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Naval Laser Safety Glossary

            These definitions are solely intended for the use in laser safety.


A-C    D-I    J-P    Q-S    T-Z

absorption

Transformation of radiant energy to a different form of energy by interaction with matter.

accessible emission limit (AEL)

The maximum accessible emission level permitted within a particular laser hazard class.

accessible optical radiation

Optical radiation to which the human eye or skin may be exposed for the condition (use, maintenance or service) specified.

amax (alpha max)

The angular limit beyond which extended source MPEs for a given exposure duration are expressed as a constant radiance or integrated radiance.

amin (alpha min)

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

aperture

An opening (typically circular) through which radiation can pass.

apparent visual angle

The angular subtense of the source as calculated from source size and distance from the eye - not a synonym for beam divergence.

attenuation

The decrease in the radiant flux (power) as radiation passes through an absorbing or scattering medium.

average power

The total energy in an exposure or emitted divided by the duration of the exposure or emission.

aversion response

Closure of the eyelid, or movement of the head to avoid an exposure to a bright light.

beam

A collection of rays characterized by direction, diameter (or dimensions) and divergence (or convergence).

beam diameter (1/e)

The distance between diametrically opposed points in that cross-section of a beam where the power per unit area is 1/e (0.368) times that of the peak power per unit area.

beam divergence or
beam spread

The full angle, expressed in radians, of the beam spread measured between those points which include laser energy or irradiance equal to 1/e of the maximum value.

blink reflex

Closure of the eyelid, or movement of the head to avoid an exposure to a bright light

CA

Correction factor which increases the MPE values in the near infrared (IR-A) spectral band (700-1400 nm) based upon reduced absorption properties of melanin pigment granules found in the skin and in the retina.

CB

Correction factor which increases the MPE values in the red end of the visible spectrum (550-700 nm), because of greatly reduced photochemical hazards.

CC

Correction factor which increases the MPE values for ocular exposure because of pre-retinal absorption of radiant energy in the spectral region between 1150 and 1400 nm.

CE

Correction factor used for calculating the extended source MPE for the eye from the small source MPE, when the laser source subtends a visual angle exceeding  amin.

CP

Correction factor which reduces the MPE for repetitively pulsed exposure of the eye.  This factor is not used for the skin.

carcinogen

An agent potentially capable of causing cancer.

coagulation

The process of congealing by an increase in viscosity characterized by a condensation of material from a liquid to a gelatinous or solid state.

coherent

A light beam is said to be coherent when the electric vector at any point in it is related to that at any other point by a definite, continuous function.

collateral radiation

Any electromagnetic radiation, except laser radiation, emitted by a laser or laser system which is physically necessary for its operation.

collecting optics

Lenses or optical instruments (such as telescopes, binoculars, or microscopes) having magnification and thereby producing an increase in energy or power densities.

collimated beam

A nearly ôparallelö (very low divergence or convergence) beam of light.

continuous wave (CW)

The output of a laser which is operated in a continuous (duration >0.25 s) rather than a pulsed mode.

controlled area

An area where the occupancy and activity of those within it is subject to control and supervision.

cornea

The transparent outer coat of the human eye which covers the iris and the crystalline lens.  The cornea is the main refracting element of the eye.

critical frequency

The pulse repetition frequency above which the laser output is considered continuous wave (CW).

denaturation

Functional modification of the properties of protein by structural alteration via heat or photochemical processes.

depigmentation

The removal of the pigment of melanin granules from human tissues.

diffuse reflection

Change of the spatial distribution of a beam of radiation when it is reflected in many directions by a surface or by a medium.

diopter

A measure of the power of a lens, defined as 1/fo, where fo is the focal length of the lens in meters.

divergence

The full angle, expressed in radians, of the beam spread measured between those points which include laser energy or irradiance equal to 1/e of the maximum value.

effective energy (Qeff)

Energy, in joules, through the applicable measurement aperture.

effective magnification (Meff)

The optical magnification used in laser safety calculations for aided viewing conditions. It is the reduction in beam diameter from an optical viewing system that increases the hazard.

effective power ( Feff)

Power, in watts, through the applicable measurement aperture.

electromagnetic radiation

The flow of energy consisting of orthogonally vibrating electric and magnetic fields lying transverse to the direction of propagation.  X ray, ultraviolet, visible, infrared, and radio waves occupy various portions of the electromagnetic spectrum and differ only in frequency, wavelength, or photon energy.

embedded laser

An  enclosed laser with an assigned class number higher than the inherent capability of the laser system in which it is incorporated, where the systems lower classification is appropriate due to the engineering features limiting accessible emission.

enclosed laser

A laser that  is contained within a protective housing of itself or of the laser or laser system in which it is incorporated. Opening or removal of the protective housing provides additional access to laser radiation above the applicable MPE than possible with the protective housing in place. (An embedded laser is an example of one type of enclosed laser).

energy

The capacity for doing work.  Energy content is commonly used to characterize the output from pulsed lasers, and is generally expressed in joules (J).

epithelium (of the cornea)

The layer of cells forming the outer surface of the cornea.

erythema

Redness of the skin due to congestion of the capillaries.

extended source

A source of laser radiation with an angular subtense at the cornea larger than amin.

fail-safe interlock

An interlock where the failure of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode.

focal length

The distance from the secondary nodal point of a lens to the primary focal point.  In a thin lens, the focal length is the distance between the lens and the focal point.

focal point

The point toward which radiation converges or from which radiation diverges or appears to diverge.

fundus

The interior posterior surface of the eye (the retina), as seen upon ophthalmoscopic examination.

funduscopic

Examination of the fundus (rear) of the eye.

half-power point

The value on either the leading or trailing edge of a laser pulse at which the power is one-half of its maximum value.

hertz (Hz)

The unit which expresses the frequency of a periodic oscillation in cycles per second.

infrared

The region of the electromagnetic spectrum between the long-wavelength extreme of the visible spectrum (about 0.7 mm) and the shortest microwaves (about 1 mm).

infrared radiation

Electromagnetic radiation with wavelengths which lie within the range 0.7 mm to 1 mm.

integrated radiance

The integral of the radiance over the exposure duration, expressed in joules-per-square-centimeter -per-steradian  (J cm-2 sr-1).

intrabeam viewing

The viewing condition whereby the eye is exposed to all or part of a laser beam.

ionizing radiation

Electromagnetic radiation having a sufficiently large photon energy to directly ionize atomic or molecular systems with a single quantum event.

iris

irradiance

Radiant power incident per unit area upon a surface, expressed in watts-per-square-centimeter   (W cm-2).

Jaeger's test

Samples of type of various sizes printed on a card for testing close visual acuity.  An analogue of the Snellen chart for distant visual acuity.

joule

A unit of energy.  1 joule = 1 watt x 1 second.

Lambertian surface

An ideal surface whose emitted or reflected radiance is independent of the viewing angle.

laser

A device that produces radiant energy predominantly by stimulated emission.  Laser radiation may be highly coherent temporally, or spatially, or both.  An acronym for Light Amplification by Stimulated Emission of Radiation.

laser diode

A laser employing a forward-biased semiconductor junction as the active medium.  Synonyms: injection laser; semiconductor laser.

laser system safety officer (LSO or LSSO)

One who has authority to monitor and enforce the control of laser hazards and effect the knowledgeable evaluation and control of laser hazards.

laser system

An assembly of electrical, mechanical, and optical components which includes a laser.

lesion

An abnormal change in the structure of an organ or part due to injury or disease.

light (visible radiation)

Electromagnetic radiation which can be detected by the human eye.  This term is commonly used to describe wavelengths which lie in the range 0.4 to 0.7 mm.

amin (limiting angular subtense)

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

limiting aperture

The maximum diameter of a circle over which radiance and radiant exposure is averaged for purposes of hazard evaluation and classification.

Tmax (limiting exposure duration)

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

lossy medium

A medium which absorbs or scatters radiation passing through it.

macula

The small uniquely pigmented specialized area of the retina of the eye which in normal individual is predominantly employed for acute central vision, i.e. area of best visual acuity.

maximum permissible exposure (MPE)

The level of laser radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin.

minimum viewing distance

The minimum distance at which the eye can produce a focused image of a diffuse source, usually assumed to be 10 cm.

nominal hazard zone (NHZ)

The nominal hazard zone describes the space within which the level of the direct, reflected or scattered radiation during normal operation exceeds the applicable MPE. Exposure levels beyond the boundary of the NHZ are below the appropriate MPE level.

nominal ocular hazard distance (NOHD)

The distance along the axis of the unobstructed beam from a laser, fiber end, or connector to the human eye beyond which the irradiance or radiant exposure, during installation or service, is not expected to exceed the appropriate MPE.

ocular fundus

The interior posterior surface of the eye (the retina), as seen upon ophthalmoscopic examination.

optically aided viewing

Viewing the laser source with an optical device such as an eye loupe, hand magnifier, microscope, binoculars, telescope, etc.  Optically aided viewing does not include viewing with corrective eyewear or with indirect image converters.

optical density (OD)

Logarithm to the base ten of the reciprocal of the transmittance.  OD is a physical property of a substance.

photophobia

An unusual intolerance of light.  Also, an aversion to light usually caused by physical discomfort upon exposure to light.

photosensitizers

Substances which increase the sensitivity of a material to irradiation by electromagnetic energy.

pigment epithelium (of the retina)

The layer of cells which contain brown or black pigment granules next to and behind the rods and cones.

plasma radiation

Black body radiation generated by luminescence of matter in a laser-generated plume.

point source

A source with an angular subtense at the cornea equal to or less than amin.

power

The rate at which energy is emitted, transferred, or received.

power density

Radiant power incident per unit area upon a surface, expressed in watts-per-square-centimeter   (W cm-2).

protective housing

An enclosure that surrounds the laser or laser system that prevents access to laser radiation above the applicable MPE level.  The aperture through which the useful beam is emitted is not part of the protective housing.

pulse duration

The duration of a laser pulse; usually measured as the time interval between the half-power points on the leading and trailing edges of the pulse.

pulse-repetition frequency (PRF)

The number of pulses occurring per second, expressed in hertz (Hz).

pulsed laser

A laser which delivers its energy in the form of a single pulse or a train of pulses when the pulse duration is <0.25 s.

pupil

The variable aperture in the iris through which light travels to the interior of the eye.

Q-switch

A device for producing very short (~10-250 ns), intense laser pulses by enhancing the storage and dumping of electronic energy in and out of the lasing medium, respectively.

Q-switched laser

A laser that emits short (~10-250 ns), high power pulses by means of a Q-switch.

radian (rad)

A unit of angular measure equal to the angle subtended at the center of a circle by an arc whose length is equal to the radius of the circle.  2p radians = 360 degrees.

radiance

Radiant flux or power output per unit solid angle per unit area expressed in Watts per centimeter squared per steradian  (W cm-2 sr-1).

radiant energy

Energy emitted, transferred, or received in the form of radiation.

radiant exposure

Surface density of the radiant energy received, expressed in units of joules per centimeter squared (J x cm-2).

radiant flux

Power emitted, transferred, or received in the form of radiation.

radiant intensity

Quotient of the radiant flux leaving a source and propagated into an element of solid angle containing the direction, by the element of solid angle.  Radiant intensity is expressed in units of watts per steradian (W x sr-1).

Rayleigh scattering

Scattering of radiation in the course of its passage through a medium containing particles whose sizes are small compared with the wavelength of the radiation.

reflectance (reflectivity)

The ratio of total reflected radiant power to total incident power.

reflection

Deviation of radiation following incidence on a surface.

refraction

The bending of a beam of light in transmission through an interface between two dissimilar media or in a medium whose refractive index is a continuous function of position (graded index medium).

refractive index (index of refraction)

Denoted by n, the ratio of the velocity of light in vacuum to the phase velocity in the medium.

repetitively pulsed laser (RPL)

A laser with multiple pulses of radiant energy occurring in a sequence.

retina

The sensory membrane which receives the incident image formed by the cornea and lens of the human eye.

retinal hazard region

Optical radiation with wavelengths between 0.4 mm and 1.4 mm, where the principal hazard is usually to the retina.

safety latch

A mechanical device designed to slow direct entry to a controlled area.

scanning laser

A laser having a time-varying direction, origin, or pattern of propagation with respect to a stationary frame of reference.

scintillation

The rapid changes in irradiance levels in a cross-section of a laser beam due to atmospheric conditions.

secured enclosure

An enclosure to which casual access is impeded by an appropriate means, e.g., a door secured by a magnetically or electrically operated lock or latch, or by fasteners that need a tool to remove.

small source

A source with an angular subtense at the cornea equal to or less than amin.

small-source viewing

The viewing condition whereby the angular subtense of the source,  amin, is equal to or less than the limiting angular subtense,  amin.

solid angle

The three-dimensional angular spread at the vertex of a cone measured by the area intercepted by the cone on a unit sphere whose center is the vertex of the cone.  Solid angle is expressed in steradians (sr).

source

A laser or a laser-illuminated reflecting surface.

spectator

An individual who wishes to observe or watch a laser or laser system in operation, and who may lack the appropriate laser safety training.

specular reflection

A mirror-like reflection.

stromal haze (of the cornea)

Cloudiness in the connective tissue or main body of the cornea.

surface exfoliation (of the cornea)

A stripping or peeling off of the surface layer of cells from the cornea.

synergism

A condition in which the combined effect is greater than the sum of the effects of individual contributors.

T1

The exposure duration (time) at which MPEs based upon thermal injury are replaced by MPEs based upon photochemical injury to the retina

Tmax (limiting exposure duration)

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

tmin

For a pulsed laser, the duration for which the MPE is the same as the MPE for 1 ns.  For thermal biological effects, this corresponds to the ôthermal confinement durationö during which heat flow does not significantly change the absorbed energy content of the thermal relaxation volume of the irradiated tissue (Example: tmin is 18 ms in the spectral region 0.4 to 1.05 mm and is 50 ms between 1.050 and 1.400 mm).

transmission

Passage of radiation through a medium.

transmittance

The ratio of transmitted power to incident power.

ultraviolet radiation

Electromagnetic radiation with wavelengths shorter than those of visible radiation; for the purpose of this standard, 0.18 to 0.4 mm.

uncontrolled area

An area where the occupancy and activity of those within is not subject to control and supervision for the purpose of protection from radiation hazards.

visible radiation (light)

Electromagnetic radiation which can be detected by the human eye.  This term is commonly used to describe wavelengths which lie in the range 0.4 to 0.7 mm.

watt

The unit of power or radiant flux.

wavelength

The distance between two successive points on a periodic wave which have the same phase.