Luminosity Functions

Luminosity functions or luminous efficiency functions are standard curves that measure the response of the human eye to light. Luminosity functions vary under different lighting conditions.

There are two luminous efficiency functions in common use.
They are photopic (brightly lit) and scotopic (low-light).

CIE Photopic Luminosity Function

Generally, luminosity function is used to mean the photopic luminosity function.
This is same as the CIE Standard Observer
Denoted as: \(V(\lambda)\), \(\bar{y}(\lambda)\)

CIE Scotopic Luminosity Function

The standard scotopic luminosity function was adopted by the CIE in 1951, based on the measurements by Wald and Crawford.
Denoted as: \(V'(\lambda)\)