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Color Photography - Early Years, Additive Color Systems, Subtractive Color Systems

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JON A. KAPECKI
Imaging Consultant

Early Years

Ever since the first days of photography in the early 1800s, its practitioners sought ways to create images that reproduced the colors of the world around them. Some resorted to hand coloring of the daguerreotype, a system invented in 1833, while others looked to create direct color photographs by finding the right combination of chemicals to form colors on exposure. An American, Levi L. Hill of Westkill, New York, may have been the first to produce a color daguerreotype (which he called a hilotype) by direct exposure, but his technique was not successfully reproduced during his lifetime. In 1891, Frenchman Gabriel Lippmann, building on the work of Claude Niépce de Saint-Victor and physicist Wilhelm Zenker, used standing waves reflected off a pool of mercury on to a silver bromide emulsion. The developed image when held at a specific angle gave a colored image.

But direct color techniques failed to yield a commercially viable system. Rather, the future of color photography lay in technologies which partitioned and captured the color information of the scene in two or more (usually three) separate or adjacent color records and then recombined that information by either chemical, physical, or optical means to reproduce the final color image.

Probably the first and certainly the most famous example of such a system was demonstrated by Scottish physicist James Clerk Maxwell to a meeting of the Royal Institution of London in May of 1861. Maxwell built upon the color theory of Thomas Young, who in 1801 had proposed that the human eye contained three types of color sensors or cones, each sensitive to a portion of the white light spectrum and that by stimulating those sensors proportionally any color could be perceived. Extended by Hermann von Helmholtz, the Young-Helmholtz theory of color vision, as it came to be known, was the basis of color printing. In his classic experiment, Maxwell’s colleague, Thomas Sutton, had photographed a tartan ribbon onto separate photographic plates through a red, green, or blue filter. These black and white negatives were converted to positive lantern slides and then projected separately through the same three filters and superimposed on a screen to yield a full color image. The photographic implications of this experiment were of little interest to Maxwell, who viewed it primarily as a verification of his theories of colorimetry, and he did not pursue the work further.

Ironically, although Maxwell did not know it, his experiment worked for the wrong reasons. Because silver halide is natively sensitive only to blue and ultraviolet light, his silver iodide separation negatives were not accurate carriers of the red and green scene information and, as would be shown by R. M. Evans in 1961, the color reproduction resulted in part from ultraviolet reflectance through the red filter and some blue-green information passed through the green filter. Indeed, it would take the German photochemist Herman Vogel’s 1873 discovery of sensitizing dyes, which extended silver halide’s response to red and green light, to produce orthochromatic (and later panchromatic) black and white films that accurately reproduced colors as shades of gray and would later allow film emulsions to separately analyze the scene for its red, green, or blue content for producing color images.

Additive Color Systems

Maxwell’s color projection was an example of an additive color system. Because each of the so-called additive primaries removes two-thirds of the white light spectrum (blue, for example, is white minus red and green), each additive filter requires its own light source and cannot be superimposed except in projection. This not only creates registration problems, but is also an extremely inefficient use of light, both in originating and displaying the color image.

Beginning in 1868, the prodigious French inventor, Louis Ducos de Hauron, would propose, patent, and sometimes demon strate a wide variety of photographic systems, including a technique for making color prints on paper, a camera for creating three-color separation negatives in a single exposure (necessary to photograph moving objects) by the use of beam splitters, a camera for motion pictures (never built), and perhaps most presciently the basics of the technically superior subtractive system for color photography. Independently, Charles Cros in Paris would make a similar proposal for subtractive color photography in a article appearing in Les Mondes in 1869.

Despite this, it was the additive system that produced the first successful color photographic materials. In 1873 in Dublin, John Joly designed a screen of thin, alternating red, green, and blue filter stripes that could be positioned in front of the taking and projected film. This permitted all the color information to be recorded on a single piece of film, albeit with a loss of resolution and the other disadvantages common to additive systems. When the negative was converted to a positive and placed in front of a white light source, the image formed by black developed silver metal would block or pass light through the appropriate filter stripes, corresponding to the color of the original image. The commercial successor to Joly’s invention, Dufaycolor, developed by Louis Dufay, used a square grid of filter elements. This was introduced in 1908 and would continue to be sold until the 1940s.

But the most successful of the early additive color films was invented by the Lumière brothers, Augusta and Louis. They were prolific inventors who patented their Autochrome plates in 1904 and introduced them to the market three years later. The Autochrome system used a screen of minute (0.015 mm) potato starch grains dyed with the additive primaries (most likely orange-red, green, and violet) with the spaces in between the grains filled by powdered charcoal and superimposed over a silver bromide panchromatic emulsion. After a reversal development process to produce a positive image, the photograph was viewed through the same screen. Though expensive, the Autochrome system was relatively easy to use and achieved significant popularity, despite requiring exposures that were as much as 50 times longer than comparable black and white materials. In 1914, Agfacolor brought significant improvements to the Autochrome process, including the use of colored resin particles that could be applied in a close-packed distribution, which eliminated the need for the optically wasteful charcoal fillers and improved the effective film speed by a factor of six or more.

Though many examples of early additive color photographs have survived, perhaps the most impressive is the collection by Russian chemist Sergei Prokudin-Gorski. In 1905 he proposed and ultimately constructed a spring-operated camera that took three monochrome separations in rapid sequence. Prokudin-Gorski, who also patented prism-based techniques for projecting color slides and color motion pictures, proceeded with the tsar’s patronage to document the Russian Empire from 1909 through 1915 taking nearly 2000 images. This stunning collection, restored digitally, is currently owned by the U.S. Library of Congress.

Additive systems also served as the basis for most of the early color motion picture films. Fortunately, George Eastman, who founded the Eastman Kodak Company in Rochester, New York, in 1880, had largely solved the biggest challenge facing the motion picture pioneers in 1888. He did this by coating photographic emulsions on a flexible and transparent nitrocellulose base and making such materials commercially available. But again, a major difficulty plaguing additive systems was trying to keep the color images in register.

Kinemacolor, marketed in 1906, attacked this problem by using two color records, red-orange and blue-green, on consecutive frames that were photographed and then subsequently projected through a rotating color filter. While the projection speed of 32 frames per second was sufficient to integrate color image through persistence of vision, it was not always fast enough to capture identical images on the paired frames, which resulted in color fringing of the moving image. The Gaumont system (1913) and the similar Opticolor and Roux Color systems solved the motion fringing problem by recording three simultaneous images through three lenses on successive frames, but reintroduced registration issues and required the film to run through the projector at a very high rate of speed.

Biocolor (1912), another two-color system with records on consecutive frames, eliminated the need for a special projector with a filter wheel or device by dyeing alternate frames red-orange and blue-green, which allowed movie theaters to show color films on the same projectors used for black and white. However, this 1912 system suffered from problems in the uniform application of dye.

In 1915, Herbert Kalmus and Daniel Comstock, both graduates from the Massachusetts Institute of Technology, along with machinist W. Burton Wescott, founded the Technicolor Corporation, deriving the name in part from their alma mater. Their first motion picture product, introduced in 1917, was a two-color additive system using a beam splitter with red and blue-green filters to record images on consecutive frames, similar to Kinemacolor. A prism was used in the projector to align the overlap with minimal registration problems. Like many two-color systems, the Technicolor entry could produce believable, though often inaccurate, color because of the limited palette.

In 1928, Kodak introduced the first Kodacolor product, a three-color motion picture system using a film embossed on the back with a fine pattern of cylindrical lenses or lenticules, spaced at about 25 per millimeter. The film was exposed through a lens whose top third was a red filter, the middle third a green filter, and the bottom third a blue filter. The lenticules, which ran parallel to the filters, served to focus a miniature copy of the lens and its corresponding color information on the panchromatic film. After development, the film was projected through a similar lens system to yield the color image.

The Horst process, introduced in 1930, was one of several that placed three additive colors on a single frame, and using an optical system with three projection lenses to reassemble the color image. Not surprisingly, the reduced image size exacerbated the light problem and also yielded grainy images.

Though these systems were certainly ingenious, as technically superior subtractive color systems were invented, additive color largely disappeared from the market. In 1977, however, the Polaroid Corporation, known for its instant color print products, introduced an instant Super-8 motion picture film called Polavision based on the additive model, followed in 1982 by Polachrome, an instant color slide film. In many ways this technology was a sophisticated updating and merger of Joly’s 1873 system and Polaroid’s instant black and white systems. The source of the color was a finely detailed additive grid of alternating red, green, and blue lines, 394 color triplets per centimeter of film width in Polachrome and 590 triplets in Polavision. The slide film could be exposed in an ordinary 35 mm camera and the rewound film cartridge placed in a proprietary processing unit with a package of processing materials, which included a developer, silver complexing agent, and stripper sheet. When the unit was cranked, the chemicals were applied to the stripper sheet which was laminated to the back of the exposed film. Development immobilized the exposed grains as silver metal and the complexing agent caused the remaining ionic silver to migrate to a receiving layer adjacent to the color screen, where additional chemistry converted it into a positive image in black silver metal. After 60 seconds, continued cranking caused the stripper sheet to peel away the layers containing the negative silver image and return it to the reagent package. The technology of the movie film was similar. Because of the inherent light inefficiencies of additive technology, both systems had a relatively low ISO camera speed of 40, and the projected images were darker than those of conventional products.

Though now largely forgotten in silver halide color products, the additive color system has achieved dominance in digital display. There pixels provide red, green, and blue elements that visually fuse into a full color image at normal viewing distance.

Subtractive Color Systems

As early as 1870, the prolific photographic innovator Louis Ducos de Hauron recognized the many advantages of a subtractive color system, but the technologies of the time were not ready to exploit those ideas. Because the three subtractive primaries—cyan, magenta, and yellow—block or modulate only a third of the white light spectrum, separation positives composed from the subtractive primaries can be overlaid and viewed with a single light source. Optimally implemented, the film, not an external filter, now carries the dyes, eliminating many of the problems related to registration and special equipment. Further, because each subtractive primary passes twice as much light as an additive primary, the use of light is far more efficient, both in the capture and in the reproduction stage. Ultimately the subtractive process would become the basis for virtually all color photographic systems; in these systems the three subtractive primary dyes would either be formed in register (e.g., Kodacolor), destroyed in register (e.g., Cibachrome) or diffused in register (e.g., Polacolor instant print film, Technicolor) to form the full color image.

But significant technical hurdles remained. To implement the integral tripack envisioned by de Hauron and others—three emulsions stacked on top of one another—required technology enabling those silver halide layers to selectively analyze the scene for its red, green, and blue content and then turn that latent image into the corresponding subtractive primary dyes of cyan (controlling red), magenta (controlling green), and yellow (controlling blue). Since the additive primaries can in turn be produced by overlaying the subtractives (e.g., white light passed through cyan and yellow filters removes red and blue, leaving green), the path to a full color image is achieved.

The discovery of sensitizing dyes, which could capture the energy of red and green light and transmit it to the silver halide grain to form a latent image, was the key to solving the first challenge. Vogel’s accidental discovery in 1873, when he found that a plate treated with the dye coralline to reduce light scatter or halation also had increased the emulsion’s sensitivity to yellow-green light, led quickly to the investigation of other synthetic and natural dyes, such as chlorophyll, by de Hauron, Cos, Becqueral, Waterhouse, Adolphe Miethe (of Agfa), and others. This yielded a fully panchromatic black and white film by 1904, with the first widely used commercial plates coming from Wratten and Wainwright in 1906.

It was not until the mid-1920s, however, that Brooker and his collaborators in the Kodak Research Laboratories synthesized a systematic series of non-diffusing dyes that could be used to sensitize silver halide in gelatin to any portion of the white light spectrum. Cyanine dyes, consisting of two heterocyclic moieties linked by a variable length conjugated chain, could be tuned to the desired spectral response and would adhere well to the silver halide grain, which efficiently transmitted the energy of the received light. Cyanine dyes continue to be the most widely used sensitizing chemicals and literally thousands have been synthesized over the years.

The second key to creating a modern subtractive color film was a technology for reading back the color information held in the latent images to form the appropriate cyan, magenta, and yellow dyes. In 1912, Rudolph Fischer and colleagues patented a technique that could produce these dyes as a function of silver development by utilizing a new kind of developing agent based on para-phenylenediamines which, when oxidized by the exposed silver halide, produced a species called a quinone diamine. This single oxidized developer could then react with different dye formers, called couplers, to produce the three subtractive primary dyes in proportion to the silver development.

Fischer further anticipated how this could be assembled into a multi-layer color film. He described a film containing a top silver halide layer sensitive to blue light and containing a coupler that could form a yellow dye, a middle layer sensitized to green light and containing a magenta dye-forming coupler, and a bottom layer of red sensitized silver halide containing a cyan dye-forming coupler. Because Fischer knew the red and green sensitized layers would also retain their native sensitivity to blue light, he considered interposing between the blue and green sensitive layers, a layer containing a yellow dye which would filter out any blue light before it reached the layers below. The yellow dye would be destroyed as a function of the process.

Essentially, Fischer described the structure and chemistry of most modern color films. However, he was unable to devise a practical example because he could not find a way to keep the three dye-forming couplers as well as the sensitizing dyes from wandering within or between their respective layers.

Meanwhile, photographic inventors looked for other ways to implement subtractive color systems. In 1914 John Capstaff at the Eastman Kodak Company devised the first film to bear the Kodachrome name. The system used two negatives, one exposed through a green filter and the other through a red filter. After silver development, a bleaching process removed the silver and hardened the gelatin in inverse proportion to the silver image. The film was then dyed in complementary colors—red-orange for the green exposure and blue-green for the red exposure—with the less hardened gelatin absorbing more dye and yielding color positives that could be attached to opposite sides of a piece of glass. The film produced remarkably good flesh tones, though it did less well in outdoor scenes where blue sky and vegetation dominated. In 1916, Capstaff adapted his process to a motion picture film, but World War I interrupted the company’s plan to market the product. It would wait until the early 1930s before a few studio movies would be made using Capstaff’s process.

The first commercially successful subtractive color movie film and the only real competitor to Technicolor in the 1920s was Prizmacolor (successor to a previous additive color film of the same name). Like Capstaff’s Kodachrome it was a duplitized film with a red-orange record on one side and a blue-green one on the other and could be projected without filters or special equipment. With modification, this two-color technology would continue to be used, especially for low-budget films, as late as the 1950s in products like Cinecolor and Trucolor. Although it could give good flesh tones, cinematographers had to be careful that they did not exceed the limited color palette that the system could reproduce.

Technicolor’s first subtractive color film entered the market in 1922. Like some of its immediate predecessors, it used two separate films, an orange-red record and a blue-green one, each half the normal thickness, cemented back to back. The originals had been photographed on consecutive frames of Kodak black and white film stock using a beam splitter. Unfortunately, in the projector the thick duplitized film with gelatin images on both sides was scratched easily and also deformed due to unequal heating of the two sides. Six years later Technicolor would replace this product with its first dye transfer film with red and green images printed on the same side of the film and carrying a silver sound track, considered significantly superior to dye-based sound tracks, to accommodate the increasing popularity of the “talkies.”

Technicolor’s inventors, however, recognized the limitations inherent in any two-color system, and in 1932 they introduced the first version of what would become their flagship product: an ingenious process for creating a three-color subtractive projection film produced from three black and white negatives exposed in a single camera. The camera contained a partially silvered mirror or beam splitter. Light passing through the mirror was directed to a piece of black and white film moving behind a green filter, while light directed to the side reached a magenta filter which passed blue and red light to a bipack composed of an orthochromatic film bonded, emulsion to emulsion, to a panchromatic film. The orthochromatic film, insensitive to red light, recorded only the blue information while the panchromatic film recorded the remaining red light. In later years, Technicolor would add a red-orange filter dye that could be removed during processing to the surface of the blue-sensitive film to keep any stray blue light from reaching the red recording film. The Technicolor camera, designed by William Young, a machinist from Springfield, Illinois, cost about $30,000 and used Kodak film stock with an effective film speed of ISO 5.

To produce the projection positive, each negative was printed onto a special black and white film, called a matrix. After development, the silver image was removed to leave behind a positive relief map of the image in hardened gelatin, which could absorb dye proportionally to image density. The three matrices, which could be reused, were imbibed with the corresponding cyan, magenta, or yellow subtractive primary dyes and then in succession forced under high pressure against a prepared receiver film to transfer the dye and create the full color image. The receiver was a black and white film coated with dye-absorbing compounds called mordants. Prior to the
dye transfer the receiver film was exposed and processed to create the silver sound track, frame lines, and a low density “key image” of the green record, which improved contrast and perceived sharpness of the final print. The process demanded registration tolerances of better than .0001 inch at each step. The new Technicolor process gave dramatically improved color reproduction and was a commercial success, despite its complexity and cost.

Meanwhile, two classical musicians with a long-standing interest in color photography, Leopold Mannes and Leo Godowsky, were exploring the chromogenic methodology initially proposed by Fischer in 1912. The two had created an additive film in 1919, but decided it would not give satisfactory color reproduction. After setting up a home laboratory in New York City in 1922, they attracted the attention of C. E. K. Mees, director of the Eastman Kodak Research Laboratory. In 1930 he invited them to continue their work at the company headquarters in Rochester, New York. Mannes and Godowsky, or “Man and God” as they were nicknamed by some of their co-workers, decided to solve Fischer’s coupler wandering problem by diffusing the dye-formers into the film during the process rather than incorporating them in the emulsion during manufacture. Once the dyes were formed by reaction with oxidized developer, they were sufficiently insoluble so as to remain in the film while the unused coupler was washed out during the process. Researchers at the Kodak Laboratories provided the other piece of the puzzle: sensitizing dyes that adhered to the silver halide grain and did not wander. The final film, which again took the name Kodachrome, had five layers, each between 1 and 3 µ thick, on a celluloid support. From the top, there was a blue-sensitized layer with a yellow filter dye (to make sure no blue light reached the layers below), a gelatin interlayer, a green-sensitized layer, another interlayer, and a red-sensitized layer.

However, the decision to diffuse the three subtractive dye-formers into the film as a function of development created an extremely complicated process involving as many as 28 steps. All versions of the Kodachrome process involved an initial black and white development step. This created a negative image in silver metal, which was removed by a chemical called a bleach and left behind three positive records in unreacted silver halide.

After light re-exposure to turn that into a positive latent image, color developer and a water-soluble cyan dye-former diffused into the film and formed a positive cyan image in all three color records. After washing and drying, a special bleaching solution was carefully diffused into only the upper blue and green records, where it oxidized the silver back to silver bromide and decolorized the dye. The depth of penetration was controlled by the bleach viscosity and time and temperature profile of the step. Since the red layer no longer contained any silver halide, the process could be repeated now for the green and blue layers, but using a developer containing a magenta dye-forming coupler. Again, the selective bleaching was carried out, this time only in the blue layer. Finally, the blue was developed to form yellow, and all the remaining silver metal was bleached and fixed out of the film.

The new Kodachrome was originally marketed as a 16 mm home movie film, but it was quickly adapted into professional versions, including one sensitized for artificial lighting and also in an 8 mm format. By 1936, Kodak was selling the product in 35 mm cartridges and other formats for still photographers, ushering in a new era of color photography among amateurs and professionals.

By 1939, Kodak had significantly modified the Kodachrome process to eliminate the touchy differential bleaching steps, replacing them with selective re-exposure, made possible by the invention of sensitizing dyes that would remain on the grain during most of the process and reduce the number of processing steps to 18. Following the black and white development and bleaching steps, the film was re-exposed through its base with red light. Treatment with a developer containing a cyan dye-forming coupler resulted in silver development and a dye image only in the red record. Re-exposure from the top with blue light and development with a yellow dye-forming developer created the blue record. The green record remaining in the middle was re-exposed, initially with just white light, later with a chemical fogging agent, and then processed with a developer containing a magenta dye-forming coupler. The film was then fully silver bleached and fixed to produce a dye-only color image. Over the years, color reversal films from companies like Ilford, Dynacolor/3M/Ferrania, and Sakura/Konishiroku would adopt this process. Ultimately, the Kodachrome process would be shortened from its original 3.5 hours to a 6-minute time while the film speed would be increased by more than 20-fold.

Meanwhile, Wilhelm Schneider and Gustav Wilmanns of Agfa in Germany were pursuing a film design closer to Rudolph Fischer’s original vision. Agfacolor Neue, introduced in 1936, used couplers bearing a long hydrocarbon chain, called a ballast, terminating in a carboxylic or sulfonic acid group. The ballast served to immobilize the couplers in their corresponding red-, green-, and blue-sensitized layers while the acid groups enabled them to form micelles that could be dispersed in the aqueous gelatin as well as loosely tether the couplers to the gelatin chains. This vastly simplified the processing to the extent that amateur photographers could do it in a home darkroom. After the black and white development step, the film went into a para-phenylenediamine color developer to produce the full color image in a single step, followed by bleaching, fixing, and washing to remove the silver. In 1942, with the outbreak of World War II, the American company Ansco introduced a near identical film sold in the United States, France, and other Allied countries.

The advantages of what became known as incorporated coupler processes were hardly lost on Eastman Kodak. In fact, several early patents of Mannes and Godowsky included claims for such films. But there were advantages to the Kodachrome technology as well. Because the film layers did not have to be thick enough to hold coupler in addition to the silver halide, they could be much thinner, which reduced optical scatter and improved sharpness. Further, because incorporated couplers were in the film before processing and remained there for the life of the processed image, any coupler instability could result in problems. Kodachrome did not have that issue.

Nonetheless, in late 1941 when Kodak introduced Kodacolor print film, a film that produced negatives in complementary colors to be subsequently printed as positives onto reflective photosensitive paper, it had turned to incorporated coupler technology with its much simpler process. The method used to anchor the dye-formers in Kodacolor was different from Agfa’s technology, using much shorter carbon chains with no ionic solubilization. These couplers were suspended in the gelatin layer in small droplets of a water-immiscible, high-boiling organic liquid termed a “coupler solvent” (though it really acted more like a plasticizer). Initially this approach was used to avoid the Agfa patents, but it was quickly realized that these couplers were more easily manufactured and purified and afforded greater opportunity for “tuning” photographic parameters. Ultimately, even Agfa, as well as other film companies, would adopt this technology.

With time, the structure of what was called an “integral tri-pack” would become more complicated, incorporating as many as 17 layers. In many films, each of the three color-recording layers would be split into two or three layers with a more light-sensitive emulsion on top followed by less sensitive layers below. This provided technology for both increased latitude and reduced granularity. A filter layer above the blue-sensitive pack removed ultraviolet light that would otherwise be recorded by the blue record. Additionally, an anti-halation layer, designed to reduced sharpness degradation resulting from backscattered light, was coated under the bottom layer or on the back of the film.

The negatives from Kodacolor print film and similar products could be converted to paper-based prints by exposing the negative onto a piece of reflective paper coated with three photosensitized layers likewise using the incorporated coupler technology and simple process. Because the printing was done with three separate primary color exposures, significant control over the tone and color of the print was possible for both quality improvements and artistic creativity. Though the prints from what became known as the “color neg-pos process” were initially expensive, they would steadily drop in price over the next 50 years, making this the most popular photographic system in history.

It was not until 1946 that Kodak adapted its incorporated coupler technology to a color slide film, Ektachrome, whose much simpler process, with its single color development step, permitted processing in the home darkroom. By 1975 this became the E-6 process and the standard for nearly all color reversal films marketed world-wide. It consisted of a black and white reversal step followed by chemical treatment to form a positive latent image on the remaining silver halide. Single-step color development was followed by treatment with an oxidizing agent, called a bleach, which converted the uniform silver deposit into silver ion for complexation and removal by the thiosulfate fixing agent.

By 1950, with further invention, the Kodacolor technology was incorporated into a family of motion picture films called Eastman Color, including negative, print, and intermediary films for duplicating. In addition to breaking the near-monopoly Technicolor had on movie print films, the new process was cheaper and faster, though it did not provide Technicolor’s highly stable silver originals for archival purposes.

The multiple generations of film involved in the color neg-pos process (as many as four in the motion picture chain) served, however, to emphasize the colorimetric imperfections of the subtractive primary dyes. Ideally, a cyan dye, for example, should control only red light by absorbing between 600 and 700 nm. But virtually all cyan dyes also had significant unwanted absorptions in the blue-green. This led to desaturated or “muddy” colors, especially in successive generations of film. The ingenious solution to this problem was a technology called “integral color masking,” invented by W. T. “Bunny” Hanson of Kodak and introduced in Ektacolor films in 1949. The technique added “colored couplers,” which bore an attached pre-formed azo dye, to the normal colorless coupler in the layer. For example, the added cyan dye-forming colored coupler carried a blue-green dye that would be released and washed out of the film to the extent that cyan dye with its unwanted blue-green absorption was formed. The result was equivalent to a “perfect” cyan image dye overlaid with a uniform density to blue green. While this gave the negative an orange cast, it required only a longer cyan exposure in making the positive print. So revolutionary was this improvement that virtually all negative films would adopt this technology once it was free of patent restrictions.

Trends during the second half of the 20th century as color film technology matured involved continuous improvement of the image structure (grain and sharpness), color, and speed of films to enlarge what was called “photospace coverage” or the breadth of conditions under which a picture could be taken while maintaining or even improving image quality. Fuji Photofilm of Japan would introduce an ISO 400 speed color negative film in 1976 and soon thereafter film speeds from the major companies would increase to over ISO 1000. This was coupled with the increasing popularity of 35 mm cameras, the introduction of the Instamatic drop-in cartridge in 1963, and the even smaller format 110 (13 × 17 mm frame) and novel disc (8 × 11 mm frame) films in 1972 and 1983, respectively. These smaller form factors placed greater demands on the film to gather and manipulate information, which led researchers to two major breakthroughs in film technology: high aspect ratio or tabular grains (called T-grains by Kodak) and image modifying chemistries.

The information content of a film is determined at exposure. That information can be partitioned among the various display attributes (grain, sharpness, contrast) or lost, but it cannot be increased. Tabular grains replace the near-cubic shape of traditional silver halide with flat, generally hexagonal crystals of about 0.1 mm thickness having a much higher surface to volume ratio. These grains not only gather far more information per unit mass, but, because they tend to align themselves nearly parallel to the film support during coating, they can significantly reduce optical scatter and improve sharpness. T-grains were first used in a 1000 speed Kodacolor film in 1982.

Image modifying chemistries provided the tools to take that additional information stored in the latent image and selectively distribute it as appropriate for the film’s intended application. Foremost among the image modifiers was developer inhibitor releasing (DIR) couplers which, on dye formation, released a silver development inhibitor (usually a sulfur or nitrogen heterocycle that could complex with the silver halide surface) that provided a kind of chemical negative feedback loop in the information transfer process. Depending on the ultimate spatial distribution of the inhibitor, it could perform several tasks. By allowing each silver grain to develop only partially, the inhibitor could force the system to use more silver grains or information-carrying centers to produce the dye image, increasing the signal to noise ratio of the system, a result manifested in reduced granularity or the perceived non-uniformity of the dye image. Another way to achieve a similar effect is by using “competing couplers” that compete with the image-forming coupler for the oxidized developer but produce a soluble or colorless “dye” product, again forcing more silver halide centers to contribute to the image.

Released inhibitor that moved between layers could be used to provide correction for unwanted dye absorption, similar to colored couplers, but via a different mechanism. For example, inhibitor released from a DIR coupler in the cyan layer could diminish development of density in the adjacent magenta layer to compensate for the unwanted green absorption of the cyan dye. This is referred to as an interlayer interimage effect.

Finally, released inhibitor traveling laterally in the layer can be used to create a concentration gradient that would slightly suppress dye formation in the center of the image area while increasing the density change at the edge. This “edge effect” gives the visual impression of improved sharpness. DIR couplers would find their first application in a motion picture internegative film in the late 1960s and in the introduction of Kodacolor II film in 1972. Controlling the diffusion of the inhibitor between and within the layers to achieve the desired image-modifying effects was hindered by its mechanism of operation—adherence to the silver halide grain. This led to an extension of the technology, delayed release DIR couplers, which released an inhibitor precursor that could diffuse in a controlled fashion before releasing the active inhibitor.

Although the disc film in 1983 was the first product to incorporate magnetic information encoding, it was the Advanced Photo System (APS) technology, introduced in 1996, that brought this concept to fruition. A joint development by Kodak, Fuji, Canon, Minolta, and Nikon, APS combined the advantages of traditional silver halide with a recording of digital information on a transparent magnetic coating that covered the film surface and on a data disc on the end of the film canister. The Information Exchange or IX tracks included data relating to selected picture format (classic, HDTV, or panoramic), date, title, print copies desired, shutter speed, aperture, focal length information, film speed, exposure compensation, and what was called Print Quality Information, such as flash use, subject brightness, backlighting status, and other scene information dependent on camera sophistication. These data could be used during the processing and printing steps to both automate the system as well as dramatically improve print quality. The system also allowed detailed backprinting on prints and production of a keyed index print. Additional tracks were reserved for future information acquisition.

The APS system featured an autoload cartridge in which the developed negatives were returned to the customer to both protect and codify the images, as well as allow mid-roll film change. Negative size, on a new, stronger polyethylene naphthalene (PEN) film base with less “memory” for curl, was 30 × 17 mm, midway between traditional 35 mm and the 110 format, and permitted the design of smaller, thinner cameras as well as an effective increase in film speed by focusing the same amount of light on a smaller surface. Advances in both emulsion and image-modifying technologies since the disc product allowed the smaller format to produce pleasing pictures up to 8 × 10 inches. Despite these advances, with the concurrent emergence of highly automated 35 mm cameras and non-film digital capture technologies, the APS system did not meet with the market acceptance its sponsors had anticipated.

Despite the inroads of digital capture, about 2.5 billion rolls of film and over 400 million single use cameras were sold worldwide in 2004, yielding almost 90 billion prints, according to research by the Photo Marketing Association. Some 656 million film units were sold in the United States with over 22 billion prints produced.

Color Spaces, Color Encodings, and Color Image Encodings - Color Spaces, Color space encodings, Color image encodings, Digital color image workflow, Raw sensor response coordinates [next] [back] Color Measurement - Introduction, Historical Perspective, Definitions and Terminology, Components of a Spectrophotometer, Light Source, Detector, Dispersing Element

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