Radiometry

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In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power.

Radiometry is important in astronomy, especially radio astronomy, and is important for Earth remote sensing. The measurement techniques categorized as radiometry in optics are called photometry in some astronomical applications, contrary to the optics usage of the term.

SI radiometry units
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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. Sometimes confusingly called "intensity".
Irradiance	E	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. Sometimes confusingly called "intensity".
Spectral radiance	L_λ or L_ν	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	E_λ or E_ν	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