apochromaten

An apochromat (plural apochromats) is a high-performance microscope objective designed to correct chromatic and spherical aberrations across a broad portion of the visible spectrum. Compared with standard achromats, apochromats minimize lateral and axial chromatic error for multiple wavelengths, resulting in higher sharpness, improved color fidelity, and closer to diffraction-limited performance.

The correction of three or more wavelengths is a defining feature. Apochromats typically use multiple optical elements made from low-dispersion or special glasses, and may include fluorite (CaF2) or other high-quality materials to reduce dispersion. Many apochromats are also “plan” objectives, meaning they are corrected for flat field curvature so image planes remain sharp across the field of view. They are commonly offered in oil-immersion variants with high numerical apertures (often around 1.0 to 1.4) and are designed to be used with correctly matched cover slips and immersion media.

Applications include fluorescence and multicolor imaging, where accurate alignment of different fluorophores is crucial, as well as quantitative imaging, confocal microscopy, and some super-resolution setups. The superior correction properties make apochromats particularly valuable in experiments requiring faithful color rendering and precise measurement of axial and lateral distances.

Limitations include higher cost, greater physical size and weight, and increased sensitivity to misalignment or refractive-index mismatches in samples. Proper matching of immersion medium, cover-slip thickness, and mounting media is essential to realize the performance advantages of apochromats.