Physics > Photometry and Radiometry Units
The tables below show the units and qualities used to measure the brightness of light. (quantity of light) They are separated into two parts; photometry and radiometry units.
The three dimensions of light are:
- Intensity (or amplitude) - related to the perception of brightness
- Frequency (or wavelength) - The perceived colour of the light
- Polarization (or angle of vibration), which is only weakly perceptible by humans under ordinary circumstances
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 (= lm/ sr) |
cd |
an SI base unit |
| Luminance |
Lv |
candela per square metre |
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 from a surface |
| Luminous efficacy |
|
lumen per watt |
lm/W |
ratio of luminous flux to radiant flux; maximum possible is 683.002 |
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.
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 |