LASER: acronym that stands for Light Amplification by the Stimulated Emission of Radiation. Lasers emit light that is monochromatic (a single color or wavelength), coherent (in phase), and collimated.

Fluence: a measure of instantaneous power output from the laser typically measured in Watts

Wavelength: length between two identical points on subsequent electromagnetic waves. The wavelength of a laser determines it’s color and absorption properties.

Irradiance: power per unit area at a treatment surface

Dosing: most common dosing technique in laser therapy is the energy density delivered to the surface of the tissue which is typically expressed in J/cm2.

Coherence: The photons within a laser beam are extremely well organized and directional. This means that all of the photons (energy) have waves that travel in unison – they are highly parallel with a specific wavelength. true laser systems focus all of their energy in one direction in a very concentrated line. a super-luminous diode, on the other hand, diffuses its energy in all directions with only a small percentage of the energy travelling in the direction of the treatment. a true laser system will deliver 90% more power to the treatment area than a super-luminous diode system of exactly the same power rating.

Collimation: A property of light commonly associated with lasers and accomplished with focusing lenses where all the photons are traveling in the same direction.

Continuous Wave (CW): A laser with a continuous output of laser radiation for a duration that is greater than or equal to 0.25 seconds.

Dose: The term dose is an estimate of a therapy which produces a desired therapeutic action without harmful side effects. The therapeutic dose (safe and effective) range is defined by clinical evaluation of the response of a sufficient number of patients, generally 50 percent who improve without toxicity. The most important parameter in laser therapy is always the dose, often referred to as “fluence”. By dose (d) is meant the energy (e) of the light directed at a given unit of area (a) during a given session of therapy. The energy is measured in joules (j), the area in cm2 and consequently, the dose in j/cm2.

Duty Cycle: Relates to the amount of time the light source is active, usually from 10% to 100%. a laser operating in continuous wave is running at 100% duty cycle.

Energy Density: The energy density expresses the total amount of energy delivered per unit area, in joules per square centimeter, j/cm2. The energy is measured in joules, and is calculated by multiplying the power output of the laser times the amount of time elapsed during the laser treatment. (energy = power x time, and the units are joules = watts x seconds.) a 4 watt continuous wave laser would deliver 240 joules in one minute. (4 watts x 60 seconds = 240 joules) then simply divide the total energy by the area to arrive at the energy density in joules per centimeter squared.

Extracellular matrix (ECM) is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.

Extracellular matrix includes the interstitial matrix and the basement membrane. Interstitial matrix is present between various animal cells (i.e., in the intercellular spaces). Gels of polysaccharides and fibrous proteins fill theinterstitial space and act as a compression buffer against the stress placed on the ECM

Frequency: The frequency of light is inversely proportional to its wavelength, and is dependent upon the energy value of the individual photons being emitted. The higher the frequency, the higher the energy, and the shorter the wavelength.

Heat shock proteins (HSP): are a group of proteins induced by heat shock, the most prominent members of this group are a class of functionally related proteins involved in the folding and unfolding of other proteins. Their expression is increased when cells are exposed to elevated temperatures or other stress. This increase in expression is transcriptionally regulated. The dramatic upregulation of the heat shock proteins is a key part of the heat shock response and is induced primarily by heat shock factor (HSF). HSPs are found in virtually all living organisms, from bacteria to humans.

Infrared Radiation (IR): This is invisible radiation of wavelengths from 700nm – 1mm. this part of the electromagnetic spectrum is broken down into 3 bands: near infrared (IR-A) 700nm – 1400nm, mid infrared (IR-B) 1400nm – 3,000nm, and far infrared (IR-C) 3,000nm – 1mm.

Irradiance: The power per unit area expressed in watts per square centimeter (w/cm2). It is also referred to as power density and applies to cw lasers.

Laser Diode: A semiconducting device which emits monochromatic non-ionizing radiation by a process of stimulated emission. a laser beam has a number of unique properties, such as coherence, polarization and directionality. Beams emitted by laser diodes are not, as is often stated, ‘straight’ and/or ‘parallel’. Unless manipulated with additional optical devices such as lenses, a laser diode’s beam is broadly divergent along one plane and narrowly divergent along the perpendicular plane, producing an elliptical cross-section.

Laser Safety Officer (LSO): The LSO is responsible for monitoring the control of laser use and implementing the laser safety program.

Laser: Light amplification by stimulated emission of radiation; refers to the specific qualities and methods by which lasers produce light. Originally theorized and defined by Albert Einstein in 1917, it was not produced until the 1950s. Laser light is coherent, has a monochromatic wavelength, is collimated, and polarized. These four characteristics differentiate lasers from LED&SLD light sources.

Light: Is a small spectrum of electromagnetic energy with wavelengths between 380 nanometers (nm) and 760nm in length. This spectrum of energy is visible to the naked eye.

Maximum Permissible Exposure (MPE): The maximum level of laser radiation to which a human can be exposed without harmful effects to the eye or skin.MPE values for eye exposure to direct beam viewing can be found in table 5 of ANSI Z136.1 Standard.

Monochromatic: Contains one specific wavelength of light (one specific color). It is an exclusive property of laser light, setting them apart from all other light sources. Because the wavelength of laser light determines its effect on tissue, the monochromatic property of laser light allows energy to be delivered to specific tissues in specific ways. Non-laser therapies such as LED’s (light emitting diodes) are sufficient for superficial treatment (wounds), but are questionable on penetration for musculoskeletal conditions. Lasers penetrate deeper.

Nominal Hazard Zone (NHZ): An area where the MPE is exceeded for the laser radiation emitted.

Optical Density (OD): is the base ten logarithm of the reciprocal of the transmittance. The OD is calculated for protective eyewear to reduce the transmission density to the safe MPE level.

Penetration: Refers to the distance an energy wave travels into the tissue before it is absorbed and dissipated as heat or molecular vibration. Penetration is a physical and thermal phenomenon, not a therapeutic phenomenon. Penetration of laser light is dependent on the wavelength of the light. Lower wavelengths are absorbed by hemoglobin and melanin, and higher wavelengths are absorbed by water in the tissues.

Photobiomodulation: When biomodulation occurs from a photon transferring its energy to a chromophore it is referred to as photobiomodulation.

Physiological Dose of Therapy: A physiological dose of any therapy is designed to stimulate production of, or provide to the body what it needs to normalize and heal itself through biomodulation. The symptomatic response to a physiological dose of therapy is dependent of the capacity of the patient’s body to respond to the therapy. The physiological dose of any treatment has specific advantages. A physiological dose represents the body’s own response to a stimulus. A physiological dose generally improves the patient’s health.

Power Density: Is amount of power delivered per unit area. Power density indicates the degree of concentration of the laser output. it is expressed in watts per square centimeter, or milliwatts per square centimeter, w/cm2 or mw/cm2. Some studies have concluded that the power density may be of even greater significance than the dose. Example: a laser’s output is 4 watts, and it is illuminating a circle of 3 centimeter diameter. first find the area of the circle, 3.14 x 1.5 x 1.5 = 7 cm2. Then divide the power by the area, 4w / 7cm2 = 0.6 w/cm2.

Power = energy / time 1 watt = 1 joule / second. It is important not to confuse power and energy, although they are closely related. Power is the rate at which energy is delivered, not an amount of energy itself.

Pulsed (Simulated): In most modern therapeutic lasers, the pulsing is simulated by mechanically or electronically interrupting the output of a continuous beam laser. The pulse rate may be adjusted up or down without significantly affecting treatment time. This is accomplished by modulating pulse duration and/or the space between pulses.

Pulsed Laser: A laser that delivers energy in single or multiple pulses which are less than or equal to 0.25 seconds in duration.

Radiant Exposure: radiant energy per unit area expressed in joules per square centimeter (j/cm2). Radiant exposure applies to pulsed lasers.

Ultraviolet Radiation (UV): Invisible radiation that has wavelengths from 180nm – 400nm.UV radiation is broken down into 3 regions; near ultraviolet (UV- A)-315nm – 400nm, mid ultraviolet (UV-B)-280nm – 315nm, and far ultraviolet (UV-C)-100nm – 280nm.

Visible Radiation: Is radiation that is visible to the human eye. The wavelengths are from 400nm – 700nm. At these wavelengths the eye can focus the light onto the retina increasing the radiant exposure by 100,000 times.

Wavelength: the property that differentiates different spectrums of energy within the electromagnetic spectrum of energy is wavelength. The wavelength of light is measured in billionths of a meter, or nanometers (nm). The energy of a wave is inversely proportional to its wavelength. In other words, the greater the energy, the shorter (smaller) the wavelength. Light of shorter wavelength carries greater the energy of the light. As wavelength becomes longer, the energy carried is less. Some wavelengths work better than others for therapy.

Wavelength is the prime determinant of tissue penetration. The wavelength is very specific for cell absorption. In the infrared (IR) spectrum, the longer wavelengths penetrate deeper and a greater percentage of the laser light will be transmitted in a forward direction. This means less scatter and better results. Each photon contains energy and just as energy of the ocean comes to shore in waves of high and low energy, the same is true of photons. Only with photons the energy is not measured by the height of the wave but the number of waves the photon carries. These waves are measured in two ways, the number of waves that will pass a given point in one second, or wavelength, the distance between one wave and the next.


CW Continuous Wave
PW Pulsed Wave
SP Super Pulsed
J Joule
W Watts
mW milliWatts
s Second
mS Millisecond
µs Microsecond
ºC Degrees Celsius
A Amperes
mA Milliamp
Hz Hertz
AC Alternating Current
cm Centimeter
mm Millimeter
nm Nanometer
µm Micrometers


LLLT: Low-level Laser Therapy
LILT: Low-intensity Laser Therapy
Cold Laser Therapy
Soft Laser Therapy
Low Reactive Laser Therapy
Low Power Laser Therapy
Light Emitting Diode Therapy
Low Energy Photon Therapy
Biostimulation Laser
Therapeutic Laser
Bioregulating Laser
Medical Laser
Class III Laser Therapy
Class IV Laser Therapy
HPLT: High Power Laser Therapy
DTL: Deep Tissue Laser
Photobiomodulation Therapy
HILT: High Intensity Laser Therapy

American National Standards Institute,

ANSI Z136.1 “safe use of lasers”. This standard establishes occupational exposure limits and laser safety practices in the United States.