What is "achromatic lens"

Achromatic \Ach`ro*mat"ic\, a. [Gr. ? colorless; 'a priv. + ?, ?, color: cf. F. achromatique.]

1. (Opt.) Free from color; transmitting light without decomposing it into its primary colors.

2. (Biol.) Uncolored; not absorbing color from a fluid; -- said of tissue.

   Achromatic lens (Opt.), a lens composed usually of two separate lenses, a convex and concave, of substances having different refractive and dispersive powers, as crown and flint glass, with the curvatures so adjusted that the chromatic aberration produced by the one is corrected by other, and light emerges from the compound lens undecomposed.

   Achromatic prism. See Prism.

   Achromatic telescope, or microscope, one in which the chromatic aberration is corrected, usually by means of a compound or achromatic object glass, and which gives images free from extraneous color.

n. (context optics English) A lens generally composed of two separate lenses, one convex and one concave and generally of different materials, in such a way that the chromatic aberration produced by one is corrected by the other.

n. a compound lens system that forms an image free from chromatic aberration

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. Typically, one element is a negative ( concave) element made out of flint glass such as F2, which has relatively high dispersion, and the other is a positive ( convex) 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 (shown), 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.