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Photometry

 

Photometry is the science of measurement of light.

There are 4 basic quantities:

Luminous flux (F) is the measure of the total power of light emitted, at all wavelengths in the visible band. The SI unit of luminous flux is the lumen (lm).

Luminous intensity (I) is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. The SI unit of luminous intensity is the candela (cd).

Luminance (L) is the luminous intensity of a surface in the direction of the observer. The luminance indicates how much luminous flux will be perceived by an eye looking at the surface from a particular angle of view.
Luminance is measured in candela per square metre (cd/m²).

Illuminance (E) is the luminous flux measured on the surface receiving the light, per unit area. Its unit of measurement is the lux.


Consequently, a white surface and a dark surface lit by the same source have the same illuminance, but not the same luminance!

For photometric studies, we recommend measuring the luminance, in candela per square meter, since that is what the human eye perceives.

 

photometry