${\displaystyle K=\frac{{\Phi }_{\text{v}}}{{\Phi }_{\text{e}}}}$

unit: lm · W ^-1

NOTE – When applied to monochromatic radiations, the maximum value of K(λ) is denoted by the symbol K_m.
K_m = 683 lm • W ^-1 for v_m = 540 x 10¹² Hz (λ_m ≈ 555 nm) for photopic vision.
K′_m = 1700 lm • W ^-1 for λ′_m = 507 nm for scotopic vision.
For other wavelengths : K(λ) = K_m V(λ) and K′(λ) = K′_m V′(λ).