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Radiometry

Definition: the science and technology of measuring properties of electromagnetic radiation, including light

German: Photometrie

Categories: general optics, light detection and characterization

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Radiometry is the science and technology of quantifying and measuring essential properties of electromagnetic radiation. That includes visible light, infrared and ultraviolet light as well as radio waves and X-rays, for example. In contrast to photometry, the visibility of the radiation and its perceived brightness is not of interest in this field; one is dealing with purely physical quantities, not involving properties of the human eye.

Radiometry provides precisely defined quantities as the basis for further work. Various kinds of radiometric measurement instruments have been developed for measuring such quantities.

Radiometric Quantities

The development of radiometry has led to a quite systematic and well-defined system of radiometric quantities. Some of the used terms and their precise definitions had to be revised for that purpose. However, older terms and deviating meanings are still widespread not only in older literature, but also because many professionals working primarily with optics and laser technology, for example, but not specifically in radiometry, have not (or not fully) adopted the suggested terms and definitions. The following table also specifies such alternative terms which are used particularly in optics and laser technology:

QuantitySymbolAlternative termUnitsRemarks
radiant energyQeoptical energy, pulse energyjoule (J)total radiated energy, e.g. of a light pulse
radiant energy densityweoptical energy densityJ/m3applied e.g. to blackbody radiation
radiant fluxΦeradiant power, optical powerW = J/sradiant energy per unit time
spectral fluxΦe,ν or Φe,λoptical power spectral densityW/Hz or W/nmradiant flux per unit frequency or wavelength
radiant intensityIe,ΩW/srradiant flux per unit solid angle
spectral intensityIe,Ω,ν or Ie,Ω,λW sr−1 Hz−1 or W sr−1 nm−1radiant intensity per unit frequency or wavelength
radianceLe,Ωbrightness (not recommended)W sr−1 m−2radiant flux per unit area and unit solid angle
spectral radianceLe,Ω,ν or Le,Ω,λW sr−1 m−2 Hz−1 or W sr−1 m−2 nm−1radiance per unit frequency or wavelength
irradianceEeflux densityW/m2received radiant flux on a surface
spectral irradianceEe,ν or Ee,λW m−2 Hz−1 or W m−2 nm−1irradiance per unit frequency or wavelength
radiosityJeW/m2radiant flux per unit area, leaving a surface (by emission, reflection or transmission)
spectral radiosityJe,ν or Je,λW m−2 Hz−1 or W m−2 nm−1radiosity per unit frequency or wavelength
radiant exitanceMeW/m2like radiosity, but counting only emitted radiation
spectral exitanceMe,ν or Me,λW m−2 Hz−1 or W m−2 nm−1radiant exitance per unit frequency or wavelength
radiant exposureHeJ/m2received radiant energy per unit area, equal to the time-integrated irradiance
spectral exposureHe,ν or He,λJ m−2 Hz−1 or J m−2 nm−1radiant exposure per unit frequency or wavelength
hemispherical emissivityεradiant exitance relative to that of a blackbody at the same temperature
hemispherical absorptanceAfraction of absorbed radiant flux on a surface
hemispherical reflectanceRfraction of reflected radiant flux on a surface
hemispherical transmittanceTfraction of transmitted radiant flux on a surface
hemispherical attenuation coefficientμm−1fraction of absorbed or scattered radiant flux per unit length

The subscript “e” of many of those quantities (which is frequently omitted) indicates that they refer to physical energies rather than to visual impressions (“v”) as in photometry. For most radiometric quantities, there is a related photometric quantity, for example radiant fluxluminous flux, radiant intensityluminous intensity, radianceluminance, irradianceilluminance, etc.

Radiometric quantities can be applied not only to visible light, but also to infrared light, ultraviolet light and radiation in other spectral regions.

There is a related field named actinometry, which refers to photon numbers instead of energies. For example, one uses a photon flux in units of m−2 s1, where radiometry deals with a radiant flux in units of W/m2 = J m−2 s−1.

Spectral and Integral Quantities

Some of those quantities are spectral quantities, referring to some unit frequency or wavelength interval. Their symbols contain “ν” or “λ” in the subscript. By integration of those over all optical frequencies or wavelengths, respectively, one obtains the corresponding integral quantities. For example, the radiant intensity equals the frequency- or wavelength-integrated spectral radiant intensity.

See the article on spectral quantities for more details.

Quantities Related to Solid Angles

There are also various quantities like Ie,Ω which refer to unit solid angles, and their integration over all solid angles (often only over a hemispherical region, i.e., a total solid angle of 2π) one obtains the corresponding integral quantities. Note that in cases a factor like cos θ is involved in the integrand.

For some of the listed quantities, e.g. for the hemispherical absorptance, the corresponding spectral quantities or angle-resolved quantities are not listed in the table above; they are defined in completely analogous ways.

Radiometric Instruments

Various types of instruments can be used for measuring radiometric quantities:

Related quantities such as the radiance of a light source are usually calculated from other measured values, e.g. from the irradiance at a location in some distance from the source.

The term radiometer can be understood as referring to any instruments measuring radiometric quantities. However, it is most common for instruments measuring quantities of invisible radiation.

Suppliers

The RP Photonics Buyer's Guide contains 3 suppliers for radiometers.

See also: photometry, radiant flux, radiance, spectral quantities
and other articles in the categories general optics, light detection and characterization

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