Radiant Intensity

In radiometry, radiant intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. The SI unit of radiant intensity is watts per steradian (W·sr−1). Radiant intensity is distinct from irradiance and radiant exitance, which are often called intensity in branches of physics other than radiometry.

SI radiometry units
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Quantity Symbol SI unit Symbol Dimension Notes
Radiant energy Qe joule J M⋅L2⋅T−2 energy
Radiant flux Φe watt W M⋅L2⋅T−3 radiant energy per unit time, also called radiant power.
Spectral power Φ watt per metre W⋅m−1 M⋅L⋅T−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 M⋅L2⋅T−3 power per unit solid angle.
Spectral intensity I watt per steradian per metre W⋅sr−1⋅m−1 M⋅L⋅T−3 radiant intensity per wavelength.
Radiance Le watt per steradian per square metre W⋅sr−1⋅m−2 M⋅T−3 power per unit solid angle per unit projected source area.

confusingly called "intensity" in some other fields of study.

Spectral radiance L
or
L
watt per steradian per metre3
or

watt per steradian per square
metre per hertz

W⋅sr−1⋅m−3
or
W⋅sr−1⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.


Irradiance Ee watt per square metre W⋅m−2 M⋅T−3 power incident on a surface, also called radiant flux density.

sometimes confusingly called "intensity" as well.

Spectral irradiance E
or
E
watt per metre3
or
watt per square metre per hertz
W⋅m−3
or
W⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
commonly measured in W⋅m−2⋅nm−1
or 10−22W⋅m−2⋅Hz−1, known as solar flux unit.


Radiant exitance /
Radiant emittance
Me watt per square metre W⋅m−2 M⋅T−3 power emitted from a surface.
Spectral radiant exitance /
Spectral radiant emittance
M
or
M
watt per metre3
or

watt per square
metre per hertz

W⋅m−3
or
W⋅m−2⋅Hz−1
M⋅L−1⋅T−3
or
M⋅T−2
power emitted from a surface per wavelength or frequency.


Radiosity Je or J watt per square metre W⋅m−2 M⋅T−3 emitted plus reflected power leaving a surface.
Radiant exposure He joule per square metre J⋅m−2 M⋅T−2
Radiant energy density ωe joule per metre3 J⋅m−3 M⋅L−1⋅T−2
See also: SI · Radiometry · Photometry

Famous quotes containing the words radiant and/or intensity:

    No
    Praise that the spring time is all
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    Dylan Thomas (1914–1953)

    We may say that feelings have two kinds of intensity. One is the intensity of the feeling itself, by which loud sounds are distinguished from faint ones, luminous colors from dark ones, highly chromatic colors from almost neutral tints, etc. The other is the intensity of consciousness that lays hold of the feeling, which makes the ticking of a watch actually heard infinitely more vivid than a cannon shot remembered to have been heard a few minutes ago.
    Charles Sanders Peirce (1839–1914)