Thin film filters are microscopically thin deposits that are often attached to glass or plastic lenses. The most common application of these filters is in optics, where they can be used to filter out certain wavelengths of light or otherwise change the nature of a beam of light as it passes through a lens. Telescopes, microscopes and cameras frequently use thin film filters, though laser technology can also make use of them.
Like other types of thin films, thin film filters are so thin that they are measured in either nanometers or micrometers, meaning that they are somewhere between a billionth of a meter and several hundredths of a meter thick. These filters will not increase the thickness of a lens by an amount that can be noticed or measured by a human observer. In order to create thin films, atoms are encouraged to settle on a substrate material through the use of either physical or chemical deposition, which, in the case of thin film filters, usually occurs in a vacuum. During the deposition process, atoms line up along the substrate material, which is kept in constant motion, until a film of the desired thickness has been made.
Some of the most common types of thin film filters are dichroic filters which are used to prevent certain wavelengths of light from passing through them, effectively blocking out certain colors. Dichroic filters can be designed to block out any range of wavelengths of light, and more than one filter can be applied to a substrate in order to block out more than one set of wavelengths. A single filter, for example, could be made to block out blue, violet, and ultra-violet light, though two filters would be required to block out only blue and ultraviolet light. Other types of filters can block a portion of all visible light, reducing its intensity. They can also prevent unwanted reflections on a lens.
Optical devices, such as telescopes, microscopes and cameras, can use thin film filters instead of traditional filters. Though the thinness of the film is one of the benefits of using these filters, the filters also reflect unwanted light away, rather than absorbing it, allowing the filter and the lens to which it is attached to remain cool. In laser technology, thin film filters can be used to intensify and keep a laser beam pure, qualities that are essential when working with high-powered beams, such as those used in laser cutting.