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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