| quantity defined by|
P(λ) is the power spectral density radiated by the source at wavelength λ;
V(λ) is the spectral luminous efficiency for photopic vision;
Km 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 Km = 683 lm/W.