Encyclopedia … combined with a great Buyer's Guide!


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

German: Photometrie

Categories: article belongs to category general optics general optics, article belongs to category light detection and characterization light detection and characterization


Cite the article using its DOI: https://doi.org/10.61835/774

Get citation code: Endnote (RIS) BibTex plain textHTML

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 energy<$Q_\textrm{e}$>optical energy, pulse energyjoule (J)total radiated energy, e.g. of a light pulse
radiant energy density<$w_\textrm{e}$>optical energy densityJ/m3applied e.g. to blackbody radiation
radiant flux<$\Phi_\textrm{e}$>radiant power, optical powerW = J/sradiant energy per unit time
spectral flux<$\Phi_{\rm{e},\nu }$> or <$\Phi_{\rm{e},\lambda }$>optical power spectral densityW/Hz or W/nmradiant flux per unit frequency or wavelength
radiant intensity<$I_{\rm{e},\Omega }$>W/srradiant flux per unit solid angle
spectral intensity<$I_{\textrm{e},\Omega,\nu}$> or <$I_{\rm{e},\Omega ,\lambda}$>W sr−1 Hz−1 or W sr−1 nm−1radiant intensity per unit frequency or wavelength
radiance<$L_{\rm{e},\Omega }$>brightness (not recommended)W sr−1 m−2radiant flux per unit area and unit solid angle
spectral radiance<$L_{\rm{e},\Omega ,\nu }$> or <$L_{\rm{e},\Omega ,\lambda}$>W sr−1 m−2 Hz−1 or W sr−1 m−2 nm−1radiance per unit frequency or wavelength
irradiance<$E_\textrm{e}$>flux densityW/m2received radiant flux on a surface
spectral irradiance<$E_{\rm{e},\nu }$> or <$E_{\rm{e},\lambda }$>W m−2 Hz−1 or W m−2 nm−1irradiance per unit frequency or wavelength
radiosity<$J_\textrm{e}$>W/m2radiant flux per unit area, leaving a surface (by emission, reflection or transmission)
spectral radiosity<$J_{\rm{e},\nu }$> or <$J_{\rm{e},\lambda }$>W m−2 Hz−1 or W m−2 nm−1radiosity per unit frequency or wavelength
radiant exitance<$M_\rm{e}$>W/m2like radiosity, but counting only emitted radiation
spectral exitance<$M_{\rm{e},\nu }$> or <$M_{\rm{e},\lambda }$>W m−2 Hz−1 or W m−2 nm−1radiant exitance per unit frequency or wavelength
radiant exposure<$H_\textrm{e}$>J/m2received radiant energy per unit area, equal to the time-integrated irradiance
spectral exposure<$H_{\rm{e},\nu }$> or <$H_{\rm{e},\lambda }$>J m−2 Hz−1 or J m−2 nm−1radiant exposure per unit frequency or wavelength
hemispherical emissivity<$\epsilon$>radiant exitance relative to that of a blackbody at the same temperature
hemispherical absorptance<$A$>fraction of absorbed radiant flux on a surface
hemispherical reflectance<$R$>fraction of reflected radiant flux on a surface
hemispherical transmittance<$T$>fraction of transmitted radiant flux on a surface
hemispherical attenuation coefficient<$\mu$>m−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.

There are also various quantities like <$I_{\rm{e},\Omega }$> 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 <$\cos \theta$> 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.

More to Learn

Encyclopedia articles:

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Spam check:

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your network:

Follow our specific LinkedIn pages for more insights and updates: