Radiometry
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: the science and technology of measuring properties of electromagnetic radiation, including light
Categories: 
- radiometry
Related: radiometersradiant fluxradiancespectral quantitiesphotometry
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DOI: 10.61835/774 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What is Radiometry?
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:
| Quantity | Symbol | Alternative term | Units | Remarks |
|---|---|---|---|---|
| radiant energy | ($Q_\textrm{e}$) | optical energy, pulse energy | joule (J) | total radiated energy, e.g. of a light pulse |
| radiant energy density | ($w_\textrm{e}$) | optical energy density | J/m3 | applied e.g. to blackbody radiation |
| radiant flux | ($\Phi_\textrm{e}$) | radiant power, optical power | W = J/s | radiant energy per unit time |
| spectral flux | ($\Phi_{\textrm{e},\nu }$) or ($\Phi_{\textrm{e},\lambda }$) | optical power spectral density | W/Hz or W/nm | radiant flux per unit frequency or wavelength |
| radiant intensity | ($I_{\textrm{e},\Omega }$) | W/sr | radiant flux per unit solid angle | |
| spectral intensity | ($I_{\textrm{e},\Omega,\nu}$) or ($I_{\textrm{e},\Omega ,\lambda}$) | W sr−1 Hz−1 or W sr−1 nm−1 | radiant intensity per unit frequency or wavelength | |
| radiance | ($L_{\textrm{e},\Omega }$) | brightness (not recommended) | W sr−1 m−2 | radiant flux per unit area and unit solid angle |
| spectral radiance | ($L_{\textrm{e},\Omega ,\nu }$) or ($L_{\textrm{e},\Omega ,\lambda}$) | W sr−1 m−2 Hz−1 or W sr−1 m−2 nm−1 | radiance per unit frequency or wavelength | |
| irradiance | ($E_\textrm{e}$) | flux density | W/m2 | received radiant flux on a surface |
| spectral irradiance | ($E_{\textrm{e},\nu }$) or ($E_{\textrm{e},\lambda }$) | W m−2 Hz−1 or W m−2 nm−1 | irradiance per unit frequency or wavelength | |
| radiosity | ($J_\textrm{e}$) | W/m2 | radiant flux per unit area, leaving a surface (by emission, reflection or transmission) | |
| spectral radiosity | ($J_{\textrm{e},\nu }$) or ($J_{\textrm{e},\lambda }$) | W m−2 Hz−1 or W m−2 nm−1 | radiosity per unit frequency or wavelength | |
| radiant exitance | ($M_\textrm{e}$) | W/m2 | like radiosity, but counting only emitted radiation | |
| spectral exitance | ($M_{\textrm{e},\nu }$) or ($M_{\textrm{e},\lambda }$) | W m−2 Hz−1 or W m−2 nm−1 | radiant exitance per unit frequency or wavelength | |
| radiant exposure (fluence) | ($H_\textrm{e}$) | J/m2 | received radiant energy per unit area, equal to the time-integrated irradiance | |
| spectral exposure | ($H_{\textrm{e},\nu }$) or ($H_{\textrm{e},\lambda }$) | J m−2 Hz−1 or J m−2 nm−1 | radiant 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−1 | fraction 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 flux ↔ luminous flux, radiant intensity ↔ luminous intensity, radiance ↔ luminance, irradiance ↔ illuminance, 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 s−1, where radiometry deals with an irradiance 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 ($I_{\textrm{e},\Omega }$) which refer to unit solid angles, and their integral over all solid angles (often only over a hemispherical region, i.e., a total solid angle of ($2\pi$) yields the corresponding integral quantities. Note that in some 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
Radiometric instruments, often collectively referred to as radiometers, are devices used to measure radiometric quantities, such as radiant flux, radiant energy, irradiance, and radiance.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is radiometry?
Radiometry is the science and technology of quantifying and measuring the physical properties of electromagnetic radiation, such as visible, infrared, and ultraviolet light. It provides a system of precisely defined physical quantities for this purpose.
How does radiometry differ from photometry?
Radiometry deals with purely physical quantities of radiation, like radiant flux in watts. In contrast, photometry considers the visual response of the human eye, quantifying the perceived brightness with quantities like luminous flux in lumens.
What are some fundamental radiometric quantities?
Key radiometric quantities include radiant energy (total energy, in joules), radiant flux (power, in watts), irradiance (power per unit area received by a surface, in W/m²), and radiance (power per unit area and solid angle, in W sr−1 m−2).
What is the difference between radiant flux and irradiance?
Radiant flux is the total optical power radiated or received, measured in watts (W). Irradiance is the power per unit area incident on a surface, measured in watts per square meter (W/m²).
What is radiance?
Radiance is a quantity describing the 'brightness' of a source or a beam of light. It is defined as the radiant flux (power) emitted, reflected, or transmitted per unit projected source area and per unit solid angle.
What are spectral radiometric quantities?
Spectral quantities, such as spectral radiance or spectral irradiance, describe the distribution of a radiometric quantity over optical frequencies or wavelengths. They specify the amount of the quantity per unit frequency interval (e.g., in W/Hz) or per unit wavelength interval (e.g., in W/nm).