The science of measuring light:
Electromagnetic radiation as a visual stimulus

The practice of Photometry involves the measurement of electromagnetic radiation in terms of its luminous power – that is, its capacity to stimulate the human visual system, and to be perceived as light. The unit of measurement for such power, or luminous flux, is the lumen (lm). As in Radiometry, five fundamental types of photometric measurement may be enumerated, corresponding to five different types of measurement geometry:

MEASURAND	SI UNITS	NON-SI	DESCRIPTION
Luminous Flux	lumen (lm)	-	Luminous power produced by a source
Illuminance	lux [lm/m2]	footcandle (fc)[lm/ft2]	Luminous Flux incident upon a surface, per unit area
Luminous Exitance	lm/m2	lm/ft2	Luminous Flux leaving a surface, per unit area
Luminous Intensity	candela(cd) [ lm/sr ]	-	Luminous Flux leaving a point source, per unit solid angle - measured in steradians (sr)
Luminance	nit [cd/(m2)]	footlambert (fl) [cd/(pft2)]	Luminous Flux per unit solid angle leaving an extended source in a given direction, per unit projected area

Photometry is related to Radiometry, and the lumen to the watt, by a simple mathematical model of the spectral sensitivity of the human visual system. This model is represented by the spectral luminous efficiency function, V(λ). By definition, the luminous flux of a source, Φ_L, is related to its spectral radiant flux, Φ(λ), by the application of a weighted integral, as indicated in the equation below. Note that - for historical reasons - the unitless V(λ) function is normalized at a peak value of 683 (lm/W):

When spectroradiometric data is available, photometric data can be derived by weighted integration, as indicated above. An alternative approach involves the construction of a sensor with a relative spectral response which closely approximates the V(λ) function. Such a device is known as a photometric or photopic sensor.

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