Achromatic Lens

An achromatic lens, also referred to as an achromat, typically consists of two optical components cemented together, usually a positive low-index (crown) element and a negative high-index (flint) element.
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.
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.

Several different types of achromat have been devised. They differ in the shape of the included lens element as well as in the optical properties of their glass (most notably in their optical dispersion or Abbe number).
By combining different materials with different dispersion properties, achromatic lens achieve superior color correction and exceptional image quality. This makes them indispensable in fields such as microscopy, spectroscopy, photography, and laser system.