Photometry measures guide

Lighting is science, and we’ve come a long way from wooden fire and candle light. But if we look into the different lighting technologies, there has been a serious advancement in the efficiency and luminous output of the lamps and luminaires. Behind this advancement there are different measures to determine the quality of the light. Here is a quick summary of the different terms and descriptions:

Quantity Unit Dimension Notes
Name Symbol (1) Name Symbol Symbol
Luminous energy Q(2) Lumen per second lm-s T*J (3) Units are sometimes called talbots
Luminous flux/ luminous power ØV (2) Lumen (=cd*sr) Lm (3) Luminous energy per unit time
Luminous intensity lV Candela (= lm/sr) Cd (3) Luminous flux per solid angle
Luminance LV Candela per square metre Cd/m2 L-2* J Luminous flux per unit solid angle per unit projected source area. Units are sometimes called nits.
Illuminance EV Lux (= lm/m2) Lx L-2* J Luminous flux incident on a surface.
Luminous exitance / Luminous emmittance MV Lux Lx L-2* J Luminous flux emitted from a surface
Luminous exposure HV Lux per second Lx*s L-2* T * J
Luminous energy density ωV Lumen second per cubic metre lm*s*m-3 L-3* T * J
Luminous efficacy η(2) Lumen per watt lm/W M-1*L-2*T3*J Ratio of luminous Flux to Radiant flux or power consumption, depending on context.
Luminous efficiency/ Luminous coefficient V 1