*A significant exception to this is the infrared lens in the form of an airspaced doublet widely spaced from a positive singlet. This can be considered either as a triplet anastigmat or as a Petzval lens. This particular design configuration results from the high index and low dispersion of the infrared materials used—usually silicon and germanium for the mid-infrared, or germanium and zinc sulfide or selenide for the 8- to 12-mm region. See Chap. 19 for infrared lens designs.
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