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.

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Quantity Symbol SI unit Symbol Dimension Notes
Radiant energy Qe joule J M⋅L2⋅T−2 energy
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.

or
L
or

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.

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.

Me watt per square metre W⋅m−2 M⋅T−3 power emitted from a surface.
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

List Of Nikola Tesla Patents - American - Patents #51–#100
... telegraphy Wireless transmission of electric power through the natural media Cites well known radiant energy phenomena and the experiments of William ... Patent 0,685,012 - Means for Increasing the Intensity of Electrical Oscillations - 1900 March 21 - A method for producing a "great increase in the intensity and duration of the (electrical) oscillations excited in a ... Producing increase intensity and duration of electric oscillations Combination of a circuit to possess freely-vibrating excitations and of means for artificially cooling the circuit to a low temperature Low ...
Electro-optical MASINT - Spectroscopic MASINT
... A spectral plot represents radiant intensity versus wavelength at an instant in time ... A radiometric plot represents the radiant intensity versus time there can be plots at multiple bands or wavelengths ... For each point along a time-intensity radiometric plot, a spectral plot can be generated based on the number of spectral bands in the collector, such as the radiant intensity plot of a ...
Photometry (optics) - Photometric Quantities - Photometric Versus Radiometric Quantities
... and radiance (radiometric) Luminous flux (photometric) and radiant flux (radiometric) Luminous intensity (photometric) and radiant intensity (radiometric) In ... greater luminous flux than a red source with the same radiant flux would ... Radiant energy outside the visible spectrum does not contribute to photometric quantities at all, so for example a 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but ...

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