A source with an angular subtense at the cornea equal to or less than alpha-min (α_min), i.e., ≤ 1.5 mrad. Also see Extended Source.

Beam movement and divergence, often preventable by using short path-lengths.

Restriction of the vibrations of the electromagnetic field to a single plane, rather than the innumerable planes rotating about the vector axis. This prevents optical losses at interfaces between the lasing medium and optical elements. Various forms of polarization include random, linear (plane), vertical, horizontal, elliptical, and circular. Of two polarization components (so-called), S and P, the P component has zero losses at Brewster’s angle. See Brewster Windows. λ = wavelength E = electric vector H = magnetic vector.

When more molecules (atoms, ions) in a laser are in a metastable state than in the ground state (a situation needed for sustaining a high rate of stimulated emissions), a “population inversion” is said to exist. Without a population inversion, there can be no lasing action.

The amount of radiant energy concentrated on a surface.