apochromatische

Apochromatische refers to an optical design that minimizes chromatic aberration across a broad spectral range by bring­ing multiple wavelengths into substantially the same focus. The term apochromatic (often abbreviated APO) denotes correction beyond standard achromatism, which corrects two wavelengths. In an apochromat, three or more wavelengths, typically red, green, and violet or near-infrared, are corrected simultaneously, reducing axial chromatic aberration and the secondary spectrum, the residual color misalignment that can occur between wavelengths.

Achieving this level of correction relies on optical design and materials. Designers use combinations of glasses with different dispersion characteristics, sometimes including fluorite (CaF2) elements, and arrange lenses in triplets or quadruplets, sometimes with air gaps, to control curvature and dispersion. Coatings and mechanical design also help reduce stray light and maintain contrast.

Applications: Apochromatic lenses appear in high-end refracting telescopes, photographic and cinema lenses, and microscopy. In astronomy, APO refractors yield sharp images with minimal color halos across wide fields, while in photography they reduce color fringing in high-contrast scenes. The term is commonly used by manufacturers to indicate superior color correction relative to standard achromatic lenses; several brands offer dedicated APO products.

See also: Chromatic aberration, Achromatic lens, Fluorite lens, Multi-element optics.