For very low levels of intensity (scotopic vision), the sensitivity of the eye is mediated by rods, not cones, and shifts toward the violet, peaking around 507 nm for young eyes; the sensitivity is equivalent to 1699 lm/W or 1700 lm/W at this peak. The standard scotopic luminous efficiency function or V ′ (λ) was adopted by the CIE in 1951, based on measurements by Wald (1945) and by Crawford (1949).