Photometric Measurements

quantitative determinations of the values of quantities characterizing optical radiation (light in the broad sense) or such optical properties of materials as transparency and reflectivity. Photometric measurements can be made with instruments that contain optical detectors. In the simplest cases in the visible light range, the human eye is used as a detector in evaluating photometric quantities.

Table 1. Principal photometric qualities
Quantity	Symbol	Defining equation	Unit
			Name	Symbol
Note: The subscript v of Φ indicates that Φv is a photometric quantity, in contrast to the radiometric quantity Φe(radiant flux); (t) time, (dΩ) infinitesimal solid angle through which the luminous flux is radiated, (dA) element of area of the surface, (θ) angle between the normal to the element of the surface and the direction in which the luminous flux is radiated (X) any photometric quantity
Luminous flux..................	ϕv		lumen	lm
Luminous energy	Q	Q = ʃϕvdt	lumen-second	lm-s
Luminous intensity (of a light source in some direction) ..........	l	l=d Φv/dΩ	candela	cd
Luminous efficacy of radiant power ...........	K	K = Φv/Φe	lumen per watt	lm/w
Luminance (at a given point and in a given direction) ....	L		candela per square meter (formerly, nit) lux	cd/m2
Illuminance (at a point of a surface).............	E	E = dϕv/dA	lux	lx
Luminous exitance...........	M	M=dϕv/dA	lumen per square meter	lm/m2
Exposure (quantity of illumination)..........	H	H=dQ/dA = ∫E dt	lux-second	lx-s
Luminous pulse emittance .............	θ	θ=⊄ldt	candale-section	cd-s
Spectral concentration of a photometric quantity .....	Xλ	Xλ = dx/dλ