Light filters work by removing or passing wavelength bands ranging in size, from hundreds of nanometers to a single wavelength. Basically, the filter material --be it glass, gel, plastic or some other substance or combination thereof--selectively absorbs, reflects, refracts or diffract specific wavelengths in the visible light spectrum.
Absorption filters are made from colored filter glass or synthetic gels. They are the most widely used type of light filter. To work well, the filter glass or polymer should be optical grade and have uniform density and color over the surface of the filter. Also, the thicker the material, the more wavelengths it will absorb. By absorbing certain wavelengths, only certain parts of the visible spectrum can be seen. For example, if a filter that is meant to absorb all other wavelengths bar yellow is used, only yellow light will come through and be seen.
Filters that are designed to reflect light reflect back the unwanted wavelengths and allows selected wavelengths to come through. One such reflective filter is an interference filter. It is made out of successive layers of dielectric materials, with thickness values ranging between one-quarter and one-half of the target wavelength, that are deposited onto an optically flat glass or polymer surface in a vacuum. The unwanted wavelengths of light are then reflected by the dielectric materials, reduced in magnitude by destructive interference and removed from the optical path. The most sophisticated interference filters can reflect the unwanted light to a different direction. The wavelengths that are wanted pass through the filter and are transmitted as light in the desired optical path.
Refractive filters work by changing the direction of a specific light wavelength or wavelengths when the wavelength(s) hit the filter's surface. The desired wavelengths are directed to a specific optical path whilst the unwanted parts of the lights spectrum are refracted to another direction. The direction change or changes occur between the refractive layers of the filter. The filter contains at least two layers.
Color Compensating Filters
Color compensating filters work by absorbing differing amounts of the red, green and blue portions of the visible light spectrum. By doing so, microscopists are able to fine-tune the color balance of tungsten-halogen microscope light sources for photomicrography with color films and digital imaging.
Acousto-Optic Tunable Filters
Acousto-Optic Tunable Filters (AOTF) control the intensity or illumination of the light wavelengths that pass through. Microscopists use these filters to intensify certain wavelengths of light from digital images on a pixel-by-pixel basis.
- Photo Credit lenses and photo filters image by FrankU from Fotolia.com
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