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.
Doublets can come in many forms, though most commercial doublets are achromats, which are optimized to reduce chromatic aberration while also reducing spherical aberration and other optical aberrations. Apochromats can also be made as doublets.
In a cemented doublet, the lenses are held together by an adhesive, such as Canada balsam or epoxy. Some doublets use no adhesive between the lenses, relying on external fixturing to hold them together. These are called air-spaced doublets.