SECTION 6: GLOSSARY

absorption

Radiation imparts some or all of its energy to any material through which it passes

accessible emission limit (AEL)

Maximum accessible emission level which is permissible in the appropriate laser class

accessible radiation

Laser radiation that can expose human eye or skin in normal usage

aperture

The opening through which laser radiation can pass

attenuation

The decrease in radiant flux as it passes through an absorbing or scattering medium

aversion response

Action, such as closing of the eye or movement of the head, to avoid exposure to laser light.

beam

A collection of rays which may be parallel, divergent, or convergent.

beam diameter

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

beam divergence

The full angle of the beam spread between diametrically opposed 1/e irradiance points; usually measured in milliradians (1 milliradian = 3.4 minutes of arc)

continuous wave

The output of a laser, operated in a continuous rather than pulsed mode.

controlled area

An area where the occupancy and activity of those within is subject to control and supervision for the purpose of protection from laser radiation and related hazards.

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.

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.

Embedded laser

A laser enclosed in a laser system, having an assigned class number higher than the inherent capability of the laser system.  The laser system’s lower classification is appropriate because of the engineering features that limit accessible emission.

Enclosed laser

A laser contained in a protective housing.  Opening or removing the protective housing provides additional access to laser radiation above the applicable MPE.  (An embedded laser is a type of enclosed laser.)

Energy (Q)

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

Erythema

The medical term for redness of the skin due to congestion of the capillaries.

Failsafe 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.

Infrared radiation

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

Intrabeam viewing

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

Iris

The circular pigmented membrane that lies behind the cornea of the human eye. 

Irradiance (E)

Power per unit area, expressed in watts per square centimeter.

Joule (J)

A unit of energy (1 joule = 1 watt*second).

Laser

A device that produces an intense, coherent, directional beam of light by stimulated electronic or molecular transitions to lower energy levels.  Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.

Laser Safety Officer

Individual with the 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 that includes one or more lasers.

Macula

The small, uniquely pigmented and specialized area of the retina.

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.  MPE is expressed in terms of either radiant exposure (joules/cm²) or irradiance (watts/cm²). 

Nominal Hazard Zone (NHZ)

Describes the space within which the level of the direct, reflected, or scattered radiation during normal operation exceeds the MPE.  Exposure levels beyond the boundary of the NHZ are below the appropriate MPE level.

Ocular fundus

The back of the eye.  May be seen through the pupil by use of an ophalmoscope.

Optical density (D_l)

Logarithm to the base ten of the reciprocal of the transmittance: D_l = - log_T , where T is transmittance.

Power

The rate at which energy is emitted, transferred, or received, usually expressed in watts (joules per second).  Also called radiant power.

prf

Abbreviation for pulse repetition frequency.  (See repetitively pulsed laser.)

protective housing

An enclosure that surrounds a laser or laser system, preventing access to laser radiation above the applicable MPE level.

pulse duration

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.

Pulsed laser

A laser that delivers its energy in the form of a single pulse or a train of pulses which are less than or equal to 0.25 s.

Q-switch

Device that produces very short (~10-250 ns) intense laser pulses by enhancing the storage and dumping of electronic energy in and out of the lasing medium.

Q-switched laser

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

Radiant energy or flux (D)

Laser energy emitted, expressed in joules.

Radiant exposure (H)

Surface density of the radiant energy received, expressed in joules per cm².

Radiant power

Laser power emitted, expressed in watts

reflection

Deviation of radiation following incidence on a surface.

Repetitively pulsed laser

A laser with multiple pulses of radiant energy occurring in sequence with a prf of 1 Hz.

Retina

The sensory membrane that receives the incident image formed by the cornea and lens of the human eye.  The retina lines the inside of the eye.

Specular reflection

A mirror-like reflection.

Transmittance (T)

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

Ultraviolet Radiation

Electromagnetic radiation with wavelengths smaller than visible radiation.

viewing portal

An opening in an experimental system, allowing the user to observe the experimental chamber.  All viewing portals and display screens included as an integral part of a laser system must incorporate a suitable means to maintain the laser radiation at the viewing position at or below the applicable MPE (eye safe) for all conditions of operation and maintenance.  It is essential that the material used for viewing portals and display screens not support combustion or release toxic vapors following exposure to laser radiation.

Visible Radiation (light)

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

watt (W)

Unit of power or radiant flux (1 watt = 1 joule per second).

wavelength (g)

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

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