Color filters make up the largest group of photographic filters. These may be sheets of colored transparent material, such as glass, gelatin or plastic, or a dye solution contained in a glass cell. Color filters selectively transmit some colors and absorb others. Red filters, for example, transmit red light and absorb green and blue light. Magenta filters transmit red and blue light and absorb green light.
Almost every color can be matched using different mixtures of red, green, and blue light. Therefore red, green, and blue filters are used in some color photographic techniques to make separate film records of the various intensities of red, green, and blue light reflected from the actual scene. These filters can also be used in the recombination of the colors in the printing or projection of the image.
Color-correcting filters are frequently used to control the overall tint of a color photograph, particularly when the illuminating source is different from that for which the film was made. For example, since daylight is more blue than artificial light from a tungsten-filament lamp, a blue filter may be used with film designed for daylight photography when taking pictures under artificial light conditions.
Color filters are also used extensively in black-and-white photography to achieve contrast. For example, when a landscape that includes a sky with clouds is photographed without a filter, the clouds often do not show up. If a pale yellow filter is used, the extreme violet end of the spectrum is removed, resulting in a darkening in the photograph of the sky and very little darkening of the clouds. A red filter carries this much further, making the sky appear almost black in the photograph, particularly when infrared-sensitive film is used. An orange filter, used in photographing red flowers and green leaves, will make the flowers appear lighter than the leaves. A green filter can be used to make the foliage appear lighter.
Filters intended to transmit only the near infrared and ultraviolet parts of the spectrum absorb the entire visible spectrum and thus appear black to the eye.
'Neutral density filters’ are used to reduce the intensity of all light reaching the film, without changing the color.
'Polarizing filters', which transmit light vibrating in only one plane, are used to reduce the effects of glare from water and polished non-metallic surfaces.
'Interference filters', which are made up of alternate quarter-wavelength layers of high and low refractive index materials, can transmit very narrow bands of the spectrum. The remainder of the spectrum is then reflected. An interference filter known as a dichroic mirror reflects one end of the spectrum and transmits the other end of it.