RADIAN A unit of angular measure equal to the angle subtended at the center
of a circle by a chord whose length is equal to the radius of the circle.
RADIANCE Brightness; the radiant power per unit solid angle and per unit area
of a radiating surface.
RADIANT ENERGY (Q) Energy in the form of electromagnetic waves usually expressed
in units of Joules (watt-seconds).
RADIANT EXPOSURE (H) The total energy per unit area incident upon a given surface.
It is used to express exposure to pulsed laser radiation in units of J/cm(2).
RADIANT FLUX RADIANT POWER - The time rate of flow of radiant energy. Units-watts.
(One  watt = 1 Joule-per-second). The rate of emission of transmission of radiant
RADIANT INTENSITY The radiant power expressed per unit solid angle about the
direction of the light.
RADIANT POWER See Radiant flux.
RADIATION In the context of optics, electromagnetic energy is released; the
process of releasing electromagnetic energy.
RADIOMETRY A branch of science which deals with the measurement of radiation.
RAYLEIGH SCATTERING Scattering of radiation in the course of its passage through
a medium containing particles, the sizes of which are small compared with the
wavelength of the radiation.
REFLECTANCE OR REFLECTIVITY The ratio of the reflected radiant power to the
incident radiant power.
REFLECTION The return of radiant energy (incident light) by a surface, with
no change in wavelength.
REFRACTION The change of direction of propagation of any wave, such as an electromagnetic
wave, when it passes from one medium to another in which the wave velocity is
different. The bending of incident rays as they pass from one medium to another
(eg. air to glass).
REPETITIVELY PULSED LASER A laser with multiple pulses of radiant energy occurring
in sequence with a PRF greater than or equal to 1 Hz.
RESONATOR The mirrors (or reflectors) making up the laser cavity including
the laser rod or tube. The mirrors reflect light back and forth to build up amplification.
ROTATING LENS A beam delivery lens designed to move in a circle and thus rotate
the laser beam around a circle.
RUBY The first laser type; a crystal of sapphire (aluminum oxide) containing
trace amounts of chromium oxide.