Abbe number

The Abbe number is a dimensionless measure of an optical material’s dispersion, calculated from its refractive indices at specified spectral lines. Higher values indicate lower dispersion.

Refractive index: A dimensionless measure of how much light slows and changes direction in a material. Its wavelength-dependent values supply the measurements used to calculate an Abbe number.
Achromatic lens: A lens that brings two selected wavelengths of light to the same focus. Combining glasses with different Abbe numbers helps reduce chromatic aberration.
Partial dispersion: The change in refractive index between two specified wavelengths within a material. It resolves spectral behavior that a single Abbe number cannot distinguish.
Ernst Abbe: A German physicist and optical scientist who helped establish the theory and design of precision optical instruments. The dispersion measure bears his name, reflecting his work in optical theory.
Fraunhofer lines: Dark absorption lines in the solar spectrum, named for the physicist Joseph von Fraunhofer. The standard Abbe number uses refractive indices at three designated Fraunhofer lines.
Apochromatic lens: A lens system that brings three wavelengths to nearly the same focus and controls other aberrations. Its glass choices depend on balancing refractive power and dispersion.
Relative partial dispersion: A normalized measure of a material’s refractive-index change over a wavelength interval, relative to a reference interval. It supplements Abbe number when matching glasses with unusual color behavior.
Joseph von Fraunhofer: A German physicist and optician who precisely measured solar spectral lines and advanced optical glassmaking. The spectral lines used in the conventional definition carry his name.
Refractive dispersion: The variation of a material’s refractive index with the wavelength or frequency of light. The Abbe number compresses this wavelength dependence into one comparative value.
Optical glass: Glass manufactured with controlled optical properties for lenses, prisms, and other components. Abbe numbers are a standard way to compare the dispersion of optical glasses.

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