Photometry is the science of the measurement of light, in terms of its
perceived brightness to the human eye. It is distinct from radiometry, which
is the science of measurement of radiant energy (including light) in terms of
absolute power. In modern photometry, the radiant power at each wavelength is
weighted by a luminosity function that models human brightness sensitivity.
Typically, this weighting function is the photopic sensitivity function,
although the scotopic function or other functions may also be applied in the
same way. The weightings are standardized by the CIE and ISO.
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