Radiometry

{{Short description|Measurement of electromagnetic radiation (esp. optical radiation)}} {{one source|date=December 2015}} thumb|upright=1.5|Comparison of photometric and radiometric quantities '''Radiometry''' is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye.

The fundamental difference between radiometry and photometry is that radiometry can cover the entire optical radiation spectrum,<ref name="CIE">{{cite encyclopedia |author=CIE |date=2020 |edition=2 |title=e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary |at=§17-25-005, radiometry |url=https://cie.co.at/eilvterm/17-25-005 |location=Vienna |publisher=International Commission on Illumination |access-date=7 Dec 2025}}</ref> while photometry is limited to the visible spectrum.<ref name="IES">{{cite encyclopedia |author=IES |date=2022 |title=ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering |at=§8.4, photometry |url=https://ies.org/definitions/photometry/ |location=New York |publisher=Illuminating Engineering Society |access-date=7 Dec 2025}}</ref> However, some definitions of radiometry include other portions of the electromagnetic radiation spectrum,<ref>{{cite encyclopedia |author=IES |date=2022 |title=ANSI/IES LS-1-22, Lighting Science: Nomenclature And Definitions For Illuminating Engineering |at=§8.3, radiometry |url=https://ies.org/definitions/radiometry/ |location=New York |publisher=Illuminating Engineering Society |access-date=7 Dec 2025}}</ref><ref name="IUPAC">{{cite encyclopedia |author=IUPAC |date=2019 |edition=2 |title=IUPAC Recommendations: Compendium of Chemical Terminology (Gold Book) |at=§R05115, radiometry |url=https://goldbook.iupac.org/terms/view/R05115 |doi=10.1351/goldbook |location=Research Triangle Park, NC |publisher=International Union of Pure and Applied Chemistry |access-date=7 Dec 2025}}</ref> and some glossaries define photometry such that associated quantities are weighted by wavelength according to the spectral sensitivity of the human visual system.<ref>{{cite encyclopedia |author=CIE |date=2020 |edition=2 |title=e-ILV, online version of CIE S 017:2020, International Lighting Vocabulary |at=§17-25-013, photometry |url=https://cie.co.at/eilvterm/17-25-013 |location=Vienna |publisher=International Commission on Illumination |access-date=7 Dec 2025}}</ref> Photometry can therefore be considered a kind of radiometry.<ref name="HB-10">{{cite book |last1=DiLaura |first1=David L. |last2=Houser |first2=Kevin W. |last3=Mistrick |first3=Richard G. |last4=Steffy |first4=Gary R. |title=The Lighting Handbook: Reference and Application |publisher=Illuminating Engineering Society |edition=10 |location=New York |date=2011 |page=9.1 |isbn=978-087995-241-9 |oclc=739932332 |url=https://archive.org/details/lightinghandbook0000unse}}</ref> Radiometry is distinct from quantum techniques such as photon counting.

The use of radiometers to determine the temperature of objects and gasses by measuring radiation flux is called pyrometry. Handheld pyrometer devices are often marketed as infrared thermometers.

Radiometry is important in astronomy, especially radio astronomy, and plays a significant role in Earth remote sensing. The measurement techniques categorized as ''radiometry'' in optics are called ''photometry'' in some astronomical applications, contrary to the optics usage of the term.

'''Spectroradiometry''' is the measurement of absolute radiometric quantities in narrow bands of wavelength.<ref>{{cite book|title=Focal Encyclopedia of Photography|publisher=Focal Press | author=Leslie D. Stroebel | author2=Richard D. Zakia | name-list-style=amp | date=1993 | edition=3rd|page=[https://archive.org/details/focalencyclopedi00lesl/page/115 115] |isbn=0-240-51417-3 | url = https://archive.org/details/focalencyclopedi00lesl |url-access=registration|quote=spectroradiometry Focal Encyclopedia of Photography.}}</ref>

== Radiometric quantities == {{SI radiometry units}} {{Radiometry coefficients}}

== Integral and spectral radiometric quantities == Integral quantities (like radiant flux) describe the total effect of radiation of all wavelengths or frequencies, while spectral quantities (like spectral power) describe the effect of radiation of a single wavelength {{mvar|λ}} or frequency {{mvar|ν}}. To each '''integral quantity''' there are corresponding '''spectral quantities''', defined as the quotient of the integrated quantity by the range of frequency or wavelength considered.<ref name="ISO 2013 i869">{{cite web | title=ISO 80000-7:2019 - Quantities and units, Part 7: Light and radiation | website=ISO | date=2013-08-20 | url=https://www.iso.org/standard/64977.html | access-date=2023-12-09}}</ref> For example, the radiant flux Φe corresponds to the spectral power Φe,{{mvar|λ}} and Φe,{{mvar|ν}}.

Getting an integral quantity's spectral counterpart requires a limit transition. This comes from the idea that the precisely requested wavelength photon existence probability is zero. Let us show the relation between them using the radiant flux as an example:

Integral flux, whose unit is W: <math display=block>\Phi_\mathrm{e}.</math> Spectral flux by wavelength, whose unit is {{nobreak|W/m}}: <math display=block>\Phi_{\mathrm{e},\lambda} = {d\Phi_\mathrm{e} \over d\lambda},</math> where <math>d\Phi_\mathrm{e}</math> is the radiant flux of the radiation in a small wavelength interval <math>[\lambda - {d\lambda \over 2}, \lambda + {d\lambda \over 2}]</math>. The area under a plot with wavelength horizontal axis equals to the total radiant flux.

Spectral flux by frequency, whose unit is {{nobreak|W/Hz}}: <math display=block>\Phi_{\mathrm{e},\nu} = {d\Phi_\mathrm{e} \over d\nu},</math> where <math>d\Phi_\mathrm{e}</math> is the radiant flux of the radiation in a small frequency interval <math>[\nu - {d\nu \over 2}, \nu + {d\nu \over 2}]</math>. The area under a plot with frequency horizontal axis equals to the total radiant flux.

The spectral quantities by wavelength {{mvar|λ}} and frequency {{mvar|ν}} are related to each other, since the product of the two variables is the speed of light (<math>\lambda \cdot \nu = c</math>): :<math>\Phi_{\mathrm{e},\lambda} = {c \over \lambda^2} \Phi_{\mathrm{e},\nu},</math> or <math>\Phi_{\mathrm{e},\nu} = {c \over \nu^2} \Phi_{\mathrm{e},\lambda},</math> or <math>\lambda \Phi_{\mathrm{e},\lambda} = \nu \Phi_{\mathrm{e},\nu}.</math>

The integral quantity can be obtained by the spectral quantity's integration:

<math display=block>\Phi_\mathrm{e} = \int_0^\infty \Phi_{\mathrm{e},\lambda}\, d\lambda = \int_0^\infty \Phi_{\mathrm{e},\nu}\, d\nu = \int_0^\infty \lambda \Phi_{\mathrm{e},\lambda}\, d \ln \lambda = \int_0^\infty \nu \Phi_{\mathrm{e},\nu}\, d \ln \nu.</math>

==References== {{Reflist}}

==External links== *[https://web.archive.org/web/20130313095139/http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page (The University of Arizona College of Optical Sciences).

Category:Radiometry Category:Measurement Category:Optical metrology Category:Telecommunications engineering Category:Observational astronomy Category:Electromagnetic radiation