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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.

Absorption Coefficient Factor describing light's ability to be absorbed per unit of path length.

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.

Angle of Incidence (See Incident Light)

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.

Articulated Arm CO2 laser beam delivery device consisting of a series of hollow tubes and mirrors interconnected in such a manner as to maintain alignment of the laser beam along the path of the arm.

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.

Aversion Response Movement of the eyelid or the head to avoid an exposure to a noxious stimulant, bright light. It can occur within 0.25 seconds, and it includes the blink reflex time.

Axial-Flow Laser A laser in which an axial flow of gas is maintained through the tube to replace those gas molecules depleted by the electrical discharge used to excite the gas molecules to the lasing. (See Gas Discharge Laser.)

Axicon Lens A conical lens which, when followed by a conventional lens, can focus laser light to a ring shape.

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.