achromatisch

Achromatisch is a term used in optics and color theory to describe the absence of hue or color. In color theory, achromatic colors are black, white, and gray—colors without saturation or chromatic content. The word derives from Greek roots meaning “not color.”

In optics, an achromatic system is designed to reduce chromatic aberration, the tendency of a lens to focus different wavelengths of light at different distances. This is accomplished by combining lenses made from glasses with different dispersion properties, typically a crown glass element and a flint glass element. When arranged as a cemented doublet or a sequence of lenses, the design balances refractive powers for two wavelengths (often red and blue), bringing those colors to nearly the same focal plane and thus minimizing color fringing in the resulting image. Despite this correction, many achromatic lenses still leave some residual aberration outside the corrected wavelengths. The concept emerged in the early 18th century, with developers such as Chester Moor Hall and later John Dollond contributing to its refinement. Achromatic designs have long been central to refracting telescopes, microscope objectives, and many photographic lenses.

In modern usage, the term can also describe any neutral optical system or filter that transmits all wavelengths equally well, though in practice it most often refers to achromatic lenses and optical assemblies. The term contrasts with apochromatic designs, which aim to correct chromatic aberration across three or more wavelengths and often improve performance further by reducing other aberrations.