In optics, a doublet is a type of lens made up of two simple lenses attached together. The lenses are made from glasses with different refractive indices and different amounts of dispersion. Often one element is made from crown glass and the other from flint glass. This combination produces a better image than a simple lens. Trilobites, which are now extinct, had natural doublet lenses.
In a cemented doublet, the lenses are held together by an adhesive, such as optically transparent epoxy. Canada balsam was traditionally used for this purpose. Some doublets use no adhesive between the lenses, relying on external fixturing to hold them together. These are called air-spaced doublets.
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