A medium in which lasing will take place, rather than absorption, at a given wavelength.

Literally, “without a focal length”; an optical system with its object and image point at infinity.

The angular subtense of an extended source beyond which additional subtense does not contribute to the hazard and need not be considered. This value is 100 milliradians (mrad) for retinal thermal effects and 110 mrad for retinal photochemical effects. Symbol: α_max.

The angular subtense of a source below which the source can effectively be considered as a point source. The value of alpha min is equal to or less than (≤) 1.5 mrad. Symbol: α_min.