations of stimuli affect the eye. This relationship is illustrated in Figure 1which summarizes color oerceution in terms of both the additiue primary color; (blue, green, and red) and the subtractive primary colors (yellow, magenta, and cyan, which result when one of the additive primary colors is removed from white light). The subtractive primary colors are often mistakenly labeled yellow, "red," and "blue"; the source of this confusion arises from the similarity in color of blue and cyan (actually a bluish green or turquoise) and of red and magenta (actually a reddish purple). White light (whether sunlight or artificial light) consists of "all" of the colors of the visible spectrum, and it is the absorption (or subtraction) of various components of the white light which results in perception of different colors. This can be illustrated in terms of Figure 1by considering that combination of red, green and blue light (the three additiue primary colors) results in the perception of white (Fig. la). Since combination of just two of these, red and green light, results in the perception of yellow it is evident that subtraction of the blue component from white light would also result in the perception of yellow. Thus there

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are a t least three ways in which the eye can be stimulated to produce the sensation of yellow 1) with a monochromatic light source having a wavelength of

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aooroximatelv 580 nm (vellow) . with a ctmbinarion of two monochromatic light sources having wnveiengths of approximately 660 and 530 nm Ired and green, respectively) 3) with a source of white light from which the blue component (the band between -40&500 nm) has been subtracted 2)

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A verv. imnortant feature of Fieure 1which must be rec. ognized is the complementary relationship between the additive and subtractive orimarv colors: each of the subtmctiue primary colors is chat wLich is perceived when one of the additive primary colors is suhtracted from white light. Removal of red from white light results in the perception of cvan. removal of areen results in the perceotion of magenta, and removal 07 blue results in the-perc;ption of yeliow. As a result of this correspondence hetween the additive and suhtractive primary coiors, each of the three pairs is described as complementary. Note that removal of any two additive primary colors leaves only the third: for example, subtraction of both red and green (using cyan and magenta filters, respectively) results in the perception of blue (Fig. Ih). The application of these primary color schemes to phntography is illustrated in Figure 2, which shows several methods for the reproduction of a yellow subject. Figure 2a depicts the additive method in which individual red and green images are superimposed on a viewing screen; Figures 2h and 2c depict the subtractive method used in modern photography. In the case of a color slide or transparency (Fig. 2b) the hlue component of the incident light is absorbed by a yellow dye, and the resulting image on the viewing screen is perceived as yellow. Similarly, the color print represented in Figure 2c appears yellow because dye molecules absorb the blue component of the incident light while the other components are reflected by the white

paper hacking. A multicolored slide or photograph requires the use of three dyes: yellow, magenta, and cyan, each of which regulates (by absorption) the intensity of one of the additive primary colors. Note that the necessity of regulating the additive primary colors requires that the dyes have ,A, corresponding to light of those colors; consequently, dyes whose colors are yellow, magenta, and cyan are the three types which must he used in color photography. Proner combination of these dves results in the formation of a-photograph which is perce&ed by the viewer as having the same (or nearly the same) colors as the original scene. The Construction of Color Films

Modem photographic films generally are manufactured by coating multiple layers of photographic emulsion on a flexible strip of transparent polymer. The emulsion consists of a dispersion of silver halide crystals (and certain organic dyes which cause them to he sensitive to light of the desired spectral region), and in the case of some color films a variety of other organic molecules which undergo suhsequent reaction t o form the dyes that ultimately produce the color image. These components are usually dispersed in eelatin. Color films consist of three layers of photographic emulsion. with a total thickness of about 2 0 4 0 u. which are sensitive to hlue, green, and red light, respectivkly. As in black and white photography (2), exposure to light results in the absorption of energy by molecules in one or more of the three layers, causing changes in some of the silver halide grains and forming the so-called latent image. These exposed grains are chemically different from the other crystals, and they are reduced hy photographic developers which do not affect the unexposed or insufficiently exposed silver halide crystals. Thus a permanent record of the light striking each of the three emulsion layers is provided. An idealized view of a typical color film is illustrated in Figure 3. Since silver halide is naturally sensitive to blue

-. Fornation d o Colored Imope by the Additive Method.

White Source

YeIIw Image is Perceived

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light the first layer (hlue sensitive emulsion) contains only a dispersion of silver halide. Directly beneath this is a layer (yellow filter) which absorbs any hlue light that passes through the first layer. Removal of this hlue light is essential since it would otherwise activate the silver halide in the remainine emulsion lavers and result in an incorrect record of the c o l k of light &king the film. Beneath the yellow filter layer are the green and red sensitive emulsions. As the silver halide in these two layers is not directlv sensitive to anv - liaht - which reaches them.. sne. cia1 organic dyes (sensitizing dyes) are employed. Green or red light is absorbed by the appropriate sensitizing dye, and transfer of energy from the dye to a silver halide grain then results in crystal im~erfections.Among.the most important sensitizing dyes are those of the cyanine class, such as the thiacyanin~sI and 11. Compound 1 is a magenta dye (ahsorbs green light), and the vinylog I1 is a cyan dye tabsorbs red light) (10)

A ca10r print.

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CH,CH, Figure 2. Addtiwe and subhactive methods f a tion of a yellow subject.

photographic color reproduc-

I: n n: n

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I; Am,, 2, Am,,

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-- 550

CH,CH, nrn

650 nrn

Volume 52, Number 10, October 1975 / 623

The bottom layer of the film, called the antihalation layer, contains a substance such as an organic dye or colloidal silver to absorb all colors, thereby preventing any light from heine reflected back into the emulsion lavers. Finallv. there is a transparent polymeric support onto ahich all tl;; layers of the film are coated. The manufacture of such multilayered films is clearly a delicate and complicated operation: the high quality of today's films is a . ohotograohic . - . tribute to modern technology. Color films are generally manufactured for a specific photographic use: either for the production of color transparencies (slides) or for color prints. In the case of transparencies a positive image is desired; that is, the colors transmitted by the slide should be the same as the colors of the original scene. In the case of color prints it is desirable to have a "negative" from which more than one print can he made. Consequently the exposed film is treated chemically to give a negative transparency in which each color is the com~lementof the one in the orieinal scene. Color prints are then made by projecting the negative image onto a Daoer which is coated with ohotoera~hicemulsion. This "dh&ographic paper" is essential1;thL same as the color film originally used to record the image except that paper rather than a strip of flexible plastic is used as a support. The overall process from color film to color print via a negative involves two separate conversions of each color into its complement, the net result being an image which has the same colors as the original scene. Both color transparency and color print films are constructed similarly (Fig. 3), and the major differences between them lie in the techniques hy which they are processed. Color print films, like films for making hlack and white prints, are designed for processing by way of a negative. Color slide films, however, are designed so that processing gives a positive transparency; as a result they are called reversal films. Mwt color print films are marketed under names ending in "-color" (e.g., Agfacolor, Fujicolor, and Kodacolor), while the reversal or slide films have the typical suffix "-chrome" (e.g., Agfachrome, Anscochrome, Ektaehrome, Fujichrome, and Kodachrome). Although each type of film is manufactured with a particular kind of processing in mind, some flexibility exists for the photographer: for example, color prints can be made from color slides by a modification of the process ordinarily used to make a print from a negative; similarly positive transparencies can he obtained from color negatives (4a, e).

The reaction proceeds a t a useful rate only under alkaline conditions, and treatment with dilute acid (stop bath) is used to terminate the development reaction after the desired effect bas been obtained. When the activated silver halide crystals have been reduced to silver metal, the remaining silver halide is removed in a step called fixing. As silver halide is highly insoluble in aqueous solution, a reagent such as sodium thiosulfate is employed to produce a water soluble complex. The result of these operations is a negative, from which photographic prints can be made by a similar series of steps: light is passed through the negative onto a sheet of paper coated with a photographic emulsion; repetition of the developing, stopping, and fixing steps then affords a positive image of the original scene. Color photography is very similar, hut there are two major differences: (1) an approprialely colored dye must be formed in each of the three emulsion layers, and (2) all of the silver metal must he removed so that the color image is not obscured. The overall color process is summarized in Figure 5, which describes the production of a photograph of a green, white, and yellow scene. For the sake of clarity only the emulsion layers of the film are shown. Initial exposure results when white light (e.g., sunlight) strikes the scene, and the reflected light is focused onto the film by the camera lens. The diagram illustrates this situation for three adjacent segments which have been exposed to green, white, and yellow light, respectively. In the segment which is struck by green light (an additive primary color) only the silver halide in the green sensitive emulsion is activated, while in the center segment the silver halide in all three emulsion layers is activated by the white light.

The Chemistry of the Color Process

In order to understand color ~ h o t o a a o h va discussion of the hlack and white process is i e l p f i +hi latter topic has been treated recentlv in this Journal (2aJ so it will onlv be outlined briefly her; As summarized in Figure 4, the exposure of hlack and white film to light reflected from some scene and focused by the lens of a camera activates some of the silver halide erains. These activated crvstals are then allowed to react &h an organic reducing agent such ashydroquinone (eqn. (1)).

Figwe 4. Outline 01

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black and white photographic process.

Journal of Chemical Education

Figure 5. Outline of the color photagraphic process,

However, in the third segment yellow light causes activation of silver halide in both the green and red sensitive emulsion layers (Note that yellow is the subtractive primary color which results when the blue component of white light is removed.) Subsequent treatment of the exposed film differs for the reversal and negative types, and these will be discussed in turn. For both t m s ., color control is achieved by formation of dyes in the three emulsion layers, and in everv instance the color of the dve is comolementary to the color to which the emulsion layer is sensitive. Thus the control over blue light is brought about by the formation (or lack of formation) in the blue sensitive emulsion layer of a yellow dye; analogous mechanisms operate for control of green (with a magenta dye) and of red (with a cyan dye).

..

Reversal Processing

In order to directly obtain a positive transparency i t is necessary to form dyes in each segment of the film which will remove from white light the appropriate additive primary colors; light passing through the slide will then he perceived as having the same colors as the original scene. Thus in the segment of film which is struck by green light, dyes must form which absorb red and hlue; in the segment struck by white light no dyes should form, and in the segment struck by yellow light a dye which absorbs blue must form. Note that in each case it is necessary for dyes to form in precisely those layers which were not activated by the original exposure; formation of a dye in the activated layers is not desired. The first step in reversal processing (Fig. 5, left) is reduction without dve formation of all activated silver halide crystals. This is accomplished by treatment of the exposed film with an ordinarv black and white tvne .. develooer. The remaining silver halide, which forms a record of those portions of the film which were not activated. is in turn activated either by exposure to light or by chemical means. Reduction with color develooer (see below) results in the formation of both Ago and (he appropriate dye in each emulsion layer. All of the silver metal is then oxidized ("bleached") back to the +1 oxidation state using reagents such as ferricyanide or dichromate, and the resulting silver salts are removed ("fixed") by complexation with thiosulfate. Modifications of this procedure permit these last two steps to be combined into a single operation: An additional step termed "stabilization" is generally carried out hy treatment with formaldehyde to inactivate any dye precursors which remain in the emulsion layers. The resulting color transparency is now complete: all of the silver metal has been removed, and the sensitizing dyes have been extracted into the aqueous medium during the various processing steps. The remaining dyes now transmit only the desired colors: the first segment contains both yellow and cyan dyes which remove blue and red so that the viewer oerceives ereen: no dves are oreseut in the center segment so the c z o r white isperceived; and the yellow dye in the third seement removes hlue so that the viewer nerceives yellow.

propriate dye in each emulsion layer. Removal of all silver from the film is accomplished by oxidation of Ago (bleaching) and complexation of the silver salts (fixing). As in reversal processing treatment with formaldehyde (stahilization) is often the final step, resulting in this case in a completed negative. The production of a color photograph or orint is accomolished bv exnosure of the nrint material kith light that has passed: thrbugh the negative followed by a re~etitionof the above stens. The net result is that each color of the original scene has been first transformed into its complement and then hack to the original color: Light which has passed through the negative is focused onto a photographic emulsion (usually coated on paper), and color development, bleaching, fixing, and stabilization are carried out as for the negative. Reference to Figure 5 (right) shows that the first segment of the negative, which contains magenta dye, prevents green light from striking the photographic paper, so that only the blue and red sensitive emulsion lavers are activated with the subsequent formation of yellow and cyan dyes. When the finished print is viewed in white licht these two dyes subtract hlue and red, respectively, and h e viewer perceives the green color of the original scene. The center segment.of the negative contains all three dyes (which subtract red, green, and blue) and permits no light to strike the photographic paper; development then causes no dye formation, and the center segment of the finished print appears white. The third segment of the negative, which contains both magenta and cyan dyes, suhtracts green and red hut permits hlue light to strike the emulsion. Color development then produces a yellow dye, and when the finished print is viewed in white light, the yellow color of the original scene is perceived. Dye-Forming .Reactions

As in the case of black and white development the reduction of activated silver halide in color nrocesses is effected by reducing agents analogous to hydroquinone (eqn. 2). However, the structures of color developers are designed such that subsequent reaction with organic compounds called couplers results in the formation of appropriately colored dyes. The color developers are usually p-phenylenediamine derivatives (III), although other classes of compounds are occasionally used. NH,

NH

N R/ \R 111

N

Oxidation of the p-phenylenediamine 111yields the quinoid species IV, which is highly susceptible to nucleophilic attack on nitrogen by a coupler anion [X-CH-Y]- as illustrated in eqn. (3). X

Negative Processing

The production of a negative requires formation in each segment of dyes whose colors are complementary to the color of light striking the film. In contrast to the reversal process this corresponds precisely to dye formation in those emulsion layers which are activated by the original exposure. Thus in the seement struck bv ereen light a maeenta dye must form, in &e segment str"uik hy wwhite lighz dye must form in each of the three lavers. and in the seement struck by yellow light both cyan-and magenta dyesmust form. The first step in negative processing (Fig. 5, right) is treatment with a color developer to form Ago and the ap-

This unusual mode of addition (i.e., attack a t the nitrogen rather than carbon of a carbon-nitrogen double bond) gives rise to the intermediate V, termed a leuco dye. The leuco dye then undergoes oxidation (by another molecule of IV) t o produce the dye VI which forms the permanent color image. Volume 52, Number 10, October 1975 / 625

The couplers used in color photography are incorporated into the emulsion lavers in all negative films and in manv reversal films (e.g.,-~ktachromec In some reversal films 1e.e.. . - . Kodachrome) the c o u ~ l e r sare not incoroorated into the film structurebut are instead introduced-during processing as components of three seoarate developer solutions; development of such films requires very carefully controlled conditions. In contrast, those films containing incorporated couplers can be processed much more easily and therefore have gained considerable popularity among. photographers who'do their own processing.The couplers used in color photography can be divided into three structural types according to the colors of the dyes they produce. The couplers employed to form yellow dyes contain an active methylene group; this is usually part of an acyclic moiety as illustrated by the benzoylacetanilides (VII). 0

contain a leaving group which permits formation of the dye (XII) from the leuco dye (XI) by means of a base-catalyzed elimination reaction (eqn. (4)).

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Mneenta - -dves are usuallv formed from couplers which contain an active methyleie group as part i f a heterocyclic rine. -. ex.. - . the ovrazoline derivatives, VIII. The formation of cyan dyes generally relies on couplers containing an active methine group as in the case of the a-naphthols (IX). The dye-forming reactions of these three types of couplers with developer 111are outlined in Figure 6. ~

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"

RT N-R'

0

IX

Vlll

There is considerable diversity in the structures of the couplers, and even within the limited group represented by VII-IX, the patent literature (11) demonstrates the large number of derivatives which are availahle by variation of the R-groups. The use of long-chain alkyl suhstituents on the couplers serves to decrease the water solubility and mobility of these organic molecules. This is especially important for films with incorporated couplers, since the couplers must not diffuse into other layers. Inspection of eqns. (2) and (3) reveals that a total of four silver halide molecules must he reduced to yield a single IV and V VI are two-elecmolecule of dye: both I11 tron oxidations. The overall efficiency of the photographic process can be increased by the use of "two-equivalent couplers" (e.g., X) (12) which requires only two silver halide molecules to produce a molecule of dye (13). Such couplers

-

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ImDrovemenl o f Color Fidelity

The photographic reproduction of color is made more difficult by the fact that "perfect" dyes are not availahle. A perfect d i e would comple~elyabsorb all of the wavelengths of light corresponding to one of the additive primary colors. However, the dyes used in photography show typical absorption spectra (Figure 7a), and some wavelengths are absorbed more effectively than others. Secondary absorptions outside the desired region are also common. As illustrated in Fieure 7a. a maeenta dve mav ahsorh some hlue lieht (-486 nm) in addicon to its major absorption in the green reeion ~" of the snectrum (centered a t -530 nm). In the case bf color negative films this p;oblem can be controlled by the use of couplers which are themselves colored (14). For example, the unwanted hlue (400400 nm) absorption of the magenta dye shown in Figure 7a could he corrected by the use of a coupler (Fig. 7b) which also ahsorbs light in that region (and which therefore has a yellow color). Since formation of a molecule of magenta dye coincides with disappearance of a molecule of the coupler, the. overall absorption of the blue light throughout the emulsion laver hv dye and couoler remains constant and uniform; &reciion for this absorption can be effected by increasing the intensity of the hlue component of the "white"

400 I

W Figure 6. Dye forming reactions.

626 / Journal of Chemical Education

I

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500

600

700

wavelength fnm) Figure 7 . Absorption spectra for a hypothetical magenta dye (curve a) and lor a colored coupler (cwve b) which is a precursor of the dye.

light from comm lens strikes film

light source used in printing. The unwanted green and blue absorptions of the cyan dye can be controlled similarly, and the yellow dye usually does not require color correction. The use of colored couplers results in a uniform colored "mask" which is familiar to most photographers as the distinct orange cast of a color negative. Thecolored couplers (e.g., XIII) are analogs of the normal couplers, hut contain an azo group which is lost during the dye forming reaction (eqn. (5)).

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Figure 8. Idealized view ol the construdan of Polaroid SX-70 Land Film.

I NR,

XIV

Clearly the orange mask which results from the use of colored couplers precludes this method of color correction in the case of transparencies. The improvement of color fidelity in reversal films is accomplished by other methods such as the use of "development inhibitor releasing couplers" (15). Other Color Processes

Althoueh snace limitations orevent a detailed discussion of other Getcods of color reproduction, the widespread use of Polaroid cameras and films mandates a brief description of the Polacolor process. Figure 8 depicts an idealized-view of the construction of the new color film recently introduced by Polaroid; in contrast to conventional photography this process provides a color print which is released directly from the camera (16). The intermediate state of a neeative is bmassed, and all the chemical species necessary fo;image fo&ation are contained within the film. Since the first four layers are initially transparent, incident light activates silver halide in each emulsion layer as in conveutional color films. However, a t this point the subsequent treatment ~ - - ~ -diverws " drasticallv. When the undeveloped Polaroid print is ejected from the camera it passes through a set of rollers which causes the rupture of a pod of chemicals located on the perimeter of the-film and distributes these reagents evenly between the image receiving layer and the first emulsion layer (Fig. 8). The reagent pod contains three major ingredients: alkali, titanium dioxide, and an indicator dye. The latter two substances prevent any additional light from reaching the emulsion layers, and the titanium dioxide also serves as a white background for the colored image which is eventually formed. The basis of the Polaroid color process lies in the structure of the dves (16b). While a dye such as XVI is water insoluble, the increased water solubility of the conjugate base allows it to diffuse through other layers of the film into the image receiving layer. At the same time the hydroquinone moiety of XVI is capable of reducing (developing) activated silver halide under alkaline conditions. The oxidized form (XVII) is neutral, and like the neutral form of the hydroquinone XVI will not undergo diffusion. Thus the action of alkali on a "dye-developer" molecule (e.g., XVI) converts i t t o a species which can migrate to the image receiving layer unless i t is first oxidized to an immobile derivative. ~

Land has reported (16~)that the oxidation of XVI need not proceed all the way to the quinone state (XVII) in order t o prevent its diffusion. In the presence of certain quaternary ammonium salts, one-electron oxidation to the semiquinone XVIII apparently is adequate t o prevent migration of the dye-derivative to the image receiving layer.

0-

XVlII

Furthermore the oxidation of dye-developer is not necessarily the result of direct interaction with an activated silver halide grain. Rather, a simple hydroquinone derivative XIX (termed a "messenger developer" by Land) which is incorporated in the film, can serve as an electron transfer agent, first being oxidized by activated silver halide and then in turn oxidizing a dye-developer molecule. CH,

OH

XIX

Figure 8 shows that beneath each emulsion layer is a layer of dye-developer; in each case the color of the dye is complementary to that of the light to which the emulsion layer is sensitive. This affords a positive image, by allowing transfer to the image-receiving layer of only the appropriate colored dyes. Thus the injection of alkali into the exposed film causes deprotonation of the dye-developer molecules (XVI) which then begin diffusing toward the image-receiving layer. Wherever incident light caused activation of silver halide Volume 52, Number 10, October 1975 / 627

in one of the emulsion layers, dye-developer molecules of the complementary color are prevented from reaching the image-receiving layer by oxidation to an immobile species. Consequently those dyes which are complementary to colors of light striking the film (and would absorb those colors) never reach the image receiving layer. In contrast the dye-developer anions which reach the image receiving layer are those which absorb the colors of light which did not originally strike the film. The net result is the reflection of light of precisely the colors of the original subject. The diffusion of a dye-developer is either terminated hy oxidation or is greatly inhibited by interaction with a stationary polymer in the image receiving layer. As the development sequence nears completion, the water generated hy reactions in the film (cf. eqn. (1))breaks down the timing layer, and the acidic polymer neutralizes the remaining alkali. This also converts the indicator dye to its colorless form. and orotonates the hvdroauinone residues of all unoxidized .dye-developer moiecules. The resulting photographic print is then both visually and chemically stable. Conclmlon

In this brief account we have shown that modern color photography relies on relatively straightforward chemistry. This includes redox reactions involving silver halide and hydroquinone analogs, nucleophilic additions to quinone derivatives. and simvle acid-base reactions. Nevertheless, the reprod"ction of color images depends upon the interaction of chemistry with a variety of other scientific disciplines in a manner which is complex, yet at the same time elegant. For example, the fabrication of cameras and films requires sophisticated technology. Similarly the design and production of dyes and dye precursors has demanded tremendous research efforts by organic chemists. When these

628 / Journal of Chemical Education

various aspects are considered as a whole, color photography emerges as a fascinating and exciting field of study. Literature Cited

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",..".","%., ",.

( 8 ) Robert$.

J. O..and Cssserio, M., "Basic P11ndplesotO~g~~i~Chcmistry.l' W. A. Benjamin, lnc. New Y ~ r k1964.p~. , 1059-1066, (hi Keller.E.,Chrmirlry, &.No. 11.8 (1970). (4) Sword h k s and reviews are available: see for example: la) Hunt, R. W. G.. "The Reproduction of Colaur," John W h y and Sona. Inc.. New York. 196.7. (hi Katz, J. and Fogel. S. J.. '"Photographic A n a l ~ i s , "Morgan and Morgan, New York, 1971. (cl Kowaliski. P., "Applied Photogcaphic Theory," John Wiley and Son% Lnc., New Yurk. 1972. id) Mees, C. E. K, and James, T. H. (Editors) "The Theory of the Photographic Proasr," 3rd ed.. MecMillan & Co.. New York, 1966. (el Thirtie. J. R.. and Zwick. D. M., in "The Encyclopedia of Chemical Technology." John Wiley and Sons, Inc., New York, 1964. Val. 5 , pp. 812-813. (0Weinsherger, A , Am~ricanScientist, 58, 648 (19701 (gl Tmutweiler, F., Chimio, 26. 661 11972). J., and Williams.. L. A... "The Photunraohie Color Develooment Pmfh! Railev. . ~. -~ ~, ceis: Chapter VI, in Venkafaraman, 'Themistry of Synthetic 0ypn.l' Academic Pross.New York. 1971. (5) (a) Evans,R. M., "The Peraption of Color." John Wiley and Sow, Inc, New York, 1974. (b) Evans. R. M.. '"An lntmduetian to Color." John Wiley and Sona, Ine., Nev York. 1948. (el Sheppard. J. J., "Human Color Perception." American Elaevier Publishing Co., New Ymk, 1968. (6) Holmhaltz, H., "Treatise on Physiolngiesl Optics: The Percebtions of Vision." IEditar: Southall. J. P. C..) Dowr Publications. Inc,Nelu York, 1962, Val. 11. (7) Wald. G., Seieneo, 162,230 (1968). (8) Land. E. H.. American Scirnfi?l. 52.247 (1960. (9) (a) Wall. E. J.. '"The History of Three Color Photography." F o d Press Ltd.. London. 1970, pp. 2 4 . (b) Mason, R. G. (Editor), '"Color." Life Library of Phot0g.sphy. Time-Life Bwks, New York, 1970. (10) Brwker, L. S.G.. snd VanLsre. E. J.. in "The Encyclopedia of C h e m i d Technolow." John Wilev and Sons. New York. 1964. Vol. 5. p 774. f4bJ. ~ p p e n d i x5. (11) &mnm (12) For example, s e e Fernandez, J. M., U.S. Patent 3,785,829(19741. osee461. fl3I Reference (&I. W. T : j .Zpt. SN. A ~ ~ ,4. 0, . 1 (~L W . (15) Bar., C. R..Thinle. J.R..endVitturn, P. W..Phot. Sci. E n g , 13,74 (19691. (16) (a) Land, E. H., Ths Phofogmphir Joumol, July, 388 11974). (b) Grssshoff, 3. M., and Taylar, L. V., U.S. Patent 8,674,178(19721.

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