extralowdispersion

Extralow dispersion refers to a class of optical materials, typically special glass or crystalline materials, formulated to minimize refractive dispersion across a range of wavelengths. In practice, these materials have a high Abbe number and provide very small variation of refractive index with wavelength, which helps reduce chromatic aberration in imaging systems.

The concept is central to lens design, especially when high color fidelity and sharpness are required. Materials with extralow dispersion are used to construct optical elements that correct or lessen the separation of colors, enabling higher performance in apochromatic or near-apochromatic assemblies. Designers often pair extralow-dispersion elements with other glasses or crystals to achieve broad spectral correction from the visible into the near-infrared.

Common examples include certain fluorite crystals and specialized low-dispersion glasses developed by major optical manufacturers. Calcium fluoride (CaF2) and fluorite-based materials are classical sources of extralow dispersion due to their inherently low dispersion across the visible spectrum. In consumer and professional optics, extralow dispersion glasses are labeled or marketed as ED (extra-low dispersion) or XLD (extralow-dispersion) materials, sometimes with designations such as “special” or “ultra” low-dispersion variants.

Applications span camera lenses, microscope objectives, telescopes, and other precision imaging systems where minimizing chromatic aberration improves resolution and color accuracy. While advantageous, extralow dispersion materials can be more expensive, harder to manufacture, and may present mechanical or thermal challenges compared to standard glasses.

See also: Abbe number, chromatic aberration, apochromatic lens, fluorite crystal, optical glass.