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Thin film filters are coatings on glass or clear plastic that interfere with the light that passes through them. They can be designed to amplify or dampen light waves in any specific or wide range. Though light behaves as both a particle and a wave, the theories involved in thin film filter technology treat light as a wave.
Thin film filters are made by covering a piece of glass with various optical coatings. Filters are made with one or more thin films designed to interfere with light in specific ways. These filters are commonly used in optical instruments, such as telescopes and photographic lenses.
The dichroic filter is the most commonly used type of thin film filter. These filters can single out a specific wavelength of light to amplify it or cancel it out. Dichroic filters usually work by reflecting certain wavelengths away and allowing only certain wavelengths to pass through. Each wavelength of light corresponds to a specific visual color, so these lenses filter out certain colors. A dichroic filter can function over a wide range of visible light.
The monochromatic thin film filter is similar in design to the dichroic filter. The main difference between the two is that the monochromatic filter targets only a narrow range of light waves. This narrow range usually corresponds to one color of light.
A thin film filter can also be used to construct an ultraviolet or infrared filter. These filters allow visible light to pass through but block invisible light at either end of the spectrum. Natural density filters may also be made from thin film filters. These block a certain portion of all visible light, which dampens the light as it passes through the lens.
In order to function, a thin film filter must follow exacting specifications. When light moves through a barrier, such as a pane of glass, it is refracted by the barrier in a specific way. Refraction can include a change in the angle, speed or both in a light wave. Controlling how light will refract makes it possible to build filters that amplify or dampen a wave of light to the degree desired.
Light waves opposite to each other will cancel out when they attempt to occupy the same space. Some filters are designed to cancel out certain wave lengths of light, such as a filter on a telescope that blocks dangerous levels of light emitting from the sun. Conversely, light waves that are the same will interfere to amplify the wave. Filters that amplify light have uses as well. For example, a photographer who wants to take pictures in very low light can use a specialized filter that amplifies the available light.