Absorbed dose, Gy
The quantity of ionising radiation absorbed by a body, most commonly measured as the energy absorbed per unit mass, Gray.
The reduction in power in a beam of radiation passing through an object.
Acceptance test (Pre-Use test / Test Before First Use)
The acceptance test shall demonstrate that the instrument conforms to type-test data. The acceptance test checks for any potential faults and provides a reference of performance for comparison with subsequent routine tests for the lifetime of the instrument. Further information on the tests required can be found in current UK guidance, such as the NPL Measurement Good Practice Guide 14 (GPG14).
Accessible Emission Limit (AEL)
The maximum accessible emission permitted within a particular laser class.
Active medium
The material which acts as the light amplifier in a laser. It is typically excited by optical, chemical or electrical means.
A measure of the amount of radioactive material. Describes the rate at which radioactive decay occurs. Unit Becquerel, symbol Bq. 1 Bq = 1 radioactive decay per second.
Administrative controls
Safety measures of a non-engineering type such as: key control, safety training, and warning notices.
See Accessible Emission Limit
"As Low As Reasonably Practicable" The principle that radiation exposures must be reduced to the lowest level that can reasonably be achieved.
Alignment aids
Tools used to assist with the alignment of a beam path from an optical radiation source
Alpha radiation
The emission of an alpha particle from an atom. Alpha Particle = 2 protons and 2 neutrons.
Ambient dose equivalent rate H*(10)
Ratio of dH*(10) by dt, where dH*(10) is the increment of ambient dose equivalent in the time interval dt. The SI unit of ambient dose equivalent rate is the Sievert per second (Sv·s-1). Units of ambient dose equivalent rate are any quotient of the Sievert or its multiples or sub-multiples by a suitable unit of time (e.g. μSv h-1).
Angular Subtense (α)
The visual linear angle subtended by the apparent source (including diffuse reflections) at the eye of the observer or at the point of measurement.
Any opening in a protective housing or other enclosure through which radiation is emitted.
Aperture Stop
An aperture stop is an opening serving to define the area over which radiation is measured.
Apparent Source
The real or virtual object that forms the smallest possible image on the retina.
Approved Dosimetry Service (ADS)
A dosimetry service that is approved by the Health and Safety Executive under the Ionising Radiations Regulations 2017 for assessing and/ or recording the doses to classified persons.
Atomic Mass
The mass of an isotope of an element expressed in atomic mass units, which are defined as one-twelfth of the mass of an atom of carbon-12.
Atomic Number
The number of protons in the nucleus of an atom. Symbol Z.
A systematic and, wherever possible, independent examination to determine whether activities and related results conform to planned arrangements and whether these arrangements are implemented efficiently and are suitable to advise the organisation's policies.
Automatic Power Reduction
A feature of an optical fibre communication system by which the accessible power is reduced to a specified level within a specified time, whenever there is an event that could result in human exposure to radiation, eg. a fibre cable break.


Bayesian analysis
A statistical method relying on combination of prior information (distributions, probabilities) about one or more parameters with new observations (experimental data) in order to make statistical inferences.
Beam Attenuator
A device to be introduced into the path of a beam that reduces the intensity of radiation by a specified factor.
Beam Expander
A combination of optical elements that will increase the diameter of an optical beam.
Beam Path Component
An optical component that lies on a defined beam path such as a beam steering mirror or a focussing lens.
Beam Splitter
Any device intended to split a beam into two or more components.
Beam Stop
Any device intended to terminate a beam.
Becquerel, (symbol Bq). Is the SI derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.
Beta Particle
An electron emitted by the nucleus of a radionuclide. The electric charge may be positive, in which case the beta particle is called a positron.
Biological dosimetry / biodosimetry
Assessment of damage to biological material, cells or tissues, most commonly in samples of blood lymphocytes, to give an indication of radiation dose received by the individual.
Term applied to the use of radiation sources in or on the body for treating certain types of cancer.
X-rays generated when high energy electrons such as beta particles are slowed down in a medium.


Care Quality Commission (CQC)
A non-departmental government body that has responsibility for regulating health and social care in England.
Cavity (laser)
The optical assembly of a laser usually containing two or more highly reflecting mirrors which reflect radiation back into the active medium of the laser.
CE Marking
A mandatory conformity mark placed on products distributed in the European Economic Area.
Comite European de Normalisation (European Committee for Standardisation)
Chemical Laser
A laser where the energy source is a chemical reaction.
Rod-shaped bodies found in the nucleus of cells in the body. They contain the genes, or hereditary constituents. Human beings possess 23 pairs.
Classified Persons
Person who is designated as classified under the Ionising Radiations Regulations, 2017, on the basis of the dose they are likely to receive. Must have their dose properly assessed, e.g., by personal dosimetry, doses recorded in long-term dose records, and have an appropriate health record.
Phenomenon in which light waves are in phase.
Collateral Radiation
Any radiation produced by a source other than that the source is intended to produce.
Collective Dose
Frequently used for collective effective dose.
Collective Effective Dose
The quantity obtained by multiplying the average effective dose by the number of people exposed to a given source of ionising radiation. Unit man Sievert, symbol man Sv. Frequently abbreviated to collective dose.
Collimated Beam
A "parallel" beam of radiation with a very small angular divergence or convergence.
Any device used to collimate radiation into a beam.
Complementary Colours
Pure spectral colours which when mixed produce white light.
Consumer Products
Personal and household goods such as timepieces, smoke alarms, and gas mantles that contain radioactive material for functional reasons.
Loose, unsealed radioactive material.
Continuous Wave (CW)
An uninterrupted radiation output. Note: a laser operating with an output period equal to or greater than 0.25 s is considered to be a continuous wave laser.
Controlled Area
Area designated in accordance with the Ionising Radiations Regulations, 2017. Must be physically demarcated, have access restricted and be described in the Local Rules. Entry into controlled areas allowed for classified persons, and non-classified persons who are working under written arrangements.
Cosmic Rays
High energy ionising radiations from outer space. Complex composition at the surface of the earth.
Pertaining to very low temperatures.
Current Density
The electric current or flow of electric charge through a conducting medium, such as tissue, per unit cross-sectional area. Unit ampere per square metre, symbol A/m2.


Damage Threshold
The minimum power or energy density, which if incident on a surface will result in damage.
The process of spontaneous transformation of a radionuclide. The decrease in the activity of a radioactive substance.
Decay Product
A nuclide or radionuclide produced by decay. It may be formed directly from a radionuclide or as a result of a series of successive decays through several radionuclides.
Defined Beam Path
An intended path of a radiation beam.
Degradation Products
The material ejected during material processing as a result of radiation/material interaction causing breakdown products.
Delivery System
An optical system designed to guide optical radiation from one point to another.
Deterministic Effects
Health effects that only appear if a threshold level of dose is exceeded, e.g. radiation-induced erythema (burns). Deterministic effects will appear within the hours, days or weeks following a high radiation exposure.
Diagnostic Radiology
Term usually applied to the use of x-rays in medicine for identifying disease or injury in patients.
Chromosome with two centromeres; an unstable chromosomal aberration caused chiefly by ionising radiation and used for biological dosimetry.
The apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects are observed when light waves travel through a material with a varying refractive index.
Diffuse Reflection
Reflection that disrupts the collimated nature of the beam due to scattering at a surface.
Divergent Beam
A beam whose cross-section increases with distance from the source.
Deoxyribonucleic acid. The compound that controls the structure and function of cells and is the material of inheritance.
General term for quantity of ionising radiation. See absorbed dose, equivalent dose, effective dose and collective effective dose. Frequently used for effective dose.
Dosimetry Service
A service that systematically measures and/or records workers' radiation doses, usually by means of personal dosemeters.
Dye Laser
Laser in which the active medium is an organic chemical compound ('dye').


Effective Dose
The quantity obtained by multiplying the equivalent dose to various issues and organs by a weighting factor appropriate to each and summing the products. Unit Sievert, symbol Sv. Frequently abbreviated to dose.
Electrical Interaction
A force of repulsion acting between electric charges of like sign and a force of attraction acting between electric charges of unlike sign.
Electromagnetic Radiation
Electromagnetic radiation is a form of radiation with both electric and magnetic field components, which can be described as waves propagating at the speed of light. Under some circumstances electromagnetic radiation can be considered to exist as particles called photons.
Electromagnetic Spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The spectrum ranges from short wavelengths such as x-rays, through visible radiation to longer wavelength radiations of microwaves, television and radio waves.
An elementary particle with low mass, 1/1836 that of a proton, and unit negative electric charge. Positively charged electrons, called positrons, also exist. See also beta particle.
Electron volt
Unit of energy employed in radiation physics. Equal to the energy gained by an electron in passing through a potential difference of 1 volt. Symbol eV. 1 eV = 1.6 x 10-19 joule approximately.
A substance with atoms all of the same atomic number.
Embedded laser product
A laser that has been built into a product which incorporates engineered safety features to lower the accessible amount of laser radiation, designed to ensure the classification of the product is lower than the classification of the laser within.
Emission duration
The temporal duration of a pulse, of a train or series of pulses, or of continuous operation, during which human access to radiation could occur as a result of operation, maintenance or servicing of a equipment.
Emission warning device
Any warning device that gives an audible or visible warning that radiation either could be emitted or is being emitted.
Enclosed system
System in which, during normal operation, the radiation is totally enclosed.
Engineering control
Safety measures of a deliberate engineering design which should be used as the fundamental method of reducing exposure to radiation. A physical means of preventing access to radiation.
Equivalent dose
The quantity obtained by multiplying the absorbed dose by a factor to allow for the different effectiveness of the various ionising radiations in causing harm to tissue. Unit Sievert, symbol Sv.
Errant laser radiation
Laser radiation which deviates from the planned beam path. Examples include unwanted secondary reflections from optical components, deviant radiation from misaligned or damaged components and reflections from tools.
Reddening of the skin caused by dilation of blood vessels.
An excited dimer. A system of two atoms which, in their excited states, join together and form a molecule in an excited state.
Excited state
The state of an atom or molecule when it possesses more energy than in its normal or 'ground' state.
Exposure Limit Value (ELV)
Legal limit on the level of optical radiation that a person can be exposed to.
Exposure Time
The duration of a pulse, or series, or a train of pulses or of continuous emission of radiation incident upon the human body.
Extended Source
An optical source is considered an extended source when the apparent source subtends a linear angle α greater than αmin (unlike most laser sources).


A fail-safe component is one whereby its failure does not increase the hazard ie. it fails in a safe condition. In the failure mode the system is rendered inoperative or non-hazardous.
The transfer of radionuclides produced by nuclear weapons from the atmosphere to earth; the material transferred.
Fast Neutrons
Conventionally, neutrons with energies in excess of 0.1 MeV. Corresponding velocity of about 4 x 10 6 ms -1
Fast Reactors
See nuclear reactor.
Nuclear fission. A process in which a nucleus splits into two or more nuclei and energy is released. Frequently refers to the splitting of a nucleus of uranium-235 into two approximately equal parts by a thermal neutron with emission of other neutrons.
Fission Products
Nuclides or radionuclides produced as a result of fission.
Fluorescence In Situ Hybridisation (FISH)
A method of detecting chromosomal translocations.
Free Radical
A grouping of atoms that normally exists in combination with other atoms but can sometimes exist independently. Generally very reactive in a chemical sense.
The number of cycles per unit time of an oscillation. Symbol: f Unit:Hz
The airborne emission of the material released by breakdown of a target during radiation/material processing or interaction.
Thermonuclear fusion. A process in which two or more light nuclei are formed into a heavier nucleus and energy is released.


A marker of DNA damage response within cells, that is used for biological dosimetry.
Gamma Ray
A discrete quantity of electromagnetic energy without mass or charge. Emitted by a radionuclide. See x-ray.
Gas Laser
A laser where the active medium is a gas.
Geiger-Műller Tube
A glass or metal envelope containing a gas at low pressure and two electrodes. Ionising radiation causes discharges, which are registered as electric pulses in a counter. The number of pulses is related to dose.
See absorbed dose.


The time taken for the activity of a radionuclide to lose half its value by decay. Symbol t ½.
Something with the potential to cause harm. The hazard can be to people, property or the environment.
Hazard Level
The potential hazard at any accessible location within an optical fibre communication system. It is based on the level of optical radiation that could become accessible in reasonably foreseeable circumstances, eg. a fibre cable break. It is closely related to the laser classification procedure in BS EN 60825-1.
Health and Safety Executive (HSE)
The Health and Safety Executive (HSE) is a non-departmental public body in the United Kingdom responsible for the encouragement, regulation and enforcement of workplace health, safety and welfare, and for research into occupational risks.


International Atomic Energy Agency. International organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons.
See International Commission on Non-Ionising Radiation Protection.
International Commission on Radiological Protection. International body of experts which makes recommendations on radiation protection standards.
Region of the spectrum comprising wavelengths in the range 700 nm to 1 mm.
Interlock (see Safety Interlock)
A mechanical, electrical or other type of device, the purpose of which is to prevent the operation of equipment under specified conditions.
International Commission on Non-Ionising Radiation Protection (ICNIRP)
A body of independent scientific experts that aims to disseminate information and advice on the potential health hazards of exposure to non-ionizing radiation.
Intrabeam Viewing
All viewing (and measurement) conditions in which the eye is exposed to optical radiation, other than extended source viewing. Examples are viewing of collimated beams and of point-type sources.
Electrically charged atom or grouping of atoms.
The process by which a neutral atom or molecule acquires or loses an electric charge. The production of ions.
Ionising Radiation
Radiation composed of particles that individually carry enough kinetic energy to liberate an electron from an atom or molecule, ionising it. Ionising radiation is generated through nuclear reactions, either artificial or natural, by very high temperature (e.g. plasma discharge or the corona of the Sun), via production of high energy particles in particle accelerators, or due to acceleration of charged particles by the electromagnetic fields produced by natural processes, from lightning to supernova explosions.
See Infrared
IRR 17
The Ionising Radiations Regulations 2017. Legislation enforced by the Health and Safety Executive, that aims to protect workers and others from the health effects of exposure to ionising radiation in the workplace.
The power of optical radiation averaged over the area on which it is incident or such other as may be specified. (W/m2)
Nuclides with the same number of protons but different numbers of neutrons. Not a synonym for nuclide.


The unit of energy, equivalent to work done by a force of one newton moving an object through one metre. Symbol J.


Key control
Key operated control on activation of equipment or systems, so that operation is only possible by authorised personnel. The key is a coded control device, which may be mechanical, password or magnetic in nature. Class 3B and 4 lasers must be key operated.


Acronym: Light Amplification by Stimulated Emission of Radiation. A device that can be made to produce or amplify electromagnetic radiation in the wavelength range from 180 nm to 1 mm primarily by the process of controlled stimulated emission.
Laser Associated Hazard
Hazards arising directly from the operation or installation of the laser or laser system.
Laser Classification
A scheme by which lasers are classified according to their laser radiation hazard. Defined in BS EN 608251.
Laser Controlled Area
An area within which persons are subject to control and supervision for the purpose of protection from laser radiation hazards.
Laser Product
Any product or assembly of components that constitutes incorporates or is intended to incorporate a laser or laser system, and which is not sold to another manufacturer for use as a component (or replacement for such component) of an electronic product.
Laser Protection Adviser
A person who gives advice to the employer on laser safety and is generally considered to be an expert. In many situations the Laser Protection Adviser will be an external consultant.
Laser Protection Supervisor
A person who provides local supervision of laser use within the healthcare and aesthetic environments. Fulfils some of the roles of the Laser Safety Officer.
Laser Safety Officer
A person who is knowledgeable in the evaluation and control of laser hazards and has responsibility for the oversight of such.
Laser System
A laser in combination with an appropriate laser energy source with or without additional incorporated components.
Term normally used to refer to the visible region of the electromagnetic spectrum.
Light Emitting Diode (LED)
Any semiconductor pn junction device which is designed to produce electromagnetic radiation by radiative recombination in a semiconductor in the wavelength region from 180 nm to 1 mm. Radiation is produced primarily by the process of spontaneous emission, although some stimulated emission may be present.
Limiting Aperture
The circular area over which irradiance and radiant exposure are averaged.
Local Rules
Set of working procedures written in accordance with the Ionising Radiations Regulations, 2017, to enable work with ionising radiations to proceed safely, and in accordance with the Health and Safety at Work Act, 1974.
Limit of Detection (normally refers to sensitivity of analytical measurements)
LSA scale
Low Specific Activity scale
Luminous transmittance/ Visible light transmittance (VLT)
The amount of white light transmitted by a filter. Expressed as a percentage.


Maintenance (laser)
The performance of those adjustments or procedures specified in user information provided by the manufacturer with the laser product, which are to be performed by the user for the purpose of assuring the intended performance of the product. It does not include operation or service.
Acronym: Microwave Amplifcation by Stimulated Emission of Radiation. A device that can be made to produce or amplify electromagnetic radiation in the microwave region of the spectrum primarily by the process of controlled stimulated emission.
Mass Number
The number of protons plus neutrons in the nucleus of an atom. Symbol A.
Maximum Angular Subtense (αmax)
The value of angular subtense of the apparent source above which the MPEs and AELs are independent of the source size. αmax = 0.1 rad = 100 mrad
Maximum Permissible Exposure (MPE)
The level of laser radiation to which under normal circumstances, persons may be exposed without suffering adverse effects. MPE levels are the maximum levels to which eye and skin can be exposed without injury and are related to wavelength, exposure duration, tissue type, viewing conditions.
Medicines and Healthcare products Regulatory Agency
The UK government agency that is responsible for ensuring that medicines and medical devices work and are acceptably safe.
See Medicines and Healthcare products Regulatory Agency
10-6 metre (1μm, one-millionth of a metre).
American spelling of micrometre.
10-6 second (1μs, one-millionth of a second).
10-3 metre (1mm, one-thousandth of a metre).
10-3 second (1ms, one-thousandth of a second).
Minimum Angular Subtense (αmin)
The value of the angular subtense of the apparent source above which a source is considered an extended source. MPEs, AELs and ELVs are independent of the source size where the angle subtended by the source is less than αmin.
Mode Locking
The production of a very short pulse of laser radiation emitted from the laser as a result of one round-trip of the laser cavity. Pulses are produced at regular intervals.
An electrically neutral group of at least two atoms.
Referring to electromagnetic radiation composed entirely of a single wavelength.
A chemical change in the DNA in the nucleus of a cell. Mutations in sperm or egg cells or their precursors may lead to inherited effects in children. Mutations in body cells may lead to effects in the individual.


10-9 metre (1nm, one thousand millionth of a metre).
10-9 second (1ns, one thousand millionth of a second).
Narrowband signals
Signals which occupy a narrow range of frequencies
An elementary particle with unit atomic mass approximately and no electric charge.
See Non-Ionising Radiation.
Nominal Ocular Hazard Distance (NOHD)
The distance at which the laser beam irradiance equals the appropriate MPE value. An indication of a 'safe viewing' distance for certain criteria. An equivalent term for skin exposure is 'Nominal Skin Hazard Distance' (NSHD).
Non-Ionising Radiation
Radiation that does not produce ionisation in biological tissue. Examples are ultraviolet radiation, light, infrared radiation and radiofrequency radiation.
Naturally Occurring Radioactive Material.
Nuclear fuel cycle
The stages in which the fuel for nuclear reactors is first prepared, then used, and later reprocessed for possible use again. Waste management is also considered part of the cycle.
Nuclear medicine
Term usually applied to the use of radionuclides for diagnosing or treating disease in patients.
Nuclear power
Power obtained from the operation of a nuclear reactor.
Nuclear power industry
The industry associated with the production of nuclear power. In the UK, the preparation of fuel for nuclear reactors, the operation of reactors, the subsequent reprocessing of the fuel, and the disposal of radioactive wastes.
Nuclear reactor
A device in which nuclear fission can be sustained in a self-supporting chain reaction involving neutrons. In thermal reactors, fission is brought about by thermal neutrons.
The core of an atom, occupying little of the volume, containing most of the mass, and bearing positive electric charge.
Nucleus of a cell
The controlling centre of the basic unit of tissue. Contains the important material DNA.
A species of atom characterised by the number of protons and neutrons and, in some cases, by the energy state of the nucleus.


Optical density (OD)
A measure of a material's ability to absorb optical radiation. It is dependent on wavelength, transmittance of the material and thickness of the material. Normally specified on a logarithmic scale.
Optical Fibre Communication System
An engineered assembly for the generation, transference and reception of optical radiation arising from lasers in which the transference is by means of optical fibre for communication and/or control purposes.
Optical gain
The optical amplification of radiation due to the focusing properties of the eye or other photodetector.
Optical radiation
Electromagnetic radiation comprising ultraviolet, visible and infrared radiations.
Optically stimulated luminescence
A physical retrospective method of dosimetry being developed for routine or emergency radiation dosimetry of individuals.
Order of Magnitude
A term used to describe when two numbers differ by a factor of ten.
The result of a hazardous event being realised. Can also be known as consequence or harm and can range from financial loss, damage, injury or even death.
A radiation exposure to a member of the public or a radiation worker which is higher than the individual should normally receive on a day to day basis.


Peak power
The maximum energy output measured during an observation period.
The time for one complete oscillation, wave motion, or other regularly repeated process. Symbol:Τ It is the reciprocal of frequency.
Periodic test
This test confirms that the performance of the instrument is typical of type and that it remains fit for purpose. It is more than a simple check. Further information on the tests required can be found in current UK guidance, such as the NPL Measurement Good Practice Guide 14 (GPG14).
It is recommended that the performance of the instruments electrical and mechanical systems is also inspected during the routine test. For example, batteries, cables, connectors and controls shall be inspected and repaired or replaced where necessary. Depending on the severity of the repair it may be necessary to repeat the acceptance tests if, for example, the detector has been repaired or replaced.
Personal Protective Equipment (PPE)
Safety equipment designed to protect the person from a specific hazard such as optical radiation. Includes gloves, goggles, ear defenders. Necessary when other methods (engineering controls, administrative controls) are not sufficient to adequately control the risks.
Photographic film
Film with emulsion sensitive to ionising radiation. The degree of blackening is related to dose.
Measurement of properties of visible radiation relative to the response of an average human eye.
A quantum of electromagnetic radiation.
Emissions of biological material into air caused by ablation or modification of tissue with a laser.
Pockel's Cell
An electro-optical device used to rotate the plane of polarisation of light by applying an electric field or pulse. Used inside a laser cavity to Q-switch to produce a pulsed output.
The direction of vibration of the electromagnetic wave in the laser beam. Polarisation may be plane, circular, elliptical or random.
Poly-allyl-diglycol-carbonate (PADC)
A type of plastic that can be used as a radiation dosemeter for radon and neutron radiations.
Population inversion
The condition needed for lasing action whereby the number of atoms in an excited state is greater than the number of atoms in a lower energy state. Usually achieved by rapidly supplying energy to the active medium.
Power density
Power of radiation incident on a surface unit of area (W/m2)
Pre-Use test
See Acceptance test
Prior Risk assessment
Defined in the Ionising Radiations Regulations 2017, an assessment made by the employer to determine whether he should take any further steps to restrict radiation exposures.
The measure or estimate of how likely it is that something will occur.
Process Associated Hazard
Hazards arising as a consequence of the particular use or application of a laser or laser product.
An elementary particle with unit atomic mass approximately and unit positive electric charge.
PT exercise
Proficiency Test exercise
Pulse duration
The time increment measured between the half peak power points at the leading and trailing edges of a pulse.
Pulsed laser
A laser which delivers its energy in the form of a single pulse or a train of pulses. Under BS EN 60825-1, the duration of a pulse is less than 0.25 s.
The act of energy transfer from an external source into the active medium of a laser.


Device used to prevent lasing action whilst the population inversion increases. The increased energy level is then rapidly released to produce an enhanced energy in a very short pulse.
Quantum Detector (Photodetector)
Detects radiation incident upon it via an element sensitive to the number of photons incident upon it. Examples of photodetectors are photodiodes and photomultiplier tubes.


Unit of angle equal to 180/π degrees
The amount of optical radiation that passes through a unit surface and falls within a given solid angle of the source (unit W/m2/sr)
Radiant Exposure
Total energy of optical radiation incident on a surface per unit area (J/m2)
The emission of energy from a source, either as waves (light, sound) or as moving particles (beta rays, alpha rays).
Radiation Protection Adviser
Radiation Protection Adviser. Person deemed to be competent to give radiation protection advice, under one of the schemes recognised by HSE.
Radiation Protection Adviser Body
A body recognised by HSE as having the requisite collective experience and quality assurance systems to provide sound radiation protection advice.
Possessing the property of radioactivity.
The property of radionuclides of spontaneously emitting ionising radiation.
The study of the effects of ionising radiation on living things.
Radiofrequency Radiation
Electromagnetic radiation often defined as having frequencies between 100 kHz and 300 GHz.
Radiological Protection
The science and practice of limiting the harm to human beings from radiation.
Measurement of properties of optical radiation (UV, Visible, IR). Concerned with power content of radiation, eg. radiant power.
An unstable nuclide that emits ionising radiation.
Term applied to the use of radiation beams for treating disease, usually cancers, in patients.
Reasonably Foreseeable Event
The occurrence of an event which under given circumstances can be predicted fairly accurately, and the occurrence probability or frequency of which is not low or very low.
The process in which radiation meeting a boundary between two media 'bounces back' to stay in the first medium.
The 'bending' of light as it passes through a medium.
Remote Interlock Connector
A connector which permits the connection of external controls placed apart from other components of a piece of equipment.
The probability of injury, harm or damage.
Risk factor
The product of the likelihood of a hazardous event occurring and the outcome or harm that arises as a result.
Radiation Protection Supervisor. Person appointed by the employer to supervise the radiation work, to ensure that local rules are followed.
See the Regulation and Quality Improvement Authority


Safety Interlock
A mechanical, electrical or other type of device, the purpose of which is to prevent the operation of equipment under specified conditions.
Scale Number, L
The maximum spectral transmittance and stability to laser radiation of filters and/or eye protectors against laser radiation.
Scanning Laser Radiation
Laser radiation having a time-varying direction, origin or pattern of propagation with respect to a stationary frame of reference.
The 'spreading out' of a beam of radiation as it passes through matter reducing the energy moving in the original direction.
Scintillation Counter
A device containing material that emits light flashes when exposed to ionising radiation. The flashes are converted to electric pulses and counted. The number of pulses is related to dose.
Sealed Source
A radioactive source that is contained, or otherwise constructed so as to prevent loss of the radioactive material.
Semiconductor Laser
A laser where the active medium is a semiconductor.
SI Units
The units of the 'International System of Units'.
The SI unit of dose equivalent is the joule per kilogram (J/kg), which has been named the Sievert (Sv) by the International Commission on Radiological Protection (ICRP).
Silicon diode detector
A device made of a silicon compound in which current flows when exposed to radiation. The current is proportional to the radiation incident on the detector.
Solid-State Laser
A laser where the active medium is a crystal.
Specular Reflection
A reflection which maintains the collimated and point-source characteristics of the source and occurs when the beam is incident on a mirror-like surface.
Stimulated Emission
When a population inversion exists in a material, an emitted photon may cause the release of an identical photon from another atom or molecule in an excited energy state. Emission is thus described as 'stimulated'.
Stochastic Effect
Health effect whose probability of occurrence depends on the dose received. Occurrence is usually many years after the exposure, and there is believed to be no threshold level of dose below which no effect will occur.
Supervised Area
Area designated in accordance with the Ionising Radiations Regulations, 2017. Supervised area need not be physically demarcated and access is unrestricted. Supervised areas must be described in the local rules.


Test before first use
As acceptance test: The acceptance test shall demonstrate that the instrument conforms to type-test data. The acceptance test checks for any potential faults and provides a reference of performance for comparison with subsequent routine tests for the lifetime of the instrument. Further information on the tests required can be found in current UK guidance, such as the NPL Measurement Good Practice Guide 14 (GPG14).
The Regulation and Quality Improvement Authority
The government body responsible for monitoring and inspecting the availability and quality of health and social care services in Northern Ireland
Thermal Detector
Detects radiation incident upon it via a temperature sensitive element. Examples of thermal detectors are calorimeters and bolometers.
Thermal Neutrons
Neutrons that have been slowed to the degree that they have the same average thermal energy as the atoms or molecules through which they are passing. The average energy of neutrons at ordinary temperatures is about 0.025 eV, corresponding to an average velocity of 2.2 x 10 3 ms -1
Thermal Reactor
See nuclear reactor.
Thermoluminescent Dosemeter (TLD)
A dosemeter that works by storing the energy it gets from the ionising radiation, and releasing it, when heated, in the form of light.
Thermoluminescent Material
Material which, having been irradiated, releases light in proportion to the ionising radiation absorbed when it is subsequently heated.
A device which converts a non-electrical parameter, eg light, into electrical signals, the variations in the electrical signals being a function of the input parameter.
Transfer of DNA material from one chromosome to another; a chromosomal aberration formed in response to ionising radiation and several other factors, which is detected by FISH and used for biological dosimetry.
The passage of radiation through a medium. If not all radiation is absorbed, that which passes through is said to be transmitted. Dependent upon wavelength, polarisation, radiation intensity and transmitting material.
The transmittance of a material is the amount of light that it allows to pass. Normally expressed as a percentage.
Triage (radiation emergency)
The sorting of potentially exposed individuals according to the urgency of their need for care.
Type test
This test is performed on at least one or more standard production instruments picked at random. Ideally all non-destructive tests should be performed on at least three standard production instruments. Destructive tests, however, may be performed on just a single instrument. The type-test investigates all aspects of the instruments design to show the extent of compliance with pre-defined specifications.


United Kingdom Accreditation Service
Ultraviolet radiation
Region of the spectrum comprising wavelengths in the range 100 nm to 400 nm. It can be further sub-divided into three regions: UV-A (315 - 400 nm), UV-B (280 - 315 nm) and UV-C (100 - 280 nm).
Uncertainty (statistical):
An estimate of the degree of potential variation of a scientifically estimated quantity, for instance radiation dose, taking in to account all the sources of error in the measurement. Low uncertainty is desirable as this indicates a high degree of confidence in the results.
Unsealed radioactive materials
Radioactive material that is not designed to be contained. e.g. radioactive gas, liquids or powder.
See Ultraviolet radiation
See ultraviolet radiation.


Visible light
Electromagnetic radiation with wavelengths between 400 and 700nm. Capable of directly causing a visual sensation in a typical human eye.
The unit of electrical potential difference, symbol V.


Waste management
The control of radioactive waste from creation to disposal.
The unit of power, equivalent to one joule of energy per second. Symbol W.
The distance between similar points on successive cycles of a wave. Unit metre, symbol m.


A discrete quantity of electromagnetic energy without mass or charge. Emitted by an x-ray machine. See gamma ray.
X-ray tube
Device that produces X-rays by accelerating electrons through an evacuated tube, onto a heavy target. Requires an electric current.


Yttrium aluminium garnet. A form of crystal used in neodymium lasers.