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Laser Safety - Glossary of Terms

Glossary of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Absorption

That light which when incident upon the filter material is neither reflected or transmitted. The degree of absorption of laser radiation in a filter material ultimately determines the maximum power density that the filter can handle.

AEL

Acronym: Accessible Exposure Limit. The maximum accessible emission level permitted within a particular laser class, as defined in EN 60825.

ANSI Z136.1 (2000)

The US standard pertaining to the safe use of lasers. In Europe, the equivalent standard in EN 60825.

Blink Reflex

The human eye's automatic aversion response if subjected to bright light - the eyelid can close within 0.25 seconds. The blink reflex affords protection against low powered, visible lasers such that laser safety eyewear is not required.

Class 1

A laser class defined in EN 60825 for those lasers which do not produce dangerous radiation. Such lasers are "eye safe" and protective equipment is not required.

Class 1M

A laser class defined in EN 60825 for those lasers which do not produce dangerous radiation unless viewed through an optical instrument (for example, a microscope or magnifying glass).

Class 2

A laser class defined in EN 60825 for those lasers which are eye safe as a result of normal human aversion responses, including the blink reflex. There is no need for eye protection with this class of laser.

Class 2M

A laser class defined in EN 60825 for those lasers which are eye safe as a result of normal aversion responses but whose beam divergence and/or diameter may render the beam unsafe if viewed through an optical instrument (for example, a microscope or magnifying glass).

Class 3R

A laser class defined in EN 60825 for those lasers which emit radiation that exceeds the maximum permissible exposure (MPE) level. The radiation is a maximum of five times the AEL of Class 1 (invisible) or five times the AEL of Class 2 (visible) lasers. The risk is slightly lower than that of Class 3B. The radiation from Class 3R lasers is dangerous to the eyes and laser safety eyewear is recommended.

Class 3B

A laser class defined in EN 60825 for those lasers for which the view into the laser is dangerous. Diffuse reflections (reflections from matte surfaces) are not considered as dangerous. Radiation from Class 3B lasers is dangerous to the eyes and laser safety eyewear is obligatory.

Class 4

A laser class defined in EN 60825 for those lasers which are inherently unsafe. Even diffuse reflections can be dangerous. There is also a danger of fire and a danger to the skin. Personal safety equipment is necessary (safety glasses, large area screens).

Coherence

Describing the property of laser radiation as travelling as waves which are in phase in both time and space. Monochromaticity and low divergence are two further properties of coherent light. The coherence of many lasers is in contrast to ordinary light sources (e.g. filament lamps) and explains why laser radiation is many times more dangerous to the eye.

Collimated

Describing rays of light that are parallel. Collimated light is emitted naturally by many lasers, but not by semiconductor diode lasers. Diverging light may be collimated by a lens or other device.

Continous Wave (CW)

A laser that produces a constant emission, one that is not pulsed. In EN 207 & 208, a continuous lasers is denoted by the symbol 'D' and is defined as any laser that emits pulses that are longer than 200ms (> 2 x 10-1 seconds).

Cornea

The front surface of the eye, transparent in the range from approximately 300-1400nm. Light in the range from 300-350nm passes through the cornea but is absorbed in the lens. The cornea performs the primary focussing of light into the eye, with the lens of the eye left to provide fine focal adjustment.

D

The nomenclature adopted in EN 207 & EN 208 to denote a laser that is continuous wave (CW), that is one that is not pulsed. Defined as any laser that emits pulses that are longer than 200ms (>0.2 seconds).

Diffuse

Describing the scattering (non-specular reflection or transmission) of light from or through a material. A material is described as being a perfect diffuser if it exhibits Lambertian-like reflectance or transmittance, in other words when the scattered light obeys Lambert’s Cosine Law. Reflections of certain classes of laser radiation from diffuse reflecting surfaces are considered less of a danger than from specular reflecting (i.e. mirror-like) surfaces.

Divergence

The property of a laser beam to increase in diameter with distance from the exit aperture as the beam propogates. The divergence is expressed in units of milliradians (mrad) and describes the full angle extent of the laser beam.

EN 207

The European laser safety standard pertaining to laser safety eyewear intended for full protection against lasers. (BS) EN 207: 1999, "Personal Eye Protection - Filters and Eye Protectors Against Laser Radiation (Laser Eye Protectors)" defines the construction, testing and selection criteria for laser safety eyewear that is designed to provide full protection against lasers. For alignment protection eyewear, refer to EN 208.

EN 208

The European laser safety standard pertaining to laser safety eyewear intended for alignment protection against lasers. (BS) EN 208: 1999, "Personal Eye Protection - Eye Protectors for Adjustment Work on Lasers and Laser Systems (Laser Adjustment Eye Protectors)" defines the construction, testing and selection criteria for laser safety eyewear that is designed to provide alignment protection against lasers. For full protection eyewear, refer to EN 207.

EN 60825

The European laser safety standard, equivalent to the US ANSI Z.136.1. (BS) EN 60825-1:2007 (Edition 2), "Safety of Laser Products – Part 1: Equipment Classification and Requirements" is the primary standard for laser safety to which other laser safety standards and guidelines refer. EN 60825 defines the laser safety classification system. The 2007 revision removed light emitting diodes (LEDs) from the scope of the laser safety standard - LEDs are now covered by other standards.

Energy

The rate of change of power, or in lay terms the capacity for doing work. Energy is used to express the output from pulsed lasers and is expressed in Joules (J). Energy is the product of power (Watts) and time (seconds). One Joule equals one Watt second.

Giant Pulsed

A type of laser (normally Q-switched) that produces short pulses, either a single pulse or a sequence of periodically repeated pulses. In EN 207 & 208, a giant pulsed laser is denoted by the symbol 'R' and is defined as any laser that emits pulses with pulse durations between 1ns and 1µs (1 x 10-9 to 1 x 10-6 seconds). Lasers with pulse durations of between 1µs and 200ms are termed ' Pulsed' and are given the notation 'I'.

I

The nomenclature adopted in EN 207 & EN 208 to denote a pulsed laser that produces a single or sequence of periodically repeated pulses. Defined as any laser that emits pulses that are longer than 1µs to 200ms (1 x 10-6 to 2 x 10-1 seconds). Lasers with pulse durations of between 1ns and 1µs are termed 'Giant Pulsed' and are given the notation 'R'. 

Infrared (Radiation), IR

That part of the electromagnetic spectrum comprising invisible optical radiation having wavelengths longer than 780nm but shorter than 1mm. In the wavelength ranges from 780nm to 1400nm, IR radiation is transmitted through the eye to the retina. Lasers operating in the 780-1400nm band are considered amoungst the most dangerous because one cannot see the beam but even relatively low levels of laser radiation in this band can lead to ocular injury and possible loss of sight. IR radiation at wavelengths longer than 1400nm is absorbed in the eye’s cornea.

Intrabeam (Viewing)

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

Irradiance

The radiant flux (or power) received at a surface per unit area. The irradiance from a laser is expressed in Watts per square meter (W/m2). The irradiance from a laser normally remains nearly constant with the distance travelled from the laser (apart from a small reduction as a result of beam divergence). This contrasts with the irradiance from ordinary light sources (e.g. a filament lamp) which decreases with the square of the distance travelled (described by the inverse squared law).

Laser

Acronym: Light Amplification by the Stimulated Emission of Radiation. A light source typically characterised by a combination of coherence, narrow spectral linewidth and a collimated beam. The term "laser light" refers to electromagnetic radiation with wavelengths between 150nm up to 11µm. A laser was first demonstrated in 1960 by Theodore H Maiman working at the Hughes Corporation, although the term ‘laser’ was first coined by Gordon Gould of Columbia University.

Lx

The nomenclature adopted in EN 207 to denote the protection level (scale number) of full protection laser safety eyewear, where 'x' is an integer from 1 to 10 which corresponds to a maximum irradiance that the filter and/or frame can withstand for 10 seconds or 100 pulses without a reduction in optical density.

M

The nomenclature adopted in EN 207 & EN 208 to denote a modelocked laser that produces a sequence of periodically repeated ultrashort pulses. Defined as any laser that emits pulses that are shorter that 1ns (1 x 10-9 seconds). Ultrafast lasers behave differently to those with longer pulse lengths. When a picosecond or femtosecond laser strikes a filter, a non-linear optical effect called induced transmission can occur. This results in a momentary increase in the spectral transmittance of the filter material. Even those filters which provide high protection levels with nanosecond or longer pulses can exhibit induced transmittance with shorter pulses and hence suffer from reduced protection. Laser safety eyewear intended for use with ultrafast lasers must be tested for stability and are assigned an 'M' rating.

MPE

Acronym: Maximum Permissible Exposure. The level of laser radiation defined in EN 60825 to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin. The MPE is the radiant flux (power) per unit area incident upon a surface expressed in Watts per square meter.

Micrometer (µm)

A unit of wavelength, normally used to describe infrared light. Often abbreviated to 'micron'. One micrometer or micron (µm) equals 1 x 10-6 meters. One micrometer also equals 1000 nanometers.

Modelocked

A type of laser that emits ultrashort pulses. In EN 207 & 208, a modelocked laser is denoted by the symbol 'M' and is defined as any laser that emits pulses with pulse durations shorter than 1ns (1 x 10-9 seconds).

Nanometer (nm)

A unit of wavelength, normally used to describe visble and ultraviolet light. One nanometer (nm) equals 1 x 10-9 meters. One nanometer also equals 0.001 micrometers.

NHZ

Acronym: Nominal Hazard Zone. Defined in ANSI Z136.1 as the space within which the level of direct, reflected or scattered laser radiation emitted during normal operation exceeds the applicable MPE. Exposure levels beyond the boundary of the NHZ are below the appropriate MPE level. Note that NHZ is a concept unique to the US standard and a nominal hazard zone is not allowed in the European standard EN 60825.

Optical Density (OD)

Describes the level of attenuation of light as it passes through an optical filter. The higher the OD value, the higher the attenuation (and the higher the protection). OD increases with increasing thickness of absorptive filters. Optical density is the logarithm (to the base ten) of the reciprocal of the transmittance. For example, an OD of 1 means 10%, an OD of 2 means 1% and an OD of 3 means 0.1% transmittance (and so on). Laser safety eyewear can be selected on the basis of OD according to the US standard ANSI Z.136.1. In Europe, EN 207 & EN 208 require that laser safety eyewear is selected on the basis of the ability of the eyewear to retain its OD for at least 10 seconds or 100 pulses when exposed to direct laser radiation of the power or energy density specified.

Peak Power

The maximum power delivered in a laser pulse. Peak power is an important metric in laser safety as the higher the peak power of a pulsed laser, the more damage it can cause to a safety filter. Consider a laser that generates pulses with an energy of 100mJ (1 x 10-1 J). If the pulse duration is 200µs (2 x 10-4 S), the peak power is 1 x 10-1 / 2 x 10-4 equals 500 Watts. If the pulse duration were to reduce to 10ns (1 x 10-8 S), the peak power would rise to 1 x 10-1 / 1 x 10-8 equals 10MW (1 x 107 Watts).

Power

The rate of energy delivery. Power (or more correctly, radiant flux) is used to express the output of continuous (CW) lasers and is measured in Watts (W). One Watt is a Joule per second. The average power from a pulsed laser is the product of energy per pulse (in Joules) and pulse repetition frequency (in Hertz). For example, if a giant pulsed laser produces 10 pulses per second (i.e. 10Hz) and the energy of each pulse is 100mJ (1 x 10-1 J), the average power is 10 * 1 x 10-1 equals 1 Watt.

Pulsed

A type of laser that produces a single or sequence of periodically repeated pulses. In EN 207 & 208, a pulsed laser is denoted by the symbol 'I' and is defined as any laser that emits pulses with pulse durations between 1µs to 200ms (1 x 10-6 to 2 x 10-1 seconds). Lasers with pulse durations of between 1ns and 1µs are termed 'Giant Pulsed' and are given the notation 'R'.

Pulse Duration

Defining the length of emission of a single pulse emitted by a laser that produces a single or sequence of periodically repeated pulses. Pulse duration may be expressed in units of milliseconds (ms = 1 x 10-3 seconds), microseconds (µs = 1 x 10-6 seconds), or nanoseconds (ns = 1 x 10-9 seconds). 

Pulse Repetition Frequency (PRF)

The number of pulses emitted per second from a repetitively pulsed laser. Expressed in units of Hertz (Hz). Ten pulses per second equals 10Hz. The average power from a pulsed laser is the product of energy per pulse (in Joules) and pulse repetition frequency.

Q-Switched

A type of laser (sometimes called 'Giant Pulsed') that produces short pulses, either a single pulse or a sequence of periodically repeated pulses. In EN 207 & 208, a giant pulsed laser is denoted by the symbol 'R' and is defined as any laser that emits pulses with pulse durations between 1ns and 1µs (1 x 10-9 to 1 x 10-6 seconds). Lasers with pulse durations of between 1µs and 200ms are termed ' Pulsed' and are given the notation 'I'.

R

The nomenclature adopted in EN 207 & EN 208 to denote a giant pulsed laser that produces a single or sequence of periodically repeated pulses. Defined as any laser that emits pulses with a duration between 1ns and 1µs (1 x 10-9 to 1 x 10-6 seconds). Lasers with pulse durations of between 1µs and 200ms are termed 'Pulsed' and are given the notation 'I'.

Reflectance

That light which when incident upon the filter material is neither transmitted nor absorbed. The degree of reflectance of laser radiation from a filter material determines both the optical density and the power handling capability of dielectric coated filter materials.

Retina

The light sensitive area on the rear of the eye. Produces a visual stimulus when exposed to visible electromagnetic radiation in the wavelength range from 380-780nm.

Rx

The nomenclature adopted in EN 208 to denote the protection level (scale number) of alignment protection laser safety eyewear, where 'x' is an integer from 1 to 5 which corresponds to a maximum irradiance that the filter and/or frame can withstand for 10 seconds or 100 pulses without a reduction in optical density. Alignment filters provide partial blocking of low power, visible wavelength laser beams. This means that the wearer can safely view the visible beam spot for laser beam alignment. The principle of alignment protection filters is that they attenuate the power of the laser such that if the beam were to accidentially hit the filter directly, the only light that would be transmitted would be equivalent to a class 2 exposure.

Scale Number

The alphanumeric protection rating assigned to the filter and frame of laser safety eyewear in accordance with EN 207 and 208. Rx is the scale number defined in EN 207 to denote the protection level of full protection laser safety eyewear, where 'x' is an integer from 1 to 10 which corresponds to a maximum irradiance that the filter and/or frame can withstand for 10 seconds or 100 pulses without a reduction in optical density. Rx is the scale number defined in EN 208 to denote the protection level of alignment protection laser safety eyewear, where 'x' is an integer from 1 to 5 which corresponds to a maximum irradiance that the filter and/or frame can withstand for 10 seconds or 100 pulses without a reduction in the optical density. In both cases, the integer 'x' is the lower of optical density or damage threshold, in other words the optical density will be al least the value of x.

Transmittance

That light which when incident upon the filter material is neither reflected nor absorbed. The degree of transmission of laser radiation through a filter material determines the optical density of the material. The spectral transmittance of the filter determines the colour and the visible light transmission of the material.

Ultraviolet (Radiation), UV

Describing that part of the electromagnetic spectrum comprising optical radiation having wavelengths between 100 and 400nm. Ultraviolet (UV) radiation is absorbed in the lens or cornea and is not visible to the naked human eye. The UV band is sub-divided into three bands based upon the biological effect of the radiation: UVC (100-280nm); UVB (280-320nm); and UVA (320-400nm).

Visible (Radiation)

Describing that part of the electromagnetic spectrum comprising optical radiation having wavelengths between 380 and 780nm. Light in this range is visible to the eye and promotes a stimulus when received on the retina.

Visible Light Transmission (VLT)

A measure of the attenuation of visible light by a laser safety filter. VLT is determined from the spectral transmittance factor of a filter, normalised to the CIE standard daylight illuminant D65 and adjusted for the CIE standard observer function for photopic vision. Should the VLT value for particular laser safety eyewear be less than 20%, the user should ensure that their working environment receives additional illumination.

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