Apochromat

An apochromat, in optics, is a lens or optical system designed to bring three wavelengths of light to a single focal plane with minimal residual chromatic aberration. By correcting chromatic error across a broader portion of the visible spectrum, apochromats offer higher color fidelity and sharper contrast than standard achromatic lenses, which only align two wavelengths.

Designs typically combine special glasses—such as extra-low-dispersion (ED) glasses or fluorite—with careful element ordering and spacing. Many apochromats are implemented as triplets or quadruplets and use advanced coatings to reduce residual aberrations across the visible range and into near-UV and near-IR. The result is reduced color fringing and higher image quality, particularly at wide apertures.

Applications include astronomy, where apochromatic refractors (APOs) are prized for planetary and stellar imaging, photography and cinematography for faithful color rendition, and precision microscopy and laser optics where chromatic dispersion can degrade measurements and performance.

Etymology and terminology: The term apochromat derives from Greek roots relating to color correction; APO is a common abbreviation in marketing and catalogs, especially for telescopes.

See also: Chromatic aberration; achromat; fluorite; low-dispersion glass.