Luminous efficiency depends not only on radiant energy, i.e. the amount of light that enters the human eye, but also on its spectral properties. The human eye is not equally sensitive to all sectors of the Spectrum.

Light stimuli at different wavelengths have different degrees of luminous efficiency for the human eye. Maximum sensitivity for photopic vision is at a wavelength of 555 nm (green), declining to both the longer wavelengths (yellow, orange, red) and the shorter wavelengths (blue, violet). Beyond approx. 380 and 780 nm it is almost zero. The sensitivity functions [V(l)] of the eye differ considerably between day and night "Photopic Vision = Light Adaptation" and "Scotopic Vision = Dark Adaptation".

In the night, maximum sensitivity shifts to the shorter wavelengths (blue, violet) so that superior perception is available for the blue portions of light that predominate atnight [maximum at 507 nm (blue-green)]. The "warm" long-wave red portion of light is hardly present and is also very weak in terms of luminous efficiency. The explanation for these facts is to be found in the evolution of man and his need to adapt to daylight conditions.

The process can be observed in the field of artificial lighting, in which human beings prefer a daylight white light color appearance with a high level of blue when illuminance is high, but a warm white light when illuminance is low. Regardless of sensitivity, radiant energy outside of the range of spectral luminous efficiency for the human eye can still cause damage to the eye (UV radiation).

In addition the human organism is capable of adjusting to different spectral compositions of available light. A white surface, for example, is perceived as white both in daylight and under artificial light. This is explained by psychological processes relating to visual perception, including memory and natural protective functions.

Photometers illuminance and luminance photometry are nearly always set for phototopic vision, and the various photometric parameters are also based on the V(Λ) evaluation.