Absorb

To transform radiant energy into a different form, with a resultant rise in temperature.

Absorption

Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength.

Accessible Emission

The magnitude of accessible laser (or other collateral) radiation of a specific wavelength or emission duration at a particular point as measured by appropriate methods and devices. Also means radiation to which human access is possible in accordance with the definitions of the laser's hazard classification.

Accessible Emission Limit

The maximum accessible emission level limit (AEL) permitted within a particular class. In ANSI Z-136.1, AEL is determined as the product of Accessible Emission times the Maximum Permissible Exposure (MPE) using the area of the limiting aperture (7 mm for visible and near infrared lasers).

Active Medium

Collection of atoms or molecules capable of undergoing stimulated emission at a given wavelength.

Afocal

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

Aiming Beam

A laser (or other light source) used as a guide light. Used coaxially with infrared or other invisible light may also be a reduced level of the actual laser used for surgery or for other applications.

Amplification

The growth of the radiation field in the laser resonator cavity. As the light wave bounces back and forth between the cavity mirrors, it is amplified by stimulated emission on each pass through the active medium.

Amplitude

The maximum value of the electromagnetic wave, measured from the mean to the extreme; simply stated: the height of the wave.

Angstrom

A unit of measure of wavelength equal to 10^-10 meter, 0.1 nanometer, or 10^-4 micrometer, no longer widely used nor recognized in the SI system of units.

Anode

An electrical element in laser excitation which attracts electrons from a cathode.

Aperture

An opening through which radiation can pass.

Apparent Visual Angle

The angular subtense of the source as calculated from the source size and distance from the eye. It is not the beam divergence of the source.

AR Coatings

Antireflection coatings used on optical components to suppress unwanted reflections.

Argon

A gas used as a laser medium. It emits blue/green light primarily at 448 and 515 nm.

Attenuation

The decrease in energy (or power) as a beam passes through an absorbing or scattering medium.

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 total energy imparted during exposure divided by the exposure duration.

Axis, Optical Axis

The optical centerline for a lens system; the line passing through the centers of curvature of the optical surfaces of a lens.