In quantum theory, the elemental unit of light, having both wave and particle behavior. It has motion, but no mass or charge.

The damage mechanism for acute laser injury (i.e. for injury immediately following exposure). The radiation incident at the surface is absorbed in the underlying tissue, increasing the temperature of the tissue to the level at which damage can occur, and laser burns result. It is a power dependent process (a function of the RATE at which energy is absorbed rather than the total quantity of energy involved).

In laser welding, a metal vapor that forms above the spot where the beam reacts with the metal surface. Also used to describe the laser tube (plasma tube, discharge tube) which contains the completely ionized gas in certain lasers.

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.