lens composed usually of two separate lenses used to produce images free of chromatic aberrations
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 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. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
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