Light Measurement Glossary of Terms

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The dissipation of light (radiation) within a surface or medium, caused by the conversion of radiant (luminous) energy to a different form of energy, usually heat, by interaction with matter. The absorption is the "missing piece", when comparing the total reflected and transmitted energy with the incident energy. The ratio of the total absorbed radiant or luminous flux to the incident flux is called absorptance. The standard unit of absorptance is percent (%) or a factor between 0 an 1. Absorptance can also be inferred from the transmission through a medium. If the %transmission of a certain wavelength is 70% then the material has a 30% Absorptance.


The characteristic of radiation that indicates its capability to produce chemical change. In our industry the term is usually used with reference to UV radiation and its effects on biological systems. Actinic strips are used in UV processing to monitor the intensity of sources. The color or optical density of the strip changes with the exposure. See the Optical Radiation Hazard applications page for a listing of ILT light measurement systems used to determine the actinic hazard of a light source.

Ambient Light

Ambient light is the light diffused in the environment surrounding a detector measuring the optical radiation from another source. This light contributes to the signal measured from the source. To make valid measurements, the contribution from the ambient light or background must be subtracted from each measurement.


A hole through which radiant energy can pass. Angular aperture is the angle through which the most divergent rays can pass through a hole or lens. The aperture of a lens is often expressed in terms of an f/#. The f/# is the ratio of the focal length of the lens to its diameter. A lens with a focal length of 100mm and a diameter of 25mm would have an aperture of f/4.


A device which reduces the amount of energy reaching a sensor. Attenuators are typically used when the radiant energy would saturate a detector. The QNDS,QNDS2 and QNDS3 filters are attenuators that reduce flux density at the detector by factors of 10, 100 and 1000, respectively.


Bandwidth describes the size of a spectral segment. A bandwidth of 10nm indicates a range of 10nm of radiation. This can be, for example, between 500nm and 510nm, 1000nm and 1010nm or an equal size segment anywhere in the spectrum.

Band Eliminator Filter

A band eliminator filter allows the wavelengths above and below the filter cutoff to pass while suppressing the wavelengths within the band. These filters are also called notch filters. A 500nm band eliminator filter with a band pass of 10 nm would suppress the wavelengths between 495nm and 505nm.


The angular width of a cone of light whose apex is at the source. Beamwidth is usually defined as the angle subtended by a cone encompassing 90% of the energy.

Black Body

A black body is an object that absorbs all radiant energy that hits it. When heated a black body emits a well defined characteristic spectrum which can be used in characterizing the spectral responsivity of detectors. As there is no such thing as a perfect black body, black body simulators are used for this purpose.


The process of normalizing the signal output from a detector to that of a detector defined as a standard (usually defined by National Institute of Standards and Technology (NIST) under identical illumination conditions). Calibration can also be accomplished by the use of a standard source (lamp) whose output energy at specific wavelengths and measurement distances are traceable to the standard lamp defined by the governing standards body (NIST).

Candela (cd)

The current SI unit of luminous intensity. One candela is equivalent to 1 lumen/steradian. Used to express both Beam Intensity (Beam Candela) and Mean Spherical Intensity (Mean Spherical Candela). Also referred to as Candlepower (cp).

Candela, beam (cd or eff cd) photometric intensity measurement

Sampling a very narrow angle of the input beam, beam candela is only representative of the lumens per steradian at the peak intensity of the beam. The sampling angle need not be defined. Can be measured in Candela (cd) for steady sources or Effective Candela (eff cd) for flashing sources.

Candela,effective (eff cd)

Unit of luminous beam intensity that has been weighted to the human eye's increased sensitivity to a flashing source.

Candela, Mean Spherical (cd) photometric intensity measurement

The luminous intensity of a source expressed in candelas. Measured in an integrating sphere, Mean Spherical Candela is the total output of the source in lumens divided by 4pi steradians in a sphere.

Candlepower (cp)

Old definition of luminous intensity. One candlepower (cp) was the luminous intensity of a standard candle made of whale wax, weighing 1/6 lb, 7/8 in. in diameter, and burning 120 grains per hour. The current SI unit for luminous intensity is the candela (cd). One candela (cd) is the same as one candlepower thus a source with a luminous intensity of 10 candelas could be referred to as a 10 candlepower source.


A CCD (charge coupled device) is a light detector with high sensitivity primarily in the visible spectrum. CCDs are typically made in either linear or two-dimensional arrays consisting of up to millions of individual detector elements. The 2D versions are used for image recording and are found in most digital camera used in both scientific and consumer applications.


The aspects of color associated with hue and saturation without reference to brightness.

Chromaticity (CIE) Coordinates

The proportion of the standard tristimulus values used in color matching. Colors are compared by their CIE X, Y, and Z coordinates.


The CIE (Committee Internationale de l’Eclairage) is the international standards organization for illumination and color vision.

Color Temperature

Color temperature refers to the temperature, in degrees Kelvin, that a black body would have to be heated to in order to have a color similar to the reference. A 40W incandescent bulb has a color temperature of about 2680K while daylight at noon has a color temperature of about 5500K.

Cosine Collector

A cosine collector is a translucent light collector that compensates for normal blocking of radiation from flat surfaces. The cosine collector samples the radiation according to the cosine law through the hemisphere above the surface. A cosine collector may also be referred to as a Lambertian surface.

Cosine Law (Lambert’s cosine law)

The flux per unit solid angle leaving or entering a surface is proportional to the cosine of the angle with respect to the normal to the surface. In a cosine collector the rays that strike the surface at an angle of 60° from the vertical will have a contribution of 0.5 (cosine of 60°) that of identical rays arriving vertically.

Cutoff Filter

A filter that does not pass light of wavelengths shorter that the cutoff wavelength and passes a wide band of wavelengths above the cutoff wavelength. The cutoff wavelength is specified at some point along the transition from maximum transmission to 0 transmission. The same filter can have different cutoff wavelengths depending upon the specified % transmission. The same filter may be specified as a 500nm cutoff filter with the 50% transmission point as the reference or a 485nm cutoff filter if the specification is the 5% transmission point, where the transmission at 485nm is 5%.

Dark Adaptation

The ability of the human eye to adjust itself to low light levels.

Dark Signal (Current)

The dark signal is the signal that flows in a photodetector when there is no optical radiation impinging upon it. This signal is created within the detector and the amplification circuits due to thermionic (temperature) effects.


A densitometer measures the opacity or absorptance of a material. The measurement is usually expressed in AU (absorptance units) or OD (optical density).

Diffraction Grating

A diffraction grating is an optical component that separates light into its constituent wavelengths. Functionally equivalent to a prism, it disperses the light into its spectrum by employing grooves to diffract the light. The angle of the diffraction is a function of the wavelength.

Diffuse Reflectance

The ratio of the incident flux to the reflected flux from a scattering surface as opposed to a highly directional or specular (mirror-like) surface.

Dynamic Range

The dynamic range is ratio of the maximum measurable signal before saturation to the minimum measurable signal above the noise. Typically the dynamic range is expressed in either decades (powers of 10) or bits (power of 2). A 5 decade dynamic range indicates that there is a factor of 100,000 between the maximum and minimum signals measurable with the apparatus. A dynamic range of 16 bits (264) indicates a factor of 65,532 between the minimum and maximum measurable signals.

Dynamic range is also expressed in db (decibels), defined as 10 log10 (maximum signal / minimum signal). Five decades of dynamic range is equal to 50 db.


A unit of energy equivalent to the amount of energy absorbed by one molecule of a material undergoing a photochemical reaction as determined by the Stark-Einstein law.

Electromagnetic Radiation

Radiation emitted by vibrating charged particles. A combined oscillation of electric and magnetic fields that propagates through space at the speed of light. The electromagnetic spectrum is theoretically infinite, includes gamma, x-ray, UV, visible, IR, microwaves, and radio waves.


The ratio of an object’s radiance to that emitted by a black body at the same temperature and wavelength.

Energy Density

Radiant energy arriving at a surface per unit area, usually expressed in joules or millijoules per square centimeter (J/cm² or mJ/cm²). It is the time-integral of irradiance. (Other terms applied include "radiant exposure," "light dose," and "total effective dosage").


Also called the throughput of an optical system, it is the product of its entrance aperture and the solid angle in which light can be accepted at that aperture.


Flux leaving a surface per unit area.


A common, but loosely used, term for energy density, or radiant flux density, at a surface. (It is a precisely defined term in EB curing: 1 Gray (Gy) = 1 J/kg , a measure of absorbed energy per unit mass). In other technologies, the term usually applies to energy absorbed within the medium of interest, but in UV curing, is equated only to irradiant energy density arriving at the surface of the medium of interest. [The preferred shortened term is energy density, expressed in J/cm² or mJ/cm²]. May also be referred to as "dose" or "dosage".


A thin metal wire purposely positioned inside a lamp bulb, that generates radiation in the visible, infrared and ultraviolet ranges when electrical current is passed through it. Tungsten material is often used, as it has great tensile strength, is very durable, and can be heated very near its melting point without evaporating rapidly. Lamp filaments are offered in a variety of designs optimized for specific applications.

Filament Description

A filament description consists of a prefix letter, to indicate whether the wire is straight or coiled, followed by a number to indicate the arrangement of the filament on the supports. Prefix letters are usually one of three options

    • S – Straight, there is no coil to the filament
    • C – Coiled, the filament is wound into a helical coil
    • CC – Coiled Coiled, the helical coiled filament is wound again into another helical coil.






The energy per second (power) in a light beam expressed in Watts or Joules/second. (Radiant power). In photometric measurements the luminous power is typical expressed in lumens (lm) .

Footcandle (fc) photometric measurement

Footcandles is a unit of measurement of illuminance (how much light is striking a point on a surface). Once footcandle is equivalent to 10.764 lux.

Footlambert (fl) photometric measurement

A unit of luminance equal to 1/p candela /ft2.


All biological organisms contain DNA. DNA is essential to reproduction. Optical radiation in the UVC range is capable of breaking the molecular bonds within DNA effectively killing microorganisms. Germicidal UV lamps are used for water treatment, sterilization of foods and their containers, and air purification, especially in hospital environments.

Hefner Unit photometric measurement

A unit of luminous intensity equal to 0.9 candle.

Illuminance photometric property

Luminous flux incident per unit area of a surface. 1 lumen/m² = 1lux.

Infrared (IR)

The invisible portion of the electromagnetic spectrum that extends from 0.75 microns to 1000 microns. Radiation in the near infrared (NIR) produces the sensation of heat.

Integrating Sphere

A hollow sphere coated with a white diffusing coating in the interior. It is used for measuring the diffuse reflectance and transmittance of objects or the total flux from a source that is completely inside.


Flux per solid angle. Radiometric measurements are made in W/sr. Photopic measurements are made in lumens/sv.

Inverse Square Law

The inverse square law correlates the relative intensity at varying distances from a point source. The relative intensity will diminish to a factor of the square root of the difference in distance. For example if at 2 meters from a source the intensity is 16 W/m², it will be 4W/m² at 4meters and W/m² at 8 meters. For extended (non-point) sources the intensity fall off approaches the inverse square law at a distance equivalent to 5 times the diameter of the source.

Irradiance radiometric property

Radiant flux incident per unit area of a surface; the power incident per unit area. The radiometric unit of measure is W/m² or factors thereof (mW/cm²). The photometric units of measure are lumens/m², lux, phot, and footcandles.

Joule (J)

The Joule is the SI unit of energy.

Lambertian Surface

A surface whose emission or scattering follows Lambert's Cosine Law, in which the radiant intensity leaving a surface is proportional to the cosine of the angle from surface normal. See Cosine Collector.


The precision with which there is a direct relationship between the incident radiation and the resultant measurement value up to a point of saturation. A linearity of 1% states that the ratio of the measured value to that of the incident radiation will not vary more than 1% from the absolute.

Lumen (lm) photometric measurement

The lumen is the photometric unit of power. It is the flux emitted in a unit solid angle by a point source that has one candela luminous intensity.

Luminance photometric property

Flux density per unit solid angle.

Lux radiometric measurement

S.I. unit of illuminance equal to 1 lumen per square meter.

Mean Spherical Candlepower (MSCP)

Luminous intensity of a light source. The MSCP rating of a lamp is measured at the design voltage and is the total amount of light given off from a light source in ALL directions (measured in an integrating sphere).

One MSCP is equivalent to all the light emitted from all directions of one standard spermaceti candle. Mean spherical candlepower is the generally accepted method of rating the total light output of miniature lamps. 1 MSCP equals 12.57 (4 pi) lumens.

A unit of length equal to 10-6 m. Infrared wavelengths are typically measured in microns.


A monochromator is a devise that uses a diffraction grating or prism to disperse light into a spectrum of its constituent wavelengths. The dispersive element is rotated such that only a narrow (monochromatic) band of light is permitted to exit the monochromator through a narrow aperture or slit.


Prefix denoting 10-9. One nanowatt (nW) = 10-9 watts.

Nanometer (nm)

A unit of length equal to 10-9 m.  Abbreviated nm. Commonly used unit to define wavelength of light, particularly in the UV and visible ranges of the electromagnetic spectrum.

Narrow Band Filter

A narrow band filter allows only a limited number of wavelengths to pass through it. Narrow band filters are usually specified at a specific central wavelength, a band pass indicating the range of wavelengths that will pass through it and a % transmission at the limits of the band pass. A 500nm narrow band pass filter with a band pass of 10 nm with a 5% cut off will pass the wavelengths between 495nm and 505nm. The transmittance above and below these wavelengths will be less than 5%.

Neutral Density Filter

A filter that reduces the intensity of the light passing through it without altering the relative spectral distribution of the energy. Neutral densities are given by the log base 10 of their attenuation. An attenuation of 100 gives a Neutral Desnity (ND) of 2. See Optical Density.

Nit (nt) photometric measurement

A unit of measure of brightness (luminance) equal to one candela per square meter.

Noise Equivalent Irradiance (NEI)

The radiant flux density in W/cmrequired to produce a signal equal to the inherent noise of the detection system. The input irradiance at which the signal to noise ratio is 1.

Noise Equivalent Power (NEP)

The radiant power, at a specified wavelength and band pass that will produce a an output signal from a detector that is equivalent to the inherent noise in that detector.


The normal is the axis drawn perpendicular to an illuminated surface. The normal is the reference from which reflective, diffractive and refractive angles are measured. A ray with a zero degree angle of incidence arrives perpendicular to a surface. A ray with a 90° angle of incidence is parallel to a surface and may not strike it.

Notch Filter

See band eliminator filter.


A measure of a material's ability to block light. It is equivalent to the reciprocal of the material’s transmittance.

Optical Chopper

A mechanical or electro optical device for passing and interrupting, at a uniform frequency, a beam of light.

Optical Density (OD)

A measure of the transmittance T through an optical medium.  OD = -log10T.   An OD of 1 is equivalent to 10% transmission. A 2 OD filter would have a transmission of 1%.

Peak Irradiance UV curing

The intense peak of focussed power directly under a lamp. The maximum point of the irradiance profile. Measured in irradiance units (W/cm²).

Phot (ph) photometric measurement

A measurement unit for illuminance. One phot = 10,000 lux (lx).


A photodiode is a two electrode semiconductor device with an optical radiation sensitive junction in which the reverse current varies with the illumination. The wavelength sensitivity is a function of the materials used in the device. Silicon photodiodes are sensitive through most of the visible spectrum. InGaAs photodiodes are sensitive in the NIR region of the spectrum. GaP photodiodes are used for the UV region of the spectrum.

Photodynamic Therapy

The use of optical radiation in the cure of medical maladies. Photodynamic therapy is used in the treatment of skin conditions such as psoriasis, jaundice in newborns, and more recently in the treatment of certain types of cancer.


A device for measuring luminous intensity or luminance. A photometer employs a photopic filter which has a band pass that is matched to the human eye response. The S.I. units employed are lumens and lux.

Photomultiplier Tube (PMT)

A photomultiplier tube is a vacuum device in which a photocathode emits electrons when exposed to light. The electrons are then accelerated through electrostatic fields to metal plates where a greater number of secondary electrons are emitted. This is repeated through several stages. This amplifies the current by many thousands.


Having sensitivity characteristic similar to the human eye response. A photopic filter will have a band pass between 400 and 700nm with its highest transmission at 550nm with a spectral responsivity specified by CIE.


A chemical substance which is rendered insoluble upon exposure to light. By exposing photoresist through a mask, electrical circuits can be created by washing the unexposed areas and etching the material below. Photoresists are usually optimized for specific regions of the UV spectrum specified as UVC, UVB, and UVA depending on the type of lamp employed for the exposure.


Many chemical products both medicinal and protective coatings can degrade when exposed to light. Photostability measurements are made to determine the principal wavelengths responsible for the degradation and the amount of exposure (dose) required to create a change that will be deleterious to the product’s efficacy. In photostability studies the effects from the visible wavelengths of light must be distinguished from the UV. Typically two detectors are employed with filtration limiting each to the measurement to only one spectral region.

Pico (p)

Prefix denoting 10-12. One pW = 10-12 Watts.

Radiance radiometric measurement

Radiant power per unit source area per solid angle. W/m²/steradian.

Radiant Emittance radiometric measurement

Radiant power emitted into a full sphere (4p steradian) by a unit area of a source , expressed in W/m². An Integrating sphere is typically used to make this measurement.

Radiant Exitance radiometric measurement

The radiant flux per unit area emitted from a source.


A device for measuring the intensity or accumulation of radiant energy. Advice on choosing a Radiometer.


The science of radiation measurement. The detection and measurement of radiant energy either at specific wavelengths or band passes or as a function of wavelength over a broad spectrum. The measurement of the interaction of light with matter as to absorption, transmission, and reflectance.


The geometric representation of a light path through an optical system.


The ratio of the reflected flux to the incident flux from a surface. In some cases the measurement may be made with either the specular or diffuse component of the total reflected flux. Reflectance is expressed as a percent.

Relative Spatial Responsivity

The relative spatial responsivity of a detector indicates the acceptance angle and percent of incident radiation at that angle that will strike the detector. The measurement is usually made in comparison to a perfect Lambertian surface.

Responsivity (spectral sensitivity)

The response or sensitivity of any system as a function of incident wavelength. In radiometry, it is the output of a device versus wavelength.


A condition wherein the radiant flux density exceeds either or the capacity of a photodetector to emit electrons in a linear relationship to the incident flux and/or the current produced by the detector exceeds the capacity of the electronics to measure the current in a linear fashion.


Relating to the wavelength responsivity of the human eye under dark adapted conditions.


The ratio of the output signal from a detector to the input signal. This may also be expressed as the minimum input irradiance level which will produce an output signal that exceeds the noise level of the detector; i.e. where the S/N is greater than 1.


Systeme Internationale d’Unities; the international metric system of units.

Spectral Response

The measure of a detector’s relative sensitivity as a function of incident wavelength. A typical spectral response curve will display the responsivity as a percentage at a given wavelength to wavelength of maximum responsivity.

Spectrometer / Spectrograph

A device which measures the interaction between light and materials as a function of wavelength. A spectrometer is usually a monochromator with an integrated detector. A spectrograph does not have an exit slit, allowing a broad band of wavelengths to be measured simultaneously by means of a multi element detector or photographic plate.

Specular Reflectance

Reflection from a mirror-like surface where in the coherence of the incident beam is maintained in the reflected beam. This is opposed to diffuse reflection wherein the reflected light is spread in all directions in a Lambertian-like manner.

Steradian (sr)

The unit of solid angle subtended by an area on the surface of a sphere equal the square of the radius of the sphere. One steradian can be visualized as a conic section with a solid angle of approximately 66°.

Stilb (sb) photometric measurement

A unit of luminance equal to 1candela/cm².

"T" Number of a Lamp

The lamp "T" number is the diameter of the lamp in increments of 1/8 inch. A "T-1" lamp is 1/8 inch in diameter, a "T-2" lamp is 1/4 inch in diameter, etc.

Talbot photometric measurement

The SI unit of the quantity of light expressed in lumen-seconds.

Thermocouple Thermopile

A device made up of dissimilar metals in which a small current is produced as a function of the difference in temperature of the materials at the junction. Thermocouples can be used for measurement of radiation in the infrared region of the spectrum.


The ratio of the radiant power transmitted through a material to the incident radiant power. Transmittance is usually expressed as a percent. A filter with a 50% transmittance (at a specific wavelength) will absorb half of the light incident on it and allow half of it to pass through it.

UV (ultraviolet)

The invisible portion of the electromagnetic spectrum with wavelengths between 1nm and 400nm.


The portion of the UV spectrum covering the wavelength range between 320nm and 400nm. This spectral region is used for many medical, UV curing and photolithographic applications. The earth’s atmosphere (at sea level) absorbs all wavelengths shorter than UVA. Prolonged exposure to UVA radiation will cause sunburn.


UVB is the portion of the UV spectrum that covers the wavelength range between 280nm and 320nm. UVB radiation is typically used in UV curing and photolithographic applications as well as in certain medical applications. Exposure to UVB radiation (from lamps or electric arcs) can cause severe sunburn and cause eye damage.


UVC is the portion of the UV spectrum that extends from 190nm to 280nm. UVC is usually employed in water treatment and sterilization applications. UVC is also used in UV curing and photolithography in microelectronics applications. Exposure to UVC radiation (from lamps, arcs, or lasers) can cause severe biological damage.

VUV (vacuum ultraviolet)

The VUV is the portion of the UV spectrum below 190 nm. Electromagnetic radiation below 190nm is absorbed by oxygen in air. Physical or chemical interactions requiring VUV radiation must be performed in a nitrogen purged environment down to 160nm or in a vacuum chamber below 160nm.

Visible Spectrum (VIS)

The visible portion of the spectrum extends between 400nm and 700nm (per the CIE). It covers the wavelengths of light that the human eye can perceive.

Watt (W) radiometric measurement

The Watt is a unit of power or work. One Watt corresponds to one Joule/sec.


When electrons vibrate they produce oscillating perpendicular electric and magnetic fields. The distance between sequential field intensity maxima is defined as the wavelength. These distances for visible radiation are very small and are usually expressed in unit lengths of nanometers (nm).


1 watt (W):
      = 0.27 lm @ 400 nm
      = 25.9 lm @ 450 nm
      = 220.0 lm @ 500 nm
      = 679.0 lm @ 550 nm
      = 683.0 lm @ 555 nm
      = 430.0 lm @ 600 nm
      = 73.0 lm @ 650 nm
      = 2.78 lm @ 700 nm

l lumen (lm)
      = 1.465 x 10-3 W @ 555 nm
      = 7.958 x 10-2 candela (4p sr)

1 joule (J)
      = 1 watt*second
      = 1 x 107 erg
      = 0.2388 gram*calories

1 lm*second
      = 1 talbot (T)
      = 1.464 x 10-3 joules @ 555 nm


1 W/cm²
      = 1 x 104 W/m²
      = 6.83 x 106 lux @ 555 nm
      =14.33 g*cal/cm²/min

1 lm/m²

      = 1 lux
      = 1 x 10-4 lm/cm²
      =1 x 10-4 phot (ph)
      = 9.290 x 10-2 lm/ft²
      = 9.290 x 10-2 foot-candles (fc)



1 watt/steradian (W/sr)
      = 12.566 watts (isotropic)
      = 683 candela @ 555 nm

1 lumen/steradian (lm/sr)
      = 1 candela (cd)
      =12.566 lumens (isotropic)
      = 1.464 x 10-3 W/sr @ 555 nm


1 W/cm²/sr
      = 6.83 x 106 lm/m²/sr @ 555 nm
      = 683 cd/cm²@ 555 nm

1 lm/m2/sr
      = 1 candela/m² (cd/m²)
      = 1 nit
      = 1 x 10-4 lm/cm²/sr
      = 1 x 10-4 cd/cm²
      = 1 x 10-4 stilb (sb)
      = 9.290 x 10-2 cd/ft²
      = 9.290 x 10-2 lm/ft²/sr
      = 3.142 apostilbs (asb)
      = 3.142 x 10-4 lamberts (L)
      = 2.919 x 

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