# Photometric Measurements

## 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
QuantitySymbolDefining equationUnit
NameSymbol
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 lumenlm
Luminous energyQQ = ʃϕvdtlumen-secondlm-s
Luminous intensity (of a light source in some direction) ..........ll=d Φv/dΩcandelacd
Luminous efficacy of radiant power ...........KK = Φvelumen per wattlm/w
Luminance (at a given point and in a given direction) ....Lcandela per square meter (formerly, nit) luxcd/m2
Illuminance (at a point of a surface).............EE = dϕv/dAluxlx
Luminous exitance...........MM=dϕv/dAlumen per square meterlm/m2
Exposure (quantity of illumination)..........HH=dQ/dA = ∫E dtlux-secondlx-s
Luminous pulse emittance .............θθ=⊄ldtcandale-sectioncd-s
Spectral concentration of a photometric quantity .....XλXλ = dx/dλ
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