Color filters include a wide range of filter types that are distinguished by their specific color spectrums and wavelengths, as well as their Schott glass composition. Color filter glass, also known as Schott glass, is distinguished by its absorption of optical radiation, depending upon the color in question. Some products are considered to be color filters based simply upon their Schott-glass material design. In general, however, this product area includes many different color filter types, including neutral density, short pass, long pass, band pass, ultraviolet (UV), infrared (IR), heat absorbing, and color-temperature conversion filters. Although most color filters are designated by a prefix, some products may appear to be of that color until placed within a specific range of the spectrum of visible light.
Product specifications for color filters include shape, dimensions, thickness, correction factor, and surface quality. Circular, square and rectangular color filters are commonly available. Circular color filters are specified by diameter. Rectangular or square color filters are specified by length and width. The correction factor is the product of the transmittance of the first air-glass interface and the transmittance of the second air-glass interface. This value is then is multiplied by the internal transmittance to produce the external transmittance. Filter surface quality is expressed in terms of scratches and digs. A scratch is a defect on a polished optical surface whose length is many times its width. A dig is a defect on a polished optical surface that is nearly equal in terms of its length and width.
Color filters that are made of Schott glass are characterized by their selective absorption of optical radiation. Each of these ranges is specified by a two-letter prefix code, which indicates the wavelength range and associated color. Choices include BG, FG, GG, OG, VG, KG, NG, RG, UG, and WG. These codes are followed by a numeric designation, which further specifies the color. BG is blue-green multiband, FG is blue-brown, GG is yellow, OG is orange, and VG is green. The KG prefix is used with IR filters which are virtually transparent in the visible range. NG color filters are made of neutral-density Schott glass. The RG prefix indicates that the Schott glass is red or black with IR transmission. The UG prefix designates black color filters with UV transmission. The WG prefix is for UV long-pass products.
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