thumb|The first color photograph made by the three-color method suggested by [[James Clerk Maxwell in 1855, taken in 1861 by Thomas Sutton. The subject is a ribbon featuring a tartan pattern.]]
Color photography (also spelled as colour photography in Commonwealth English) is photography that uses media capable of capturing and reproducing colors. By contrast, black-and-white or gray-monochrome photography records only a single channel of luminance (brightness) and uses media capable only of showing shades of gray.
In color photography, electronic sensors or light-sensitive chemicals record color information at the time of exposure. This is usually done by analyzing the spectrum of colors into three channels of information, one dominated by red, another by green and the third by blue, in imitation of the way the normal human eye senses color. The recorded information is then used to reproduce the original colors by mixing various proportions of red, green and blue light (RGB color, used by video displays, digital projectors and some historical photographic processes), or by using dyes or pigments to remove various proportions of the red, green and blue which are present in white light (CMY color, used for prints on paper and transparencies on film).
Monochrome images which have been "colorized" by tinting selected areas by hand or mechanically or with the aid of a computer are "colored photographs", not "color photographs". Their colors are not dependent on the actual colors of the objects photographed and may be inaccurate.
The foundation of all practical color processes, the three-color method was first suggested in an 1855 paper by Scottish physicist James Clerk Maxwell, with the first color photograph produced by Thomas Sutton for a Maxwell lecture in 1861. Color photography has been the dominant form of photography since the 1970s, with monochrome photography mostly relegated to niche markets such as fine art photography.
History
Early experiments
thumb|A circa 1850 "Hillotype" photograph of a colored engraving. Testing in 2007 found that [[Levi Hill's process did reproduce some color photographically, but also that many specimens had been "sweetened" by the addition of hand-applied colors.]]
Color photography was attempted beginning in the 1840s. Early experiments were directed at finding a "chameleon substance" which would assume the color of the light falling on it. Some early results, typically obtained by projecting a solar spectrum directly onto the sensitive surface, seemed to promise eventual success, but the comparatively dim image formed in a camera required exposures lasting for hours or even days. The quality and range of the color was sometimes limited mainly to primary colors, as in the chemically complicated "Hillotype" process invented by American daguerreotypist Levi Hill around 1850. Other experimenters, such as Edmond Becquerel, achieved better results but could find no way to prevent the colors from quickly fading when the images were exposed to light for viewing. Over the following decades experimentation continued without practical results.
Three-color processes
The three-color method, the foundation of most color processes, chemical or electronic, was first suggested in an 1855 paper on color vision by Scottish physicist James Clerk Maxwell.
The method is based on the Young–Helmholtz theory, which states that the human eye sees color using millions of intermingled cone cells of three types on its inner surface. According to the theory, one type of cone is most sensitive to the end of the spectrum called "red", another is sensitive to the middle or "green" region, and the third is sensitive to the "blue" region. The named colors are arbitrary divisions imposed on the continuous spectrum of visible light and the theory is not an entirely accurate description of cone sensitivity. The simple description of these three colors coincides enough with the sensations experienced by the eye that when these three colors are used the three cones types are adequately and unequally stimulated to form the illusion of various intermediate wavelengths of light.
In his studies of color vision, Maxwell showed, by using a rotating disk with which he could alter the proportions, that any visible hue or gray tone could be made by mixing only three pure colors of light – red, green and blue – in proportions that would stimulate the three types of cells to the same degrees under particular lighting conditions. To emphasize that each type of cell by itself did not actually see color but was simply more or less stimulated, he drew an analogy to black-and-white photography: if three colorless photographs of the same scene were taken through red, green and blue filters, and transparencies ("slides") made from them were projected through the same filters and superimposed on a screen, the result would be an image reproducing not only red, green and blue, but all of the colors in the original scene.
The first color photograph made according to Maxwell's prescription, a set of three monochrome "color separations", was taken by Thomas Sutton in 1861 for use in illustrating a lecture on color by Maxwell, where it was shown in color by the triple projection method. The test subject was a bow made of ribbon with stripes of various colors, apparently including red and green. During the lecture, which was about physics and physiology, not photography, Maxwell commented on the inadequacy of the results and the need for a photographic material more sensitive to red and green light. A century later, historians were mystified by the reproduction of any red at all, because the photographic process used by Sutton was for all practical purposes totally insensitive to red light and only marginally sensitive to green. In 1961, researchers found that many red dyes also reflect ultraviolet light, coincidentally transmitted by Sutton's red filter, and surmised that the three images were probably due to ultra-violet, blue-green and blue wavelengths, rather than to red, green and blue.
Additive color
Creating colors by mixing colored lights (usually red, green and blue) in various proportions is the additive method of color reproduction. Liquid-crystal display (LCD), LED, plasma, and CRT (picture tube) color video displays all use this method. If one of these displays is examined with a sufficiently strong magnifier, it will be seen that each pixel is actually composed of red, green and blue sub-pixels which blend at normal viewing distances, reproducing a wide range of colors as well as white and shades of gray. This is also known as the RGB color model.
Subtractive color
The same three images taken through red, green and blue filters which are used for additive color synthesis may also be used to produce color prints and transparencies by the subtractive method, in which colors are subtracted from white light by dyes or pigments. In photography, the dye colors are normally cyan, a greenish-blue which absorbs red; magenta, a purplish-pink which absorbs green; and yellow, which absorbs blue. The red-filtered image is used to create a cyan dye image, the green-filtered image to create a magenta dye image, and the blue-filtered image to create a yellow dye image. When the three dye images are superimposed they form a complete color image.
This is also known as the CMYK color model. The "K" is a black component normally added in ink-jet and other mechanical printing processes to compensate for the imperfections of the colored inks used, which ideally should absorb or transmit various parts of the spectrum but not reflect any color, and to improve image definition.
thumb|Color print made by [[Louis Ducos du Hauron from three direct photograms, 1869 or 1870. A presentation copy of this bears an early (February?) 1870 date on the mat in DdH's handwriting. This may refer to the print date only, with the original photography possibly occurring in 1869.]]
At first it may seem that each image ought to be printed in the color of the filter used in making it, but by following any given color through the process the reason for printing in complementary colors should become apparent. A red object, for example, will be very pale in the red-filtered image but very dark in the other two images, so the result will be an area with just a trace of cyan, absorbing just a bit of red light, but a large amount of magenta and yellow, which together absorb most of the green and blue light, leaving mainly red light to be reflected back from the white paper in the case of a print, or transmitted through a clear support in the case of a transparency.
Before the technical innovations of the years 1935 to 1942, the only way to create a subtractive full-color print or transparency was by means of one of several labor-intensive and time-consuming procedures. Most commonly, three pigment images were first created separately by the so-called carbon process and then carefully combined in register. Sometimes, related processes were used to make three gelatin matrices which were dyed and assembled or used to transfer the three dye images into a single layer of gelatin coated on a final support. Chemical toning could be used to convert three black-and-white silver images into cyan, magenta and yellow images which were then assembled. In a few processes, the three images were created one on top of another by repeated coating or re-sensitizing, negative registration, exposure and development operations. A number of variations were devised and marketed during the first half of the 20th century, some of them short-lived, others, such as the Trichrome Carbro process, enduring for several decades. Because some of these processes allow very stable and light-fast coloring matter to be used, yielding images which can remain virtually unchanged for centuries, they are still not quite completely extinct.
thumb|An 1877 color photographic print on paper by [[Louis Ducos du Hauron, the foremost early French pioneer of color photography. The overlapping yellow, cyan and red subtractive color elements are apparent.]]
The production of photographic three-color prints on paper was pioneered by Louis Ducos du Hauron, whose comprehensive 1868 French patent also included the basic concepts of most of the color photographic processes which were subsequently developed. For making the three color-filtered negatives required, he was able to develop materials and methods which were not as completely blind to red and green light as those used by Thomas Sutton in 1861, but they were still very insensitive to those colors. Exposure times were impractically long, the red or orange-filtered negative requiring hours of exposure in the camera. His earliest surviving color prints are "sun prints" of pressed flowers and leaves, each of the three negatives having been made without a camera by exposing the light-sensitive surface to direct sunlight passing first through a color filter and then through the vegetation. His first attempts were based on the red-yellow-blue colors then used for pigments, with no color reversal. Later he used the primary colors of light with color reversal.
Color sensitization
As long as photographic materials were usefully sensitive only to blue-green, blue, violet and ultraviolet, three-color photography could never be practical. In 1873 German chemist Hermann Wilhelm Vogel discovered that the addition of small amounts of certain aniline dyes to a photographic emulsion could add sensitivity to colors which the dyes absorbed. He identified dyes which variously sensitized for all the previously ineffective colors except true red, to which only a marginal trace of sensitivity could be added. In the following year, Edmond Becquerel discovered that chlorophyll was a good sensitizer for red. Although it would be many more years before these sensitizers (and better ones developed later) found much use beyond scientific applications such as spectrography, they were quickly and eagerly adopted by Louis Ducos du Hauron, Charles Cros and other color photography pioneers. Exposure times for the "problem" colors could now be reduced from hours to minutes. As ever-more-sensitive gelatin emulsions replaced the old wet and dry collodion processes, the minutes became seconds. New sensitizing dyes introduced early in the 20th century eventually made so-called "instantaneous" color exposures possible.
Color cameras
thumb|A 1909 color photograph by [[Sergey Prokudin-Gorsky, documenting Russian peasant girls in a rural area along the Sheksna River near Kirillov]]
thumb|A 1937 color photograph of the [[Royal Palace of Amsterdam by Bernard F. Eilers using color separations and changing three filters.]]
Making color separations by reloading the camera and changing the filter between exposures was inconvenient, added delays to the already long exposure times and could result in the camera being accidentally shifted out of position. To improve the actual picture-taking, a number of experimenters designed one or more special cameras for color photography. They were usually of two main types.
The first type used a system of partially reflecting surfaces to divide the light coming through the lens into three parts, each part passing through a different color filter and forming a separate image, so that the three images could be photographed at the same time on three plates (flexible film had not yet replaced glass plates as the support for the emulsion) or different areas of one plate. Later known as "one-shot" cameras, refined versions continued to be used as late as the 1950s for special purposes such as commercial photography for publication, in which a set of color separations was ultimately required in order to prepare printing plates.
The second type, known variously as a multiple back, repeating back or drop back camera, still exposed the images one at a time but used a sliding holder for the filters and plates which allowed each filter and the corresponding unexposed area of emulsion to be quickly shifted into place. German photochemistry professor Adolf Miethe designed a high-quality camera of this type which was commercially introduced by Bermpohl in 1903. It was probably this Miethe-Bermpohl camera which was used by Miethe's pupil Sergei Mikhailovich Prokudin-Gorskii to make his now-celebrated color photographic surveys of Russia before the 1917 revolution. One sophisticated variant, patented by Frederic Eugene Ives in 1897, was driven by clockwork and could be adjusted to automatically make each of the exposures for a different length of time according to the particular color sensitivities of the emulsion being used.
Otherwise, simple cameras with multiple color-filtered lenses were sometimes tried, but unless everything in the scene was at a great distance, or all in a plane at the same distance, the difference in the viewpoints of the lenses (parallax) made it impossible to completely register all parts of the resulting images at the same time.
Color photography introduced to the public
Before the late 1890s, color photography was the domain of experimenters willing to build their own equipment, do their own color-sensitizing of photographic emulsions, make and test their own color filters, and otherwise devote a large amount of time and effort to their pursuits. The complicated process rarely produced images free of problems, and most photographers still regarded color photography as unrealistic.
thumb|An 1895 [[collotype print of Albert Bierstadt, photographed in color by his brother Edward Bierstadt.]]
In 1898, however, it was possible to buy the required equipment and supplies ready-made. Two adequately red-sensitive photographic plates