$\Phi =K_{\text{m}}{\displaystyle \underset{0}{\overset{\infty }{\int }}V(\lambda )P(\lambda )}\mathrm{d}\lambda$

where:

P(λ) is the power spectral density radiated by the source at wavelength λ;
V(λ) is the spectral luminous efficiency for photopic vision;
K_m is a constant

NOTE – In the SI system of units, where P(λ) is expressed in watts per metre, the luminous flux Ф is expressed in lumens, and K_m = 683 lm/W.