ACHROMATIC LENSES

An achromatic lens, also known as an achromat, typically consists of two optical components cemented together, usually a positive low-index (crown BK7) element and a negative high-index (flint F2) element. In comparison to a singlet lens, which only consists of a single piece of glass, the additional design freedom provided by using a doublet design allows for optimization of performance. An achromatic lens will have noticeable advantages over a comparable diameter and focal length singlet.

Achromatic lenses are designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths 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. Typically, one element is a negative element made out of flint glass such as F2, which has relatively high dispersion, and the other is a positive element made of crown glass such as BK7, which has lower dispersion. The lens elements are mounted next to each other, often cemented together, and shaped so that the chromatic aberration of one is counterbalanced by that of the other. In the most common type, 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. Negative doublets, in which the negative-power element predominates, are also made. Multiple-lens systems, such as this achromatic doublet, can partially correct chromatic aberrations, but they may require lenses of different materials and add to the expense of optical systems such as cameras.

An achromatic lens comes in a variety of configurations, most notably, positive, negative, triplet, and aspherized. It is important to note that it can be a doublet (two elements) or triplet (three elements); the number of elements is not related to the number of rays for which it corrects. 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/violet) into focus in the same plane.