PHOTOMETRY

The Science of Measuring Light:
Electromagnetic radiation as a visual stimulus

In the field of Photometry, electromagnetic radiation is evaluated in terms of its luminous power, that is, its capacity to stimulate the human visual system, and to be perceived as light. Such luminous power is measured as a quantity known as luminous flux; the unit of measurement is the lumen (lm). For some applications, luminous flux itself is the quantity of interest; for others, the the spatial concentration of luminous flux is measured. For example, it may be desirable to measure the luminous flux falling on a given area (illuminance), or directed within a given angular range (luminous intensity). The following table summarizes five fundamental types of photometric measurement, 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.

The forgoing is an overview of key concepts in photometry, with emphasis on generality and concise description. For a more complete explanation, please consult the UDTi Photometry Tutorial, and our companion index of resources.