Photometry - the branch of optics concerned with the measurement of quantities characterising light as perceived by the human eye, as distinct from radiometry, which deals with energy measurements. In photometry, what is important is the impression that the human eye perceives when stimulated by an electromagnetic wave. The starting point of photometry is therefore how the eye functions as a selective detector of the electromagnetic spectrum. Photometric, as opposed to radiometric, units can serve as indicatorsb in psychophysical experiments.
The human eye is not equally sensitive to light waves of all wavelengths. Photometry attempts to 'straighten out' this inconvenience by selecting indices (units), which are the product of the objectively measured magnitude of light of a given wavelength, a certain constant and a sensitivity coefficient for that wavelength, which can be read off a spectral sensitivity graph. The graph shows that the eye will react differently to light of the same objective intensity if it varies in wavelength. It is worth remembering that photometry makes the assumption that the sensitivity of the human eye corresponds to photopic vision rather than scotopic vision, so it may be the case that the predictions of photometry do not work in dark lighting.