[3.2]  \Phi
Radiant flux (radiant power). The time rate of flow of radiant energy, evaluated in terms of a standardized visual response.

\Phi_{v} = K_{m}\int \Phi _{e,\lambda} V\left ( \lambda \right )d\lambda ,


\Phi _{v} = luminous flux, (lumens, lm)
\Phi _{e,\lambda } = watts per nanometer
\lambda = wavelength (nanometers, nm)
V\left(\lambda\right) = the spectral luminous efficiency function
K_{m} = the maximum spectral luminous efficacy, (lumens per watt, lm/W)

Unless otherwise indicated, the luminous flux is defined for photopic vision. For scotopic vision, the corresponding spectral luminous efficiency function, V'(λ), and the corresponding maximum spectral luminous efficacy, K’m, are substituted in the above equation. Km and K’m are derived from the basic SI definition of luminous intensity and have the values 683 lm/W and 1,700 lm/W, respectively. (See also candela, spectral luminous efficacy of radiant flux, values of spectral luminous efficiency for photopic vision, and values of spectral luminous efficiency for scotopic vision.)

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