| SI radiometry units |
| Quantity |
Symbol |
SI unit |
Abbr. |
Notes |
| Radiant energy |
Q |
joule |
J |
energy |
| 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 J |
watt/square meter |
W.m−2 |
emitted plus reflected power leaving a surface |
| Spectral radiance |
L 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 |
E 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 |