An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
The most common type of achromat is the achromatic doublet, which is composed of two individual lenses made from glasses with different amounts of dispersion. Usually one element is a concave lens made out of flint glass, which has relatively high dispersion, while the other, convex, element is made of crown glass, which has lower dispersion. The lens elements are mounted next to each other, typically cemented together, and shaped so that the chromatic aberration of one is counterbalanced by that of the other.
In the most common type (illustrated above), the positive power of the crown lens element is not quite equalled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus. Negative doublets, in which the negative-power element predominates, are also made.
The exact date of the first achromatic doublet's creation is not known, nor is the name of the person who first accomplished the task. Theoretical considerations of the feasibility of the system were debated in the 18th century following Newton's statement that such a correction was impossible (see History of the telescope). Credit for the first invention, around 1733, of the achromatic refracting lens is often given to an English barrister named Chester Moore Hall. Some of the concepts were demonstrated with lenses made of glass and water, but the first useful lenses were not known to have been made until the early 18th century by George Bass under the direction of Hall. The first patent for an achromatic doublet was awarded to John Dollond around 1758 following his independent theoretical and experimental work.
The triple achromat, which reduced secondary colour defects, was invented in 1763 by Dollond's son Peter.