Ablation

The removal of any material (e.g., skin tissue, paint, ceramic film, etc.) by means of its interaction with laser energy or power.

Accessible Emission Limit

Abbreviated AEL.  The maximum accessible emission level permitted within a particular laser hazard class.
Note: the laser classifications consist of: Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3B, and Class 4. Generally speaking, the potential beam hazard level increases in the same order.

Alpha Max

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: αmax.

Alpha Min

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: αmin.

Angstrom

A unit of measurement for a wavelength of light (written Å), equal to one ten billionth of a meter (10-10 meter).  (Used occasionally.)

Anode

An electrical element in laser excitation which attracts electrons from a cathode. An anode can be cooled directly by water or by radiation.

AR Coatings

Anti-reflection coatings, used on the backs of laser output mirrors to suppress unwanted multiple reflections which reduce power.

Autocollimator

A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light.

Average Power

The rate at which energy (joules per second) is emitted, transferred or absorbed with its measurements described by the unit measurement of watts (W).

Aversion Response

Any motion(s) including: eyelid closure, eye movement, pupillary constriction, head movement to avoid exposure to noxious or bright visual light stimulants. In application to visible laser light, its exposure to critical retinal area(s) is established as 0.25 seconds or less.