Radiant flux

Not to be confused with Radiation flux.

"Spectral power" redirects here. Not to be confused with Spectral power density.

A flow chart describing the relationship of various physical quantities, including radiant flux and exitance.

In radiometry, radiant flux or radiant power is the radiant energy emitted, reflected, transmitted, or received per unit time, and spectral flux or spectral power is the radiant flux per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of radiant flux is the watt (W), one joule per second (J/s), while that of spectral flux in frequency is the watt per hertz (W/Hz) and that of spectral flux in wavelength is the watt per metre (W/m)—commonly the watt per nanometre (W/nm).

Mathematical definitions

Radiant flux

Radiant flux, denoted Φ_e ('e' for "energetic", to avoid confusion with photometric quantities), is defined as<ref name="ISO_9288-1989" /> <math display="block">\begin{align} \Phi_\mathrm{e} &= \frac{d Q_\mathrm{e}}{d t} \\[2pt] Q_\mathrm{e} &= \int_{\Sigma} \mathbf{S}\cdot \hat\mathbf{n}\, dA \end{align}</math> where

• t is the time;
• Q_e is the radiant energy flux of the field out of a closed surface Σ;
• S is the Poynting vector, representing the current density of radiant energy;
• n is the normal vector of a point on Σ;
• A represents the area of Σ.

But the time-average of the norm of the Poynting vector is used instead, because in radiometry it is the only quantity that radiation detectors are able to measure: <math display="block">\Phi_\mathrm{e} \approx \int_\Sigma \langle|\mathbf{S}|\rangle \cos \alpha\ dA ,</math> where ⟨-⟩ is the time-average, and α is the angle between n and <math>\langle|\mathbf{S}|\rangle.</math>

Spectral flux

Spectral flux in frequency, denoted Φ_e,ν, is defined as<ref name="ISO_9288-1989" /> <math display="block">\Phi_{\mathrm{e},\nu} = \frac{\partial \Phi_\mathrm{e}}{\partial \nu} ,</math> where ν is the frequency.

Spectral flux in wavelength, denoted Φ_e,λ, is defined as<ref name="ISO_9288-1989" /> <math display="block">\Phi_{\mathrm{e},\lambda} = \frac{\partial \Phi_\mathrm{e}}{\partial \lambda} ,</math> where λ is the wavelength.

SI radiometry units

Comparison of photometric and radiometric quantities

SI radiometry units

Quantity		Unit		Dimension	Notes
Name	Symbol<ref group="nb" name="note-suffix-e">Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.</ref>	Name	Symbol
Radiant energy	Qe<ref group="nb" name="note-alternative-symbol-radiometric">Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.</ref>	joule	J	M⋅L2⋅T−2	Energy of electromagnetic radiation.
Radiant energy density	we	joule per cubic metre	J/m3	M⋅L−1⋅T−2	Radiant energy per unit volume.
Radiant flux	Φe<ref group="nb" name="note-alternative-symbol-radiometric"></ref>	watt	W = J/s	M⋅L2⋅T−3	Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and called luminosity in Astronomy.
Spectral flux	Φe,ν<ref group="nb" name="note-suffix-nu">Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)</ref>	watt per hertz	W/Hz	M⋅L2⋅T −2	Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.
	Φe,λ<ref group="nb" name="note-suffix-lambda">Spectral quantities given per unit wavelength are denoted with suffix "λ".</ref>	watt per metre	W/m	M⋅L⋅T−3
Radiant intensity	Ie,Ω<ref group="nb" name="note-suffix-omega">Directional quantities are denoted with suffix "Ω".</ref>	watt per steradian	W/sr	M⋅L2⋅T−3	Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
Spectral intensity	Ie,Ω,ν<ref group="nb" name="note-suffix-nu"></ref>	watt per steradian per hertz	W⋅sr−1⋅Hz−1	M⋅L2⋅T−2	Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity.
	Ie,Ω,λ<ref group="nb" name="note-suffix-lambda"></ref>	watt per steradian per metre	W⋅sr−1⋅m−1	M⋅L⋅T−3
Radiance	Le,Ω<ref group="nb" name="note-suffix-omega"></ref>	watt per steradian per square metre	W⋅sr−1⋅m−2	M⋅T−3	Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
Spectral radiance Specific intensity	Le,Ω,ν<ref group="nb" name="note-suffix-nu"></ref>	watt per steradian per square metre per hertz	W⋅sr−1⋅m−2⋅Hz−1	M⋅T−2	Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".
	Le,Ω,λ<ref group="nb" name="note-suffix-lambda"></ref>	watt per steradian per square metre, per metre	W⋅sr−1⋅m−3	M⋅L−1⋅T−3
Irradiance Flux density	Ee<ref group="nb" name="note-alternative-symbol-radiometric"></ref>	watt per square metre	W/m2	M⋅T−3	Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral irradiance Spectral flux density	Ee,ν<ref group="nb" name="note-suffix-nu"></ref>	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = 10−26 W⋅m−2⋅Hz−1) and solar flux unit (1 sfu = 10−22 W⋅m−2⋅Hz−1 = 104 Jy).
	Ee,λ<ref group="nb" name="note-suffix-lambda"></ref>	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiosity	Je<ref group="nb" name="note-alternative-symbol-radiometric"></ref>	watt per square metre	W/m2	M⋅T−3	Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral radiosity	Je,ν<ref group="nb" name="note-suffix-nu"></ref>	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity".
	Je,λ<ref group="nb" name="note-suffix-lambda"></ref>	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiant exitance	Me<ref group="nb" name="note-alternative-symbol-radiometric"></ref>	watt per square metre	W/m2	M⋅T−3	Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance	Me,ν<ref group="nb" name="note-suffix-nu"></ref>	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
	Me,λ<ref group="nb" name="note-suffix-lambda"></ref>	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiant exposure	He	joule per square metre	J/m2	M⋅T−2	Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure	He,ν<ref group="nb" name="note-suffix-nu"></ref>	joule per square metre per hertz	J⋅m−2⋅Hz−1	M⋅T−1	Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence".
	He,λ<ref group="nb" name="note-suffix-lambda"></ref>	joule per square metre, per metre	J/m3	M⋅L−1⋅T−2
See also: SI Radiometry Photometry

See also

• Luminous flux
• Heat flux
• Power (physics)
• Radiosity (heat transfer)

Further reading

• Boyd, Robert (1983). Radiometry and the Detection of Optical Radiation (Pure & Applied Optics Series). Wiley-Interscience. ISBN 978-0-471-86188-1.