Radiant intensity

(electromagnetism) The energy emitted per unit time per unit solid angle about the direction considered; usually expressed in watts per steradian.

See also: Intensity (disambiguation)

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.

See also

• Candela
• Luminous intensity

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SI radiometry units

Quantity	Symbol	SI unit	Abbr.	Notes
Radiant energy	Q	joule	J	energy
Radiant flux	Φ	watt	W	radiant energy per unit time, also called radiant power
Radiant intensity	I	watt per steradian	W·sr−1	power per unit solid angle
Radiance	L	watt per steradian per square metre	W·sr−1·m−2	power per unit solid angle per unit projected source area. called intensity in some other fields of study.
Irradiance	E, I	watt per square metre	W·m−2	power incident on a surface. sometimes confusingly called "intensity".
Radiant exitance / Radiant emittance	M	watt per square metre	W·m−2	power emitted from a surface.
Radiosity	J or Jλ	watt per square metre	W·m−2	emitted plus reflected power leaving a surface
Spectral radiance	Lλ or Lν	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	commonly measured in W·sr−1·m−2·nm−1
Spectral irradiance	Eλ or Eν	watt per metre3 or watt per square metre per hertz	W·m−3 or W·m−2·Hz−1	commonly measured in W·m−2·nm−1

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