The receiver shall have a photodetector with a spectral response proportional to the photopic luminosity function of the CIE (1931) standard colorimetric observer (see table).
It has long been recognized that the ability of the receptors in the retina to absorb electromagnetic radiation depends on the wave length of the radiation. Consequently, in 1924, the Commission de l'Eclairage (CIE) created a standard photopic luminosity function or 'standard observer' (see below) for photometric measurements. For the human eye, an efficiency of 1 was assigned to the wavelength of 555 nanometers (nm). The logarithm of this function is the 'relative visual brightness'.
