R

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 (180/pi, numerically). Corresponds to an angle of 57.29578 degrees.

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 radiation, usually expressed in Joules.

Radiant exposure (H)

The total energy per unit area incident upon a given surface. Used to express exposure to pulsed laser radiation in units of J/cm².

Radiant flux

Also known as radiant power. The time rate of flow of radiant energy. Units-Watts. (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.

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 that 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 medium 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.

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.