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.