An acronym for light amplification by stimulated emission of radiation. A laser is a cavity, with mirrors at the ends, filled with material such as crystal, glass, liquid, gas or dye. A device which produces an intense beam of light with the unique properties of coherence, collimation and monochromaticity.

Laser Accessories

The hardware and options available for lasers, such as secondary gases, Brewster windows, Q-switches, and electronic shutters.

Laser Damage Threshold

The resistance of an optical component to laser induced damage is quantified in an irradiation power density expression know as laser damage threshold. The most common laser damage process this is often initiated at a dielectric surface or interface by the ionization of a contaminant molecule such as water that has been absorbed on a surface or trapped at an interface. Ionization can also occur in the interior of a pure dielectric, where the very high photon fluxes of some pulsed lasers may induce a process known as multi-photon ionization.

A rare phenomenon that results in acoustic amplification and subsequent mechanical damage.

Energy absorption at inclusions that leads to thermal fracture can occur, but it is unusual in high-quality dielectric materials.

In addition to these effects, polishing imperfections such as scratches and digs can result in constructive interference of reflected or diffracted waves in which the resultant electric fields can potentially cause damage.

Laser damage threshold is usually specified as a power per unit area for cw lasers or as energy over time per unit area, for pulsed lasers. A typical specification for a high-energy laser optic could be 4.5 J/cm2 in 10 nsec at 355 nm.

Laser Oscillation

The buildup of the coherent wave between laser cavity end mirrors producing standing waves.

Laser Rod

A solid-state, rod-shaped lasing medium in which ion excitation is caused by a source of intense light, such as a flashlamp. Various materials are used for the rod, the earliest of which was synthetic ruby crystal.

Laser System

An assembly of electrical, mechanical, and optical components which includes a laser. Under the Federal Standard, a laser in combination with its power supply (energy source).

Leading Edge Spike

The initial pulse in a series of pulsed laser emissions, often useful in starting a reaction at the target surface. The trailing edge of the laser power is used to maintain the reaction after the initial burst of energy.


A curved piece of optically transparent material which depending on its shape is used to either converge or diverge light.

Lens Maker's Formula

The standard formula for determining the focal length of a thin lens (thickness = 0) with radii of curvature r1 and r2. The focal length is given by the equation

  1/f = (n - 1)[(1/r1) - (1/r2)]

where f is the focal length and n is the index of refraction of the material.


The range of electromagnetic radiation frequencies detected by the eye, or the wavelength range from about 400 to 760 nanometers. The term is sometimes used loosely to include radiation beyond visible limits.

Limiting Angular Subtense

The apparent visual angle which divides intrabeam viewing from extended-source viewing.

Limiting Aperture

The maximum circular area over which radiance and radiant exposure can be averaged when determining safety hazards.

Limiting Exposure Duration

An exposure duration which is specifically limited by the design or intended use(s).

Linear Polarization

With respect to light radiation, the case where the polarization direction is fixed.

Longitudinal or Axial Mode

Determines the wavelength bandwidth produced by a given laser system controlled by the distance between the two mirrors of the laser cavity. Individual longitudinal modes are produced by standing waves within a laser cavity.