Definition: (1) Level of lighting at which a reduction of lighting level is not discernible: highest luminance level at which drop in luminance causes no change in appearance. * E.g. shadow detail visble in a print in good light disappears when the ambient light level drops below extinction level. (2) Full or nearly total absorption of plane-polarized light by a polarizer that has an axis at right-angles to the plane of polarization. (3) Loss of a diffracted beam's intensity caused by a nearly perfect crystal.

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