Photography and Primary Colors

Color photography was explored throughout the 1800s. Initial experiments in color could not fix the photograph and prevent the color from fading. Moreover until the 1870s the emulsions available were not sensitive to red or green light.

The first permanent color photo was taken in 1861 by the Scottish physicist James Clerk Maxwell. Several patentable methods for producing images (by either additive or subtractive methods, see below) were devised from 1862 on by two French inventors (working independently), Louis Ducos du Hauron and Charles Cros (see Coe, ref 1, for details). Practical methods to sensitize silver halide film to green and then orange light were discovered in 1873 and 1884 by Hermann W. Vogel. (Full sensitivity to red light was not achieved until the early years of the 20th century.)

The first fully practical color film, Autochrome, did not reach the market until 1907. It was based on a screen-plate method, the plate being made using dyed dots of potato starch. The screen plate lets filtered red, green or blue light through each grain to a photographic film in contact with it. This is then developed to a negative, and reversed to a positive, which when viewed through the screen plate restores the original colors in their correct proportions.

Other systems of color photography included that invented by Sergei Mikhailovich Prokudin-Gorskii, which involved three separate monochrome exposures ('separation negatives') of a still scene through red, green, and blue filters.

The first modern ('integrated tri-pack') color film, Kodachrome, was introduced in 1935 based on three colored emulsions. Most modern color films, except Kodachrome, are based on technology developed for Agfacolor (as 'Agfacolor Neue') in 1936. (In this newer technology, the color-couplers are already within the emulsion layers, rather than having to be carefully diffused in during development.) Instant color film was introduced by Polaroid in 1963.

There are basically two color systems:

1. Additive: The colors are added as colored lights. In this system, the most common set of primary colors is red, green and blue. Maxwell's experiment was of this type, as are screen-plate methods, such as Autochrome. Modern digital photographs seen on a VDG are also viewed by addition of light from an RGB phosphor array.
2. Subtractive: Colors are subtracted from white light by dyes or pigments. In this system the most common set of primary colors is cyan, magenta and yellow. Ducos du Hauron made several pictures by this method in the late 1800s.

Several commercial print methods were devised using the subtractive technique during the 1930s (see eg Coe, ref 1), for printing from 'separation negatives'. Kodachrome was the first commercially-available 'integrated tri-pack' film of this type.

There are two main types of color film in current use:

• Color negative film forms a negative image when exposed, which is fixed during developing. This is then exposed onto photographic paper to form a positive image.
• Color reversal film, also known as slide film, forms a negative image when exposed, which is reversed to a positive image during developing. The film can then be projected onto a screen.

http://en.wikipedia.org/wiki/Color_photography