achromatic lens

 Achromatic Lens is an achromatic consists of a positive low-index Crown Glass lens element cemented to a negative high-index Flint Glass lens element. The elements are chosen so as to cancel chromatic aberration at two well-separated wavelengths, usually in the blue and red region of the spectrum. Focal length is constant at those two wavelengths and focal length shifts are virtually eliminated across the visible wavelengths. One frequent use is to achieve diffraction limited focusing of a laser beam. Negative Achromats are typically used when there is a need to eliminate chromatic aberration. In addition to reducing chromatic aberration at the design wavelengths, spherical aberration and coma are greatly reduced.