The diameter of a circle over which irradiance or radiant exposure is averaged for purposes of hazard evaluation and classification. Symbol: D_f.

Commonly called illumination; the luminous or visible flux per unit area on a receiving surface at any given point.

Abbreviated MPE.  The level of laser radiation to which an unprotected person may be exposed without adverse biological changes to the eye or skin.

The lens used primarily in CO2 lasers by Coherent, Inc. It has one side convex, the other concave.