View source for Radiometry

{{Short description|Techniques for measuring electromagnetic radiation}}
{{one source|date=December 2015}}
[[File:photometry_radiometry_units.svg|thumb|upright=1.5|Comparison of photometric and radiometric quantities]]
'''Radiometry''' is a set of techniques for [[measurement|measuring]] [[electromagnetic radiation]], including [[visible light]]. Radiometric techniques in [[optics]] characterize the distribution of the radiation's [[power (physics)|power]] in space, as opposed to [[photometry (optics)|photometric]] techniques, which characterize the light's interaction with the human eye. The fundamental difference between radiometry and photometry is that radiometry gives the entire optical radiation spectrum, while photometry is limited to the visible spectrum. Radiometry is distinct from [[quantum optics|quantum]] techniques such as [[photon]] counting.

The use of [[radiometer]]s to determine the temperature of objects and gasses by measuring radiation flux is called [[pyrometry]].  Handheld pyrometer devices are often marketed as [[infrared thermometer]]s.

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 (astronomy)|''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 [[wavelength]]s or [[frequency|frequencies]], while [[electromagnetic spectrum|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 Φ<sub>e</sub> corresponds to the spectral power Φ<sub>e,{{mvar|λ}}</sub> and Φ<sub>e,{{mvar|ν}}</sub>.

Getting an integral quantity's spectral counterpart requires a [[Limit (mathematics)|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 [[watt|W]]:
<math display=block>\Phi_\mathrm{e}.</math>
Spectral flux by wavelength, whose unit is {{nobreak|W/[[metre|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/[[hertz|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>

== See also ==
* [[Reflectivity]]
* [[Microwave radiometer]]
* [[Ionizing radiation#Measurement|Measurement of ionizing radiation]]
* [[Radiometric calibration]]
* [[Radiometric resolution]]

==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).

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