Achromatic lens
From Wikipedia, the free encyclopedia
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. 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 and shaped so that the chromatic aberration of one is counterbalanced by the chromatic aberration of the other, while the positive power of the crown lens element is not quite equaled 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. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
The first achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond.
