An achromatic lens, often called an achromat, is a type of optical lens capable of correcting chromatic aberration, a distortion that occurs when glass splits white light into multiple colorwavelengths in the spectrum.
Chromatic aberration is a common type of imaging defect. When white light passes through a singlet lens, the wavelengths of light are refracted. Because different wavelengths pass through the glass at different rates, they come into focus at different points on the plane. As a result, the operator will not be able to bring all the colors into focus simultaneously. Chromatic aberration creates blurry fringes of color between dark and light parts of an image, significantly reducing image quality.
To deal with the problem of chromatic aberration, people rely on achromatic lenses, which combine two or more lenses to direct two wavelengths of light, usually red and blue, to the same focal point.