An achromatic lens, also referred to as an achromat, typically
consists of two optical components cemented together, usually a
positive low-index (crown) element and a negative high-index
(flint) element. Due to the difference in refractive indices,
chromatic aberration with respect to two selected wavelengths has
been corrected. The lenses have been corrected for on-axis
spherical aberration as well. Achromatic doublets are used as
telescope objectives, eye loupes, magnifying glasses and eyepieces.
Achromatic doublets have also been used to focus and manipulate
laser beams because their image quality is superior to single
lenses.
