Note 1 to entry: The S/P ratio can be expressed by

${\displaystyle R_{\text{SP}}=\frac{{K^{\prime }}_{\text{m}}{\displaystyle \underset{\text{0}}{\overset{\infty }{\int }}S\text{(}\lambda \text{)}{V}^{\prime }\text{(}\lambda \text{)}\text{d}\lambda }}{K_{\text{m}}{\displaystyle \underset{\text{0}}{\overset{\infty }{\int }}S\text{(}\lambda \text{)}V\text{(}\lambda \text{)}\text{d}\lambda }}}$

where K′_m ≈ 1 700 lm·W⁻¹ is the maximum luminous efficacy for scotopic vision, K_m ≈ 683 lm·W⁻¹ is the maximum luminous efficacy for photopic vision, S(λ) is the relative spectral distribution of the source, and λ is the wavelength.

Note 2 to entry: The S/P ratio has unit one.