SI radiometry units | ||||
Quantity | Symbol | SI unit | Abbr. | Notes |
Radiant energy | Q | joule | J | |
Radiant flux | watt | W | radiant energy per unit time, also called radiant power | |
Radiant intensity | I | watt/steradian | W.sr-1 | power per unit solid angle |
Radiance | L | watt/steradian/square meter | 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/square meter | W.m-2 | power incident on a surface, sometimes confusingly called intensity |
Radiant exitance / Radiant emittance | M | watt/square meter | W.m-2 | power emitted from a surface |
Radiosity | J or Jλ | watt/square meter | W.m-2 | emitted plus reflected power leaving a surface |
Spectral radiance | Lλ or Lv | watt/steradian/meter3 or watt/steradian/square meter/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 Ev | watt/meter3 or watt/square meter/hertz | W.m-3 or W.m-2.Hz-1 | commonly measured in W.m-2.nm-1 |
SI photometry units | ||||
Quantity | Symbol | SI unit | Abbr. | Notes |
Luminous energy | Qv | lumen second | lm.s | units are sometimes called talbots |
Luminous flux | F | lumen (= cd.sr) | lm | also called luminous power |
Luminous intensity | Iv | candela (=lumen/sr) | cd | an SI base unit |
Luminance | Lv | candela/square meter | cd/m2 | units are sometimes called "nits" |
Illuminance | Ev | lux (= lm/m2) | lx | used for light incident on a surface |
Luminous emittance | Mv | lux (= lm/m2) | lx | used for light emitted on a surface |
Luminous efficacy | lumen/watt | lm/W | ratio of luminouos flux to radiant flux | |
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