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An optical material is chosen for its interaction with light, including refractive index, optical absorption, scattering of light and, where relevant, birefringence or nonlinear optics. A material transparent at one wavelength can absorb strongly at another; visible transparency alone does not qualify a window for ultraviolet or infrared use.

A simple characterization compares transmitted radiant power for samples of several thicknesses. Interface reflection contributes to every sample, while bulk attenuation grows with path length. Separating these effects is essential when selecting an optical glass for an optical lens or an optical fiber.