| 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
|
