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Unless otherwise stated this page contains Version 1.0 content (Read more about versions) 2.5.3 PhotometryThe results of photometric matching experiments show that radiation may be evaluated in terms of visual effect by means of a quantity known as luminous flux, _{}_{γ}, which is related to _{}_{e, λ}(λ), the spectral power distribution of the radiation, by an equation of the form
where The following table gives the values of this function at 10 nm intervals. For the complete tabulation at 1 nm interval refer to ISO/CIE 10527 (1991). Photopic spectral luminous efficiency function V(λ)
The magnitude of the unit of luminous flux, the lumen,
is determined by assigning a value to K_{m} of equation (1).
However, for historical reasons the SI base unit for photometry is the unit of
luminous intensity, the candela or lumen per steradian; the formal definition
(16th CGPM, 1979) appears in section 1.1.1. The rounded frequency The spatial properties of light are described in photometry by quantities analogous to those used in radiometry (see section 2.5.2), with luminous flux replacing radiant power. The same symbols are used for corresponding quantities, distinguished where necessary by the suffixes e (radiation) and v (photometric). These quantities are listed in the following table, together with their radiometric equivalents to indicate the relation between the two systems of measurement. Photometric quantities
Luminance and colour temperature of selected light sources
Scotopic system of photometry At low levels of luminance the relative luminous effects of radiations of different wavelength are in general no longer represented even approximately by the values of V(λ) given in the above table. This is because a different visual mechanism known as the scotopic or darkadapted mechanism, as opposed to the photopic or lightadapted mechanism, is dominant. To evaluate electromagnetic radiation with respect to the effect it produces on the scotopic mechanism, an alternative set of weighting factors—the scotopic spectral luminous efficiency function V'(λ)—must be used, the values of which were agreed by the CIE in 1951 and accepted by the CIPM in 1976. Scotopic spectral luminous efficiency function V'(λ)
In the alternative system of photometry obtained by taking V′(λ) as basis instead of V(λ), all the formal definitions of the previous section are retained, with terms modified by the adjective ‘scotopic’ and symbols by the addition of a prime. The calculated value for the scotopic maximum luminous efficacy K'_{m} is 1700.06 lmW^{−1}, which may safely be rounded to 1700 lm W^{−1}. In the socalled mesopic region between the photopic and the scotopic, the concept of luminous flux as an additive quantity does not apply and there is at present no agreed method of spectral weighting for mesopic vision. Reference ISO/CIE 10527 (1991) CIE Standard Colorimetric Observers. O.C.Jones / T.M.Goodman 
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