Browse our Glossary of Laser Definitions:

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  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.  
Absorption of Radiation  
  Receiving electromagnetic radiation by interaction with the material, and transforming it to different form, which is usually heat (rise in temperature). The absorption process is dependent on the wavelength of the electromagnetic radiation and on the absorbing material.  
Accessible Emission Limit (AEL)  
  Maximum accessible emission level which is permissible in the appropriate class of laser.
Accessible Radiation  
  Laser radiation that can expose human eye or skin in normal usage.  
Active Medium  
  Collection of atoms or molecules which can be stimulated to a population inversion, and emit electromagnetic radiation in a stimulated emission.  
  Laser alignment is a measure of the deviation of the optical axis of the laser beam with respect to the mechanical axis of the laser housing. Some laser diode modules feature an adjustable alignment for precise positioning applications.  
  The process in which the electromagnetic radiation inside the active medium within the laser optical cavity increase by the process of stimulated emission.
  The maximum value of a wave, measured from its equilibrium.
  A unit of length equal to one ten-billionth of a meter  
  The positive electrode of a gas laser, used for electrical excitation of the gas in the tube.
  A small opening through which the electromagnetic radiation pass.  
Argon Laser  
  A gas laser in which argon ions are the active medium. This laser emits in the blue - green visible spectrum, primarily at 488(nm) thru 515(nm).
  The decrease in radiation energy (power) as a beam passes through an absorbing or scattering medium.  
Average Power  
  Total energy of an exposure divided by the duration of the exposure.
Aversion Response  
  Action, such as closing of the eye or movement of the head, to avoid exposure to laser light.  


Beam Diameter 
  Defined as the diameter of a circular beam at a certain point where the intensity drop to a fraction of its maximum value. The common definitions are 1/e (0.368) and 1/e2 (0.135) of the maximum value.
Beam Divergence  
  Angle of beam spread, measured in (milli)radians. Can be approximated for small angle by the ratio of the beam diameter to the distance from the laser aperture.  
Brewster Windows  
  Windows at the ends of a gas laser, used to produce polarized electromagnetic radiation. The window is at Brewster angle to the optical axis of the laser, so only one type of polarization can pass through.  
  The visual sensation of the luminous intensity of a light source.


Carbon Dioxide (CO2) Laser
  A gas laser in which CO2 molecules are the active medium. This laser emits in the infrared spectrum, primarily at 9-11 (µm), with the strongest emission line at 10.6 (µm).
  The negative electrode of a gas laser, used for electrical excitation of the gas in the tube.
  A chemical group that absorbs light at a specific frequency and so imparts color to a molecule; also a colored chemical compound.  
  A property of electromagnetic waves which are in phase in both time and space. Coherent light has Monochromaticity and low beam divergence, and can be concentrated to high power densities. Coherence is needed for interference processes like holography.

Continuous Wave (CW)  
  Constant, steady-state delivery of laser power. CW is the abbreviation for continuous wave; the continuous-emission mode of a laser as opposed to pulsed operation.  


  A wave property which create deviation from a straight line when the beam pass near an edge of an opaque object.
Diode Laser    
  See Semiconductor Laser  
Diode Pumped Solid State (DPSS) Lasers    
  Are lasers made by pumping a crystal, for example, a ruby or a neodymium-doped yttrium aluminum garnet (YAG) crystal with a laser diode, usually one that emits infrared light.  
  Increase in beam diameter with distance from the aperture. (See Beam Divergence)


Electric Vector  
  The electric field associated with a light wave which has both direction and amplitude.
Electromagnetic Radiation    
  A wave which propagates in a vacuum at the speed of light, and is composed of simultaneous oscillations of electric field and magnetic field perpendicular to each other, and perpendicular to the direction of propagation of the beam. Created by accelerating electric charge, and include X-rays, visible spectrum, infrared spectrum, microwave etc.  
Electromagnetic Spectrum    
  The range of frequencies and wavelengths emitted by atomic systems. The total spectrum includes radiowaves as well as short cosmic rays. Wavelengths cover a range from 1 hz to perhaps as high as 1020 hz.  
Electromagnetic Wave    
  A disturbance which propagates outward from an electric charge that oscillates or is accelerated. Includes radio waves; X-rays; gamma rays; and infrared, ultraviolet, and visible light.
  Negatively charged particle of an atom.  
Electron Volt (eV)    
  Unit of energy: The amount of energy that the electron acquire while accelerating through a potential difference of 1 (Volt). 1 (eV) = 1.6*10-19 (Joule).  
  Act of giving off radiant energy by an atom or molecule.  
  The ratio of the radiant energy emitted by a any source to that emitted by a blackbody at the same temperature.
  The rate at which emission occurs.
Excimer Laser    
  A gas laser which emits in the UV spectrum. The active medium is an "Excited Dimer" which does not have a stable ground state.  
  Energizing the active medium to a state of population inversion.
Excited State    
  Atom with an electron in a higher energy level than it normally occupies.  


Fan Angle    
  The measure of angular spread of a line-generating laser. The fan angle determines the line length produced at a certain distance. CALPAC line-generating lasers have a 90° fan angle. A 90° fan angle will produce a line length that is 2x the projection distance (e.g. a 10 foot long line when projected from 5 feet).
  10-15 seconds  
  A tube typically filled with Krypton or Xenon. Produces a high intensity white light in short duration pulses.  
  Emission of light of particular wavelength, as a result of absorption of light at shorter wavelength. It is a property of some materials, each material has a specific wavelength of absorption and emission.
  The radiant, or luminous, power of a light beam; the time rate of the flow of radiant energy across a given surface.  
Frequency (n)    
  The number of times that the wave oscillates per second (The number of periods of oscillations per second).  


  See Amplification
Gas Discharge Laser    
  A laser containing a gaseous lasing medium in a glass tube in which a constant flow of gas replenishes the molecules depleted by the electricity or chemicals used for excitation.  
Gas Laser    
  A laser in which the active medium is a gas. The gas can be composed of molecules like CO2, Atoms like Helium-Neon, or ions like Ar+.  
Gaussian Curve Normal    
  Statistical curve showing a peak with even distribution on either side. May either be a sharp peak with steep sides, or a blunt peak with shallower sides. Used to show power distribution in a beam. The concept is important in controlling the geometry of the laser impact.
Ground State    
  Lowest energy level of an atom or molecule.  


Helium-Neon (He-Ne) Laser    
  A gas laser in which Helium (He) and Neon (Ne) atoms are the active medium. This laser emits primarily in the Visible spectrum, primarily at 632.8(nm), but also have some lines in the near Infrared.
  Unit of frequency in the International System of Units (SI), abbreviated Hz; replaces cps for cycles per second.  
  An interference phenomena captured on a plate (or film). It can contain enormous amount of information and a 3 dimensional image can be constructed from it.  


Infrared Spectrum (IR)  
  Invisible electromagnetic radiation between 0.7-1,000(µm).
Injection Laser  
  See Diode Laser.  
Integrated Radiance  
  Product of the exposure duration times the radiance. Also known as pulsed radiance.  
  The magnitude of radiant energy.
Ion Laser  
  A laser in which the active medium is composed of ions of a Nobel gas (like Ar+ or Kr+). The gas is usually excited by high discharge voltage at the ends of a small bore tube.  
Irradiance (E)  
  Radiant flux (radiant power) per unit area incident upon a given surface. Units: Watts per square centimeter. (Sometimes referred to as power density, although not exactly correct).  
  Exposure to radiant energy, such as heat, X-rays, or light.  


Joule (J)  
  A unit of energy (1 watt-second) used to describe the rate of energy delivery. It is equal to one watt-second or 0.239 calorie.
  A unit of radiant exposure used in measuring the amount of energy incident upon a unit area.  


Kelly Sidebands  
  Sidebands in the spectrum of certain mode-locked lasers, related to periodic disturbance of solitons


  An acronym for Light Amplification by Stimulated Emission of Radiation. A laser device is an optical 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 Brewster windows, Q-switches and optical components used to control laser radiation.  
Laser Class  
  In order to regulate laser safety, the Center for Devices & Radiological Health (CDRH) classifies lasers into different categories based on wavelength and output power.  
Laser Diode Module  
  A complete laser assembly including all circuits, a laser diode, and optics packaged in a protective housing. All that is required for operation is an appropriate external power supply.
Laser Medium  
  See Active Medium  
Laser Oscillation    
  The buildup of the coherent wave between laser cavity end mirrors producing standing waves.  
Laser Pulse    
  A discontinuous burst of laser radiation, as opposed to a continuous beam. A true laser pulse achieves higher peak powers than that attainable in a CW output.  
Laser Rod  
  A solid-state, rod-shaped active medium in which ion excitation is caused by a source of intense light (optical pumping), such as a flash lamp. Various materials are used for the rod, the earliest of which was synthetic ruby crystal. See Solid State Laser
Laser Safety    
  A laser is a light source that can be dangerous to people exposed to it. Even low power lasers can be hazardous to a person's eyesight. The coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds. Certain wavelengths of laser light can cause cataracts or even boiling of the vitreous humour, the fluid in the eyeball. Infrared and ultraviolet lasers are particularly dangerous, since the body's "blink reflex", which can protect an eye from excessively bright light, works only if the light is visible.
Laser Safety Officer    
  Individual who has the authority to monitor and enforce the control of lasers  
  A curved piece of optically transparent material which depending on its shape, is used to either converge or diverge light.
  Usually referred to the visible spectrum. The range of electromagnetic radiation frequencies detected by the eye, or the wavelength range from about 400 to 700 nanometers. The term is sometimes used loosely to include radiation beyond visible spectrum limits.  
Limit Accessible Emission Level (AEL)    
  Permitted within a particularly class. In ANSI Z-136.1, AEL is determined as the product of Accessible Emission Maximum Permissible Exposure limit (MPE) and the area of the limiting aperture (7mm for visible and near infrared lasers).  
Limiting Aperture    
  The maximum circular area over which radiance and radiant exposure can be averaged when determining safety hazards.  
Longitudinal (Axial) Modes    
  Specific wavelengths in the laser output, determined by standing waves within the laser cavity. Only longitudinal modes under the laser gain curve, above the laser threshold are found in the laser output.  


Maximum Permissible Exposure (MPE)    
  The level of laser radiation to which person may be exposed without hazardous effect or adverse biological changes in the eye or skin.
Metastable State    
  The upper laser level. An excited state of the atom or molecule, which have a long lifetime.  
  Micro-meter, one millionth of a meter (10-6 [m]).  
Milliamperes (mA)    
  A unit of electrical current equal to one-thousandth of an ampere.
  A unit to measure angles, one thousandth of a radian. 1 milliradian (mrad) = 0.057°.  
Milliwatt (mW)    
  A unit of power equal to one-thousandth of a watt.  
  A term used to describe how the power of a laser beam is geometrically distributed across the cross-section of the beam. Also used to describe the operating mode of a laser such as continuous or pulsed.  
Mode Locked    
  A method of controlling the length of the output laser pulse . Produce very short [10-12 (sec)] burst of pulses.
Monochromatic Light    
  Theoretically, light at one specific wavelength. Practically, light with very narrow bandwidth. The light out of a laser is the most monochromatic source known to man.
  Laser emission at several closely-spaced frequencies.  


Nanometer (nm)    
  One billionth of a meter (10-9 [m]).
  One billionth (10-9) of a second. Longer than a picosecond or femtosecond, but shorter than a microsecond. Associated with Q-switched lasers.  
Nd:Glass Laser    
  A solid-state laser in which a Nd doped glass rod is used as a laser active medium, to produce 1064 (nm) wavelength.  
Nd:YAG Laser    
  A solid-state laser in which Neodymium doped Yttrium Aluminum Garnet is used as a laser active medium, to produce 1064 (nm) wavelength.. YAG is a synthetic crystal.
Neodymium (Nd)    
  The rare earth element that is the active element in Nd:YAG laser and Nd:Glass lasers.  
Nominal Hazard Zone (NHZ)    
  The zone inside which laser radiation that is direct, reflected, or scattered exceeds the MPE for the laser  
Nominal Ocular Hazard Distance (NOHD)    
  Distance along the axis of the direct laser beam to the human eye beyond which the MPE of the laser is not exceeded  


Operating Current    
  The range of specified current required to operate a laser. Laser operating current is measured in milliamperes (mA).
Operating Voltage    
  The range of specified input voltage required to operate a laser. Laser operating voltage is measured in volts (V).  
Optical Cavity (Resonator)    
  Space between the laser mirrors where lasing action occurs.  
Optical Density    
  A logarithmic expression for the attenuation produced by an attenuating medium, such as an eye protection filter.
Optical Fiber    
  A filament of quartz or other optical material, capable of transmitting light along its length by multiple internal reflection and emitting it at the end.  
Optical Pumping    
  The excitation of the active medium in a laser by the application of light, rather than electrical discharge. Light can be from a conventional source like Xenon or Krypton lamp, or from another laser.  
Optical Radiation    
  Ultraviolet, visible and infrared spectrum (0.35-1.4 mm) that falls in the region of transmittance of the human eye.  
Optical 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.
Output Coupler    
  The part of the laser which enable light to come out of the laser. Usually it is a partially reflecting mirror at the end of the laser optical cavity.
Output Power    
  The energy per second (measured in Watts) emitted from the laser in the form of coherent light.  


  Waves are in phase with each other when all the troughs and peaks coincide and are "locked" together. The result is a reinforced wave in increased amplitude (brightness).
  The elemental unit of light. Quantum of light with energy (E) proportional to the wavelength (l) (lambda) (or frequency f). E = hf = hc / l (lambda). ( l (lambda) = wavelength, c = speed of light, h = Planks constant).  
Photorefractive Keratectomy    
  PRK, an acronym for PhotoRefractive Keratectomy, is an eye surgery procedure intended to reduce a person's dependency on glasses or contact lenses. PRK is a procedure that permanently changes the shape of the central cornea using an excimer laser to ablate a small amount of tissue from the front of the eye, just under the corneal epithelium. A computer system tracks the patient's eye position 4,000 times per second, redirecting laser pulses for precise placement. PRK is also known as LASEK; Laser-Assisted Sub-Epithelial Keratectomy.  
  A period of time equal to 10-12 seconds.
  Vibration of the electric field vector in specific direction perpendicular to the direction of propagation of the wave.  
Population Inversion    
  An excited state of matter, in which more atoms (or molecules) are in upper state than in a lower one. This is a required situation for a laser action.  
  The rate of energy delivery in a unit of time, expressed in Watts (Joules per second). Thus: 1 (Watt) = 1 (Joule)/1 (sec).  
Pulse Duration    
  The "On" time of a pulsed laser.
Pulsed Laser    
  Laser which delivers energy in the form of a single or train of laser pulses.
  Addition of energy (thermal, electrical, or optical) into active laser medium. Used to produce a state of population inversion. See Optical Pumping  


Q-Switch Laser    
  A laser which store energy in the active medium, to produce short pulse with high energy. It is done by blocking the resonator ability to oscillate, keeping the "Q-Factor" of the optical cavity low.


  A unit of measurement of angles. 2 (rad) = 360°, 1 (rad) = 57.3°.
  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.  
Radiant Power    
  See radiant flux  
  In the context of optics, electromagnetic energy is released; the process of releasing electromagnetic energy.
  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).  
  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.
Ruby Laser    
  The first laser type. A solid state laser which use a crystal of sapphire (aluminum oxide) containing trace amounts of chromium oxide as an active medium.  


Scanning Laser    
  A laser having a time-varying direction, origin or pattern of propagation with respect to a stationary frame of reference.
Selective Photothermolysis    
  The process in which transfer of laser energy is restricted to a particular site because of the selective absorption of a chromophore at that site. In other words, proper selection of the wavelength and exposure time damages only the desired target tissue.  
Semiconductor Laser    
  A type of laser which produces its output from semiconductor materials such as GaAs. See Diode Laser  
Solid Angle    
  The ratio of the area on the surface of a sphere to the square of the radius of that sphere. It is expressed in steradians (sr).
Solid State Laser    
  A laser in which the active medium is in solid state (usually not including semiconductor lasers).  
Spectral Response    
  The response of a device or material to monochromatic light as a function of wavelength.  
Specular Reflection    
  Mirror-like reflection  
Spontaneous Emission    
  Random emission of a photon by decay of an excited state to a lower level. Determined by the lifetime of the excited state.
Spot Size    
  A measure of the diameter of the beam of laser radiation.
Steradian (sr)    
  The unit of measure for a solid angle.  
Stimulated Emission    
  Coherent emission of radiation, stimulated by a photon absorbed by an atom (or molecule) in its excited state.


  The lowest order transverse mode possible. The power distribution across the beam is of a gaussian shape.
Thermal Containment Time    
  The time in which no heat (and hence no thermal effect) is dissipated to surrounding tissue, and is roughly one-quarter of the thermal relaxation time. This TCT defines the ideal pulse width for treating a given chromophore.  
Thermal Relaxation Time    
  The time it takes for a target structure to dissipate 50 percent of the energy absorbed to surrounding tissue, and that this time was roughly equal to the square of the diameter of the target structure.  
Transverse Electro-Magnetic (TEM) Mode    
  Used to designate the shape of a cross section of a laser beam.
Transverse Mode    
  The geometry of the power distribution in a cross section of a laser beam.  
Tunable Dye Laser    
  A laser whose active medium is a liquid dye, pumped by another laser or flash lamps, to produce various colors of light. The color of light may be tuned by adjusting optical tuning elements or changing the dye used.  
Tunable Laser    
  A laser system that can be "tuned" to emit laser light over a continuous range of wavelengths or frequencies.  


Ultraviolet (UV) Radiation    
  Electromagnetic radiation with wavelengths between soft X-rays and visible violet light, often broken down into UV-A (315-400 [nm]), UV-B (280-315 [nm]), and UV-C (100-280 [nm]).


Visible Spectrum    
  Electromagnetic radiation which can be detected by the human eye. It is commonly used to describe wavelengths which lie in the range between 400 nm and 700-780 nm.
Volt (V)    
  The basic unit of electromotive force or potential difference.  


  A unit of power (equivalent to one Joule per second) used to express laser power.
  A unit of irradiance used in measuring the amount of power per area of absorbing surface, or per area of CW laser beam.  
Wavelength (l) (Lamda)    
  The length of the light wave. The shortest distance at which the wave pattern fully repeats itself, usually measured from crest to crest. The wavelength of light in the visible spectrum determines its color. Common units of measurement are the micrometer (micron), the nanometer, and (old unit) the Angstrom unit.  


X-Ray Lasers    
  An X-ray laser is a laser device which can emit in the spectral region of X-rays, i.e., with wavelengths of only a few nanometers. In this extreme spectral region, traditional laser gain media cannot be used. There are basically two different approaches: Free electron lasers consist basically of an undulator through which very high energy electrons are sent, and (usually) a laser cavity. Optical gain in the X-ray region can also be generated in plasmas, which themselves are typically created with laser beams or with electrical discharges. The laser transitions are between excited states of highly charged ions (e.g. Ta45+ for 4.5 nm or Se24+ for 20 nm).


YAG = Yttrium Aluminum  
  A widely used solid-state crystal which is composed of yttrium and aluminum oxides which is doped with a small amount of the rare-earth neodymium.


Zero Dispersion Wavelength  
  A wavelength where the chromatic second-order dispersion of a fiber is zero