The luminosity function is a standard function established by the Commission Internationale de l'Éclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. It converts radiant energy into luminous (ie, visible) energy.

For the standard conditions chosen in establishing this standard luminosity function (photopic vision) the luminously effective radiant intensity in lumens of radiation of spectral intensity J_λ watts/unit wavelength is given by

where

y_λ is the standard luminosity function normalized to a value of unity at 555 nanometres (see luminous coefficient).

The numerical values for y_λ are commonly given as a luminosity curve.

For very low levels of intensity (scotopic vision) the peak of the luminosity function curve shifts toward the violet for young eyes (507 m_μ) with an absolute value of 1746 lumens/watt.