Laser Helpers

LSO’s Abridged Dictionary

in Laser Safety

An LSO (Laser Safety Officer) may operate in a multitude of environments, including: health care facilities, university or corporate research & development groups, industrial manufacturing, and military installations. Laser safety nomenclature, however, is common to all.  Every LSO should be familiar with these frequently encountered terms.

Ablation:   The removal of any material (e.g., skin tissue, paint, ceramic film, etc.) by means of its interaction with laser energy or power.

Accessible Emission Limit (AEL):  The maximum accessible emission level permitted within a particular laser hazard class.
Note:  The laser classifications consist of: Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3B, and Class 4. Generally speaking, the potential beam hazard level increases in the same order.

Alpha Max:  The angular subtense of an extended source beyond which additional subtense does not contribute to the hazard and need not be considered. This value is 100 milliradians (mrad) for retinal thermal effects and 110 mrad for retinal photochemical effects. Symbol: αmax.

Alpha Min:  The angular subtense of a source below which the source can effectively be considered as a point source. The value of alpha min is equal to or less than (≤) 1.5 mrad. Symbol: αmin.

Average Power:  The rate at which energy (joules per second) is emitted, transferred or absorbed with its measurements described by the unit measurement of watts (W).

Aversion Response:  Any motion(s) including: eyelid closure, eye movement, pupillary constriction, head movement to avoid exposure to noxious or bright visual light stimulants. In application to visible laser light, its exposure to critical retinal area(s) is established as 0.25 seconds or less.

Barrier:  Any device(s) employed to partially (e.g., laser attenuative translucent window materials) or fully (e.g., opaque curtains) diminish or restrict the transmission of incipient laser light.

Beam Divergence:  The increase in the diameter of the laser beam with distance from the exit aperture, based on the full angle at the point where the irradiance (or radiant exposure for pulsed lasers) is 1/e times the maximum value. Symbol: Ф

Continuous Wave (CW) Laser:  A laser operating at a continuous output for a period of time equal to or greater than (≥) 0.25 seconds.

Controlled (Laser) Area:  Any area where the occupancy or activity of those within its confines is subject to access control and/or supervision for the purpose of protection from laser radiation hazards.

Critical Frequency:  A pulse repetition frequency above which the laser is considered to operate as a CW laser.
Example: for a short (0.25 s to 10 s) unintentional exposure to nanoseconds (or longer) pulses, the Critical Frequency is 55 kilohertz (kHz) for wavelengths between 400 nanometers (nm) to 1050 nm and 20 kHz for wavelengths between 1050 nm to 1400 nm.

Diffuse Reflection:  A change in the spatial distribution of the laser beam by virtue of its reflective interaction in many directions by a non-specular (non-mirror like) surface or other reflecting medium.

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 maximum permissible exposure (MPE) than possible with the protective housing in place.
Example: an embedded laser is one type of enclosed laser.

Engineering Control Measures:  The primary and preferred method to ensure safe laser operation. They are comprised of measures designed, engineered and incorporated into a laser or laser system which require minimal to no training for individuals who may be exposed to laser output.
Examples: protective housings, interlocks on removable protective housings, activated shutters, watch-dog timers, etc.

Extended Source:  A source of optical radiation with an angular subtense at the cornea larger than αmin. Also, see Point Source.

Focal Length:  The distance from the secondary nodal point of a lens to the secondary focal point. For a thin lens imaging a distant source, 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.

Hertz (Hz):  The unit of measurement which expresses the frequency of a periodic oscillation in cycles per second.

Infrared (IR):  The region of the electromagnetic spectrum between the long-wavelength extreme of the visible spectrum (~700nm) and the shortest microwaves (~1mm).
Note: the International Commission on Illumination (CIE) recommended the following division:
IR-A: 700nm – 1400nm
IR-B: 1400nm – 3000nm
IR-C: 3000nm – 1mm

Intrabeam Viewing:  The viewing condition within which the eye is exposed to all or part of a laser beam.

Irradiance:  Radiant power incident per unit area upon a surface, expressed in watts-per-centimeter-squared (W•cm-2). Symbol: E

Joule:  A unit of energy. 1 joule = 1 Newton • meter; 1 watt • second. Joule is a measurement frequently given for laser output in pulsed operation.

Laser Classification:  An indication of the beam hazard level of a laser or laser system during normal operation or the determination (typically established by its AEL) thereof. The classifications of which are cited in Accessible Emission Limit (AEL) above.

Laser Safety Officer (LSO):  One who possess the authority and responsibility to monitor and to enforce the control of laser hazards and to effect the knowledgeable evaluation and control of laser hazards in order to establish a laser or laser system’s safe use.

Limiting Aperture Diameter:  The diameter of a circle over which irradiance or radiant exposure is averaged for purposes of hazard evaluation and classification. Symbol: Df.

Maximum Permissible Exposure (MPE):  The level of laser radiation to which an unprotected person may be exposed without adverse biological changes to the eye or skin.

Nominal Hazard Zone (NHZ):  The space within which the level of the direct, reflected or scattered radiation may exceed the applicable MPE. Exposure levels beyond the boundary of the NHZ are below the appropriate MPE.

Nominal Ocular Hazard Distance (NOHD):  The distance along the axis of an unobstructed laser beam from a laser, a fiber end, or connector to the human eye beyond which the irradiance or radiant exposure is not expected to exceed the applicable MPE.

Non-Beam Hazard:  A class of hazards that result from factors other than direct human exposure to a laser beam.
Examples: electrical, resistive heating, collateral and plasma radiation, fire, robotic mechanical hazards, etc.

Optical Density (OD):  the logarithm to the base ten of the reciprocal of the transmittance at a particular wavelength, where: Dλ = log10 (1/τ λ) where τ λ is the transmittance at the wavelength of interest. Symbol: D(λ), D λ or OD. Optical Density is commonly used to describe the protection factor provided by a filter (such as used in eyewear, viewing windows, etc.) at a specific wavelength. Each incremental unit of OD (1, 2, 3….etc.) represents a 10x increase in protection.

Point Source:  A source with an angular subtense at the cornea equal to or less than alpha-min (αmin), i.e., ≤ 1.5 mrad. Also see Extended Source.

Pulse Duration:  The duration of a laser pulse, typically measured as the time interval between the half-power points on the leading and trailing edges of the pulse.
Typical units:
Microsecond (µs) = 10-6 s
Nanosecond (ns) = 10-9 s
Picosecond (ps) = 10-12 s
Femtosecond (fs) = 10-15 s

Radian:  A unit of angular measurement equal to the angle subtended at the center of a circle by an arc whose length is equal to the radius of the circle.
Example: 1 radian = ~ 57.3 degrees; 2π radians = 360 degrees

Shall:  As used in any American National Standard Institute (ANSI) Z136.x document, the word is to be understood as implementing a mandatory response.

Should:  As used in any ANSI Z136.x document the word is to be understood as implementing an advisory response.

Threshold Limit:  This term is applied to laser protective eyewear (LPE) filters, protective windows and barriers. The TL is an expression of the “resistance factor” for beam penetration of a laser protective device expressed in W•cm-2 or J•cm-2. It is taken to be the maximum average irradiance or radiant exposure at a given beam diameter for which a laser protective device (e.g., barriers, LPE, curtains, windows, etc.) provides adequate beam resistance. Thus, laser exposures delivered on the protective device(s) at or below the TL will limit beam penetration to levels at or below the applicable MPE.

Watt (W):  The unit of power or radiant flux. 1 watt = 1 joule-per-second.

Wavelength:  The distance in the line of advance of a sinusoidal wave from any one point to the next point of corresponding phase (e.g., the measured distance from one peak to the next peak). Wavelength is the fundamental property of light—the length of the light wave, which determines its color. Common units of measurement (which is usually from crest to crest) are the micron, the nanometer, and (earlier) the angstrom.