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Filters may be the simplest way of implementing a spectral imaging system. They can also be very complicated, particularly if they are acousto-optical filters. Filters can be used several ways. The filters can be a set of individual elements in a filter wheel, or âtheyâ can be continuously variable as with a circular or linear variable filterâCVF or LVF. The filters can be used in the collimated portion of the optics, usually in front of the entrance pupil, or in the convergent beam near the focal plane. The tradeoffs are size versus the narrowness and spectral shape of the filtration.
8.1 Types of Filters
Filters can be constructed from almost anything that has a variation of an optical property with wavelength. Perhaps the simplest example is an absorption filter. A material, like window glass, transmits light from about 0.4 μm to about 2.5 μm, and it absorbs the light everywhere else. This glass, as described, is a bandpass filter, and it is a rather wide bandpass, with a resolving power of about 0.7. Most people are familiar with the commercially available Wratten filters that are dye-impregnated materials. Many semiconductors make excellent long-wavelength cut-on filters. That is, they have almost complete absorption out to the wavelength at which the photon energy is no longer sufficient to free an electron for conduction. These cut-ons are much sharper than the cutoffs of the dye filters. Reststrahlen or residual-ray filters make use of the large variation of refractive index with wavelength and therefore reflection in the region of so-called anomalous dispersion. This special reflection is used in either single- or many-pass arrangements to accentuate the âtransmissionâ in the part of the spectrum where this phenomenon occurs. Lyot filters make use of the polarization properties of birefringent materials, and Christiansen filters make use of the variation of scattering with refractive index, which in turn varies with wavelength.
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