FiltersFilters

Interference filters are made up from many, many individual thin Fabry-Perot interferometers. Each layer creates a series of transmission windows. When these are arranged properly all of the maxima overlap and produce a bandpass that is essentially the product of each individual layer. Much of the light that is not transmitted by the filter is reflected and as a result interference filters can be used at somewhat higher power levels than colored glass filters. However, a certain amount of the energy is purposely absorbed (usually at the higher and lower ends of the filter's range) by either the filter or the substrate to block out harmonic transmission maxima at other, undesired wavelengths. This absorbed energy eventually limits the power handling ability of the filter.

Neutral density filters work by absorption or reflection. By combining several metals (such as aluminum, nickel, and chromium) in a proprietary combination, very neutral metallic absorbers can be created by vacuum evaporation onto a glass substrate. Neutral density filters may be combined to form higher density filters. If they are arranged to avoid multiple reflections between each other, the resulting optical density will be simply the sum of the individual filters (or, put another way, the transmittance will be the product of the components).

Colored glass filters also simply absorb light of a specific color, while transmitting the balance. Each specific filter is made from optical glass to which an absorbing material has been added during manufacture. The thicker the filter, the more it will absorb. Colored glass filters are excellent for applications where a general spectral region must be separated or isolated from the region of interest. Since they operate by absorption they may not be used in high power applications where the absorbed energy cannot be disposed of. They are cost effective, very uniform over their entire aperture, temperature and angle insensitive, and easy to use.

Diffusers are, of course, not really filters at all. Diffusers act as scatterers. An ideal (Lambertian) diffuser would scatter light equally in all directions and appear equally bright from wherever viewed. Most times, however, diffusers are only needed to smooth out the image of an inhomogeneous light source (such as a coiled tungsten element). In this situation uniformly ground glass will scatter most of the light in a forward direction while eliminating most of the smaller structure.

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