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/cm2.

Radiant Flux Radiant Power

The time rate of flow of radiant energy. Units-watts. (One [1] watt = 1 Joule-per-second). The rate of emission of transmission of radiant energy.

Radiant Intensity

The radiant power expressed per unit solid angle about the direction of the light.

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.

Real Image

A real image is one in which the light rays actually converge; if a screen were placed at the point of focus, an image would be formed on it.


The return of radiant energy (incident light) by a surface, with no change in wavelength.


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 (e.g., air to glass).

Repetitively Pulsed Laser

A laser with multiple pulses of radiant energy occurring in sequence with a PRF > 1 Hz.


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.