Making use of this theory, Maxwell commissioned photographer Thomas Sutton to produce a color image. Sutton made four — not three as commonly believed — individual black-and-white negatives of a tartan plaid ribbon through separate blue-violet, green, red, and yellow-colored fluids (filters).¹ Black-and-white positives were made from each negative. These positives, except for the yellow, were projected in register (all images perfectly aligned) onto a white screen from separate apparatuses, called lantern projectors or magic lanterns, with each slide conveyed through the same colored filter used to make the original negative. For example, the positive photographed through the green filter was projected through the same green filter. When all three positives were simultaneously superimposed on a screen, the result was a projected color image (not a photograph) of the multicolored ribbon. Although no practical results came until German photochemist Hermann Wilhelm Vogel was able to make emulsions more color-sensitive through the use of dyes, Maxwell’s demonstration proved the additive color theory and offered a practical projection process for producing photographic color images. [1|2|5|1449]

Later scientific investigation revealed that the early photographic emulsions Sutton used for Maxwell’s experiment were not capable of recording the full visible spectrum. Neither orthochromatic (sensitive to all colors except red and deep orange) nor panchromatic (sensitive to red, green, blue, and ultraviolet) emulsions had been invented. The test should have failed since the emulsion used was not sensitive to red and only slightly sensitive to green. Scientists took a century to figure out why Maxwell’s experiment worked with an emulsion that was not sensitive to all the primary colors. It is now thought that Maxwell’s method succeeded because of two other deficiencies in the materials that canceled out the effect of the nonsensitive emulsion: (1) the red dye of the ribbon reflected ultraviolet light that was recorded on the red negative, and (2) his green filter was faulty and let some blue light strike the plate. Both of these defects corrected for the lack of sensitivity of the emulsion to red and green light. It is also thought that Sutton made the fourth yellow exposure to help compensate for the green filter, allowing more blue light to reach the plate. Regardless, when done with contemporary panchromatic emulsions <Maxwell’s research proves to be theoretically sound and provides the basis for how digital sensors electronically capture color images. [1|2|5|1450]

Isaac Newton observed that colors could be produced by interference when a very thin film of air or liquid separates two glass plates. If a slightly convex surface of glass is placed on a flat surface, a thin film around the point of contact will produce colored circles known as Newton’s rings. Also, the colors in certain beetles, birds, and butterflies, as well as tints of mother-of-pearl and soap bubbles, are the result of the interference phenomena and not due to actual pigments. Another common example can be seen when gasoline or oil sits on a wet road. [1|2|6|1451]

In 1891 the French physicist Gabriel Lippmann introduced a direct color process based on wavelength interference principles that did not use colorants, dyes, or pigments, which gives the process excellent archival properties. Lippmann made color photographs by using a panchromatic emulsion and a mercury mirror that reflected waves of light, in a manner similar to how color is produced on oil slicks. In the camera, the emulsion plate was placed in contact with a mirror of liquid mercury, facing away from the lens. Light traveled through the plate and was reflected by the mercury, producing a latent image of the interference pattern on the plate. The plate was developed and the faint color image could be viewed by illuminating the plate with diffuse light from the same side that the viewer is positioned. The light is reflected off the surface (emulsion-side) of the plate. In order to separate the directly reflected light from the light reflected from the recorded fringes in the emulsion, a glass wedge or prism was cemented on top of the emulsion. Although the colors could be surprisingly true, the process was impractical for general commercial use because it required scientific precision, extremely long exposure times, and complex viewing methods. This process, for which Lippmann won the Nobel Prize, is considered to be a cornerstone for the future development of holography. [1|2|6|1452]Additional examples:
 
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In 1869 Louis Ducos du Hauron, a French scientist, published Les Couleurs en photographie – solution du problème, which anticipated many of the theoretical frameworks for making analog color photographs. Among his proposed solutions was one in which the additive theory could be applied in a manner that did not require the complicated separation process that Maxwell devised. Ducos Du Hauron speculated that a screen ruled with fine lines in the three primary colors would act as a filter to produce a color photograph with a single exposure instead of the theoretical three needed in Maxwell’s experiment ). At the time, he photographed each scene through green, orange, and violet filters (at the time considered the primary colors of light), then printed his three exposures on thin sheets of bichromated gelatin containing carbon pigments of red, blue, and yellow— the complementary colors. When the three positives (transparencies) were superimposed, a full-color photograph resulted. Simultaneously, Charles Cros independently demonstrated how color images could be made using three-color separation negatives/positives, confirming the path that could be followed for a practical color process to evolve. [1|2|7|1453]Additional examples:
 
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To simplify the process, in the first decade of the twentieth century, companies such as Sanger Shepherd in England began to manufacture “Repeating Back” attachments, which allowed the photographer to make separate exposures of a static subject – each time through a different colored filter – that were later combined to form a single color image. This was followed by “One-Shot” cameras that used black-and-white plates to simultaneously make three exposures of the same subject through three separate color filters. This method was also employed for the Lumiere Brothers Trichromie (a.k.a. Trichrome) process. Improvements followed, and these triple-exposure cameras were used for advertising and portrait work until the advent of multi-layered films like Kodachrome, Agfacolor, and Kodacolor. [1|2|7|1454]

In 1894 John Joly, a Dublin physicist, patented the first linescreen process for additive color photographs, based on Louis Ducos du Hauron’s concept. In this process, a glass screen with transparent ruled lines of red, green, and blue, about two hundred lines per inch, was placed against the emulsion of an orthochromatic (not sensitive to red light) plate. The exposure was made and the screen removed. The plate was processed and contact printed on another plate to make a positive black-andwhite transparency which was placed in exact register with the same screen used to make the exposure. The final result was a limited-color photographic transparency that was viewed by transmitted light. Introduced in 1896 as the Joly Color process, this method enjoyed only a brief success. It was expensive and the available emulsions still were not sensitive to the full range of the spectrum, thus the final image was not able to achieve the look of “natural” color. However, Joly’s work indicated that the additive screen process had the potential to become a commercially viable way of making color photographs. [1|2|8|1455]Additional examples:
 
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In Lyons, France, Auguste and Louis Lumière, the inventors of the first practical motion picture projector, patented a major breakthrough in the making of color photographs in 1904. The Autochrome Lumière was the first commercially viable and extensively used color photographic process. Introduced to the market in 1907, it remained in production until 1935. Autochrome was a reversal process, which produced one unique image—a positive transparency on a glass support that was viewed by projection or through a transmitted light source. [1|2|9|1456]Additional examples:
 
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An Autochrome plate was produced as follows: (1) a glass support was covered with an initial layer of varnish that remained tacky; (2) the color screen layer, composed of potato starch grains dyed orange-red, green, and violet-blue, was dusted onto the sticky varnish; (3) a fine carbon black powder was used to fill remaining gaps between the grains, and the layers were then pressed flat in a rolling press; (4) a second varnish was applied to protect the starch grains from moisture; (5) a photosensitive layer of silver gelatin emulsion was then applied ; panchromatic emulsion, which greatly extended the accuracy of recording the full range of the visible spectrum, became the emulsion of choice once it became commercially available in 1906; (6) after exposure and processing, the manufacturer recommended a final coat of varnish to further protect the plate before a cover glass was applied to preserve the entire color image. [1|2|9|1457]Additional examples:
 
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To maintain a proper color balance, a deep yellow filter was placed in front of the camera lens. The exposure was made with the plate’s filter layer pointed toward the lens, so that the dyed potato starch acted as tiny filters. After development, the plate was re-exposed to light, and finally redeveloped to form a positive transparency made up of tiny dots of the primary colors. The Autochrome was a pioneering method of utilizing the principles articulated by Ducos du Hauron and Charles Cros in which the eye mixed the colors, in a fashion much like George Seurat’s pointillist painting Sunday Afternoon on the Island of La Grande Jatte (1884 – 1886), to make a color-positive image. Alfred Stieglitz sang its praises in Camera Work, Number 20, October 1907: “Color photography is an accomplished fact. The seemingly everlasting question whether color would ever be within the reach of the photographer has been definitely answered … The possibilities of the process seem to be unlimited … In short, soon the world will be color-mad, and Lumière will be responsible.” [1|2|9|1458]

Used from 1907 to 1935, Autochrome did have its limitations. Since the light had to travel through the potato starch grains and the yellow filter on the front of the lens, exposure times were much longer than with the black-and-white films of the day. In the additive processes, it was not uncommon for 75 percent or more of the light to be absorbed by this combination of filters before reaching the emulsion. Suggested starting exposure time was between 1/5 of a second and 1 second at f/4 in direct sunlight at midday in the summer and six times longer on a cloudy day, although over time there were many suggestions on how to increase its sensitivity. The randomly applied potato grains tended to bunch up, creating blobs of color. Also, Autochromes that were not lantern projected could be difficult to see and were sometimes placed in specially designed viewers called diascopes. When they were regularly marketed in New York (ca. 1910), a box of four 3 1/4 × 4-inch plates cost $1.20 and a box of 7 × 14-inch plates sold for $7.50, making them pricey for an average working person. [1|2|9|1459]Additional examples:
 
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The advantages of this process, however, were numerous. Autochromes could be used in any regular plate camera with the addition of a special yellow-orange filter; the image was made in one exposure, not in three; although pricey, the cost was not overly prohibitive; it gave serious amateurs much easier access to color; and while the colors were not accurate by today’s standards, they did produce a warm, soft, and inviting pastel image that people considered to be quite pleasing. [1|2|9|1460]Additional examples:
 
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World War I was the first major conflict to be covered by color photography. Autochromes became the basis for publications such as L’Histoire illustrée de la guerre de 1914. By the end of World War I, magazines such as National Geographic were using Autochromes to make color reproductions for the first time in their publications. Between 1914 and 1938, National Geographic published a reported 2,355 Autochromes, more than any other journal, thus taking a leadership role in bringing the “realism” of color photography into mass circulation. Autochrome was the first color process to get beyond the novelty stage and to become successful in the market place. It cracked a major aesthetic barrier because it was taken seriously for its picture-making potentialities. This enabled photographers to begin to explore the visual possibilities of making meaningful color photographs for ambitious projects such as Albert Kahn’s Archives of the Planet . Between 1909 and 1931, the French banker financed photographic teams who visited over 50 countries and amassed some 72,000 Autochrome plates that documented the diversity of the human condition in color, not just as ethnography or reportage but also for the purpose of inspiring education and universal peace. ² [1|2|9|1461]Additional examples:
 
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Other additive screen processes followed on the heels of Autochrome. In 1906 Clare L. Finlay of England patented a process that was introduced in 1908 as the Thames Colour Screen. Made up in a precise checkerboard fashion of red, green, and blue elements, rather than the random mosaic pattern used in Autochrome, this separate screen could be used with any type of panchromatic film or plate to make a color photograph. The Thames Colour Plate, which combined an integral screen with the emulsion to form a single plate, was released in 1909. Both of these processes were abandoned after World War I, but improved versions were marketed under the name of Finlay Colour in 1929 and 1931. The Finlay Colour processes were to be the major rivals to Dufaycolor until the introduction of subtractive-process materials in the mid-1930s. [1|2|10|1462]Additional examples:
 
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In 1912 G.S. Whitfield also obtained a patent for screenplates that were marketed by the Paget Dry Plate Company. Renamed Duplex in 1920, the process was discontinued few years later. [1|2|10|1463]

From about 1909 to 1914, the French firm of Louis Dufay made the Dioptichrome Plate to compete with the Autochrome. Although withdrawn from production, the process later was improved and renamed Dufaycolor when it was introduced as a ciné film in 1932. Soon produced as both cut-sheet and roll film, thus making it simple to use, Dufaycolor gave wider public access to color photography. The process became popular, especially in the UK, because it was faster (more light sensitive) than Autochrome and did not require any additional optical device to form a viewable image. Also, some people preferred the structure of its screen, which was a mosaic of alternating blue-dye and green-dye squares that were crossed at right angles by a pattern of parallel red-dye lines. This design offered greater color accuracy and a faster emulsion; by the mid-1930s exposures of f/8 at 1/25 of a second on a sunny day were possible, but the projected image was subdued and flawed by the conspicuous mosaic pattern. Dufaycolor was marketed until the late 1940s. By this time, the quest for an easier-to-use process that would provide more realistic and natural colors brought about technical discoveries that would eventually make the additive screen processes commercially obsolete. However, various film-based versions of Autochrome, including Filmcolor (1931), Lumicolor (1933), and Atlicolor (1952), were made into the mid-1950s. [1|2|11|1464]

Between 1983 and 2002 Polaroid (see Diffusion-Transfer Process later in this chapter) marketed Polachrome, an instant 35mm color slide film that could be used in any 35mm camera, which was based on additive-color, line-screen, positive-transparency film similar to Joly Color. The film was inserted with an individual chemical pod into a small tabletop Polaroid AutoProcessor for processing. Although it was convenient, it never achieved widespread use due to issues of color accuracy and because the line-screen became highly visible when the image was projected or enlarged. [1|2|12|1465]

Today the additive method is occasionally employed in color printmaking. It is in limited use because additive enlarger systems are more complex and expensive. To make a full color print with the additive process, the enlarger is used to make three separate exposures, one through each of the three primary-colored filters. The blue filter is used first and controls the amount of blue in the print, next the green filter is used for the green content, and last is the red filter. Some people prefer this additive technique, also known as tricolor printing, because it is relatively easy to make adjustments in the filter pack, with each filter controlling its own color. [1|2|13|1466]

A major change in additive color printing is the use of digital enlargers in professional photography labs that continue to make chemical prints on silver halide paper (light-sensitive compounds in paper and film). Instead of the older bulb-based printers, this combination of chemical-based color printing and digital imaging relies on rapid bursts of laser light in three colors — red, green, and blue — to expose the photographic paper. The laser light is bounced off a rotating six-sided mirror that reflects the light dots onto the paper. As the mirror turns, it draws a line across the paper in light, making extremely sharp images. These digital enlargers have the additional advantage of high-speed scanners and the ability to work from a digital file, which allows each image to be analyzed by software that adjusts color, contrast, and exposure as needed. It also has facial recognition software for smoothing facial features so that not every skin pore is sharply apparent. Other systems, such as Durst Lambda and LightJet, use LED light printers instead of lasers. [1|2|14|1467]

The additive system is the ideal vehicle for color television since the set creates and then emits the light-forming picture. The cathode ray tube (CRT) in color television has three electron guns, each corresponding to one of the additive primaries. These guns stimulate red, green, or blue phosphors on the screen to create different combinations of the three primaries. This creates all the colors that form the images we see on the television set. Liquid crystal display (LCD) and plasma (a type of gas) flat-screen televisions use the same RGB concept to have pixels (picture element) form the color image. A typical 8-bit LCD or plasma television utilizes a 256 red × 256 green × 256 blue subpixel depth to deliver 16,777,216 color combinations. [1|2|15|1468]