Concept

Abbe number

Summary
In optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is an approximate measure of the material's dispersion (change of refractive index versus wavelength), with high values of V indicating low dispersion. It is named after Ernst Abbe (1840–1905), the German physicist who defined it. The term V-number should not be confused with the normalized frequency in fibers. The Abbe number, Vd, of a material is defined as :V_D = \frac{ n_d - 1 }{ n_F - n_C }, where nC, nd and nF are the refractive indices of the material at the wavelengths of the Fraunhofer C, d, and F spectral lines (656.3 nm, 587.56 nm, and 486.1 nm respectively). This formulation only applies to the visible spectrum. Outside this range requires the use of different spectral lines. For non-visible spectral lines the term V-number is more commonly used. The more general formulation defined as, :V = \frac{ n_\text{center} - 1 }{
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