Understanding Color Filters: How They Work and What They Do

A color filter is a sheet of transparent material that modifies a light beam by selectively absorbing some colors relative to others. It is used to create special effects in photography, signs, traffic lights, and directional signs in cars, ships, and airplanes. Color filters can also be used to help distinguish between items that are only differentiated by color. Most light sources emit a wide range of wavelengths that cover the entire spectrum of visible light.

However, in many cases, it is desirable to produce light that has a restricted wavelength spectrum. This can be easily achieved by using specialized filters that transmit some wavelengths and selectively absorb or reflect unwanted wavelengths. Color filters are generally constructed from transparent pieces of tinted glass, plastic, lacquered gelatin (for example, Wratten filters) that have been treated to selectively transmit the desired wavelengths and restrict others. The two most common types of filters used today are absorption filters and interference filters. Absorption filters absorb unwanted wavelengths while interference filters eliminate selected wavelengths through internal destructive interference and reflection.

In any filter, a small amount of incident light is reflected from the surface, regardless of the construction of the filter, and a small part of the light is also absorbed. However, these artifacts are usually minimal and do not interfere with the main function of the filter.

Absorption Filters

Absorption filters are generally made of stained glass, lacquered gelatin, or synthetic polymers (plastics) and have a wide range of applications. They are used to create special effects in a number of photography applications and are widely used in the film industry. In addition, absorption filters are commonly found in signs and traffic lights and as directional signs in cars, ships and airplanes. The following diagram (figure) illustrates a magenta filter designed to fit a camera lens.

We've also created an interactive Java tutorial that describes how lacquered glass and gelatin filters work. In Figure 2 above, absorption is plotted against the visible wavelengths that pass through the magenta filter. The maximum intensity of the absorbed light falls to about 550 nanometers, right in the center of the green region of visible wavelengths. The filter also absorbs some light in the blue and red regions, indicating that this filter is not perfect and that a small part of all the wavelengths do not pass through it. A perfect filter would have a very sharp peak centered in the green region that would have zero absorption at non-green wavelengths, but this is practically impossible to achieve with absorption filters visible in the real world that can be manufactured at reasonable prices. This type of unwanted absorption is often called secondary absorption and is common in most filters.

Learn how gelatin and glass absorption filters are used to pass a specific band of wavelengths. Just follow the arrows from the vertex to the opposite side or from the side to the opposite vertex. You can also check Table 2 to see the correct color of the CC filter.

Color Correction

For example, a green hue can be removed by using a magenta CC filter. The appropriate density of the chosen DC filter must be determined using test exposures.

See Photography Through The Microscope, by John Delly, for color illustrations of colored molds. When we perform experiments with photomicrography (photography through a microscope), we often add color-compensating filters to the light path. This is most easily achieved by shaping the filter into a circle with scissors and inserting it into the light path just behind the diffusion filter. As an alternative, Kodak sells small metal frames containing Wratten filters which can be placed in the light port of the microscope just above the field diaphragm. This allows for global color correction in resulting photomicrographs.

RGBW Matrix

Color filters filter light by wavelength range so that separate filtered intensities include information about color of light.

For example, Bayer filter (shown in image) provides information about intensity of light in red, green and blue (RGB) wavelength regions. Raw image data captured by image sensor then converted into full-color image (with intensities three primary colors represented each pixel) using demosaication algorithm that adapts each type color filter. Spectral transmittance CFA elements together with demosaication algorithm together determine color reproduction. Quantum efficiency sensor's passband and range CFA's spectral responses usually wider than visible spectrum so all visible colors can be distinguished. Filter responses generally do not correspond CIE color matching functions so color translation required convert tristimulus values into common absolute color space.

Dichroic Filters

Photographers and microscopists often have to make minor corrections to color illumination for photographic magnifiers and optical paths for microscopes to ensure accurate color reproduction.

This is usually done with Kodak color compensation filters (abbreviated CC) that can be placed in light path magnifier or microscope. Diazonaphthoquinone photoresist (DNQ) -novolac is material used as support to make color filters from dyes or color pigments. An RGBW matrix (in red, green, blue, white) is CFA that includes white or transparent filter elements that allow photodiode to respond all colors light; some cells panchromatic more light detected instead absorbing compared Bayer matrix. Any color image not transmitted by filter will be absorbed it will not be seen. Term dichroic arises from fact that filter appears one color under illumination with transmitted light another with reflected light. Term also used reference...

Clément Vermeulen
Clément Vermeulen

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