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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.