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Photonics Dictionary


1. The decrease of the absorption (or gain) coefficient of a medium near some transition frequency when the power of the incident radiation near that frequency exceeds a certain value. As long as the absorption (or gain) coefficient is constant, the power absorbed (or emitted) by the medium is proportional to the incident power. However, there is always a limit to the rate at which the medium can absorb (or emit) power that is determined by the lifetimes of the energy levels involved. As this limit is reached, the induced transitions become rapid enough to affect the energy level populations, making them more nearly equal. 2. With respect to color, attribute of a visual sensation that permits a judgment to be made of the proportion of pure chromatic (in contrast to achromatic) color in the total sensation. The psychosensorial correlate, or nearly so, of colorimetric purity. See also chroma; Munsell chroma.
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