Light and filtering

Color printing uses the three primary colors: green, red, and blue. By adding three primary colors, you get white light. Remove one primary color and you get its complementary color: blue-yellow, green-magenta, and red-cyan. To make a successful print, you must know the primary and complementary colors and how they affect your final image. Color filters affect light passing through them by absorbing light of all colors except their own. Primary color filters pass only one primary, but block the other two. Complementary color filters, absorb light of their primary colors, but allow the other two to pass.

Color papers include three silver halide layers, and each respond to only one primary color. To make a good print, you can use complementary filters, since each affects only one layer of color materials without affecting the other two. This allows you to make one exposure rather than three with primary colors. Filters are used to control color balance of your prints. You can use different combinations of complimentary filters to control color balance with one exposure or make three separate exposures using red, green, and blue filters. You must use either method to produce a good print. Without filtered light, you'll get a brownish result. Filter strength also affects color balance. 40 magenta for example, subtracts twice as much green light as a 20 magenta filter. This can determine how small or large color balance will be. It is best to use two complementary filters at a time. If you use all three complimentary filters, you can produce neutral density. It is best to zero one filter to achieve a good color balance. This also gives a brighter illumination and shorter exposure time. UV filters should also be used to block ultraviolet light. Ultraviolet light is invisible to the naked eye, but printing papers are sensitive to it. Without UV filters, colors can be affected. To correct color balance, you must add or subtract filters. With positive/positive papers, color control is the opposite of negative/positive. If a print is too yellow for example, you reduce yellow filter with positive/positive papers, but add yellow filter with negative/positive. It is always best to reduce filtration to correct color balance with either paper if possible.