A basic silver halide emulsion reacts simply to light - across approximately the whole spectrum. The developed image is also monochromatic. Most photography, however, is in color, and the steps needed to create a color image from black-and-white emulsion are fairly complex. What makes color reproduction possible is that the full white-light spectrum can be broken down into three equally spaced wavelengths: blue, green and red. These colors are each located in a third of the spectrum. Combined equally they make white light, but combined in other proportions they can reproduce almost any other color. This is the three-color principle of reproduction.
There are two ways of using these three colors for reproduction: additive and subtractive. In the less commonly used additive method, blue, green and red are combined. The subtractive process, which is the basis for most color films, uses dyes or pigments to subtract selectively from white light. To do this, it makes use of the three colors that are complementary to red, blue and green - these are cyan (blue-green) yellow and magenta (purple-pink). Each one absorbs its opposite primary color; thus, cyan absorbs, or blocks, red light; yellow absorbs blue, and magenta absorbs green.
Using this three-color principle, a color film can be constructed with the same basic emulsion as black-and-white film (i.e., using a silver compound that is sensitive only to the amount of light reaching it). Color film has three layers of emulsion, each one of which has been made sensitive to one of the three primary colors - blue, green, red - by means of built-in dye filters. The technical term is integral tri-pack film.
Color negative film: During exposure, three separate latent images are formed. When developed, they become three negatives, corresponding respectively to the blue, green and red light in the subject. (This is very similar to the separation negatives necessary to print the color images in this book.) There is just one development, and the process is completed by bleaching out the unwanted silver to leave three color dye images. When projected in an enlarger, the three transparent layers combine to give an image that is reversed not only in tone, but also in color. Color negative film has a distinct orange appearance; this acts as a mask, designed to compensate for color distortions that would otherwise occur in printing.
