Spectral sensitivity of the human eye

Spectral sensitivity is the relative efficiency of detection of light or other signal, as a function of the frequency or wavelength of the signal.

There are two basic types of retinal photo-receptors
 * Cones cell -> It's responding to bright-light condition (Photopic vision).
 * Rod cell -> It's responding to low-intensity light condition (Scotopic vision).

Depending on their spectral sensitivity, the former belong to either By combining their separate inputs, the brain creates colors. The cones are concentrated in the center of the retina, which is called fovea.
 * Long-wavelengths sensitive (L)
 * Mid-wavelengths sensitive (M)
 * Short-wavelengths sensitive (S)

Eye spectral response is directly related and influenced by light intensity to which it is exposed. Light intensity determines the level of activity of cones and rods, and with it main characteristic of human vision.

Sensitivity of cones and rods varies with the wavelength, so called visible spectrum, extending from 370 nm to 730 nm. Energy level corresponding to the wavelength of light wave -Inversely proportion to the wavelength, and in proportion to frequency - stimulates eye photo-receptors, which send received stimulus to the brain. Specific combinations of stimuli from the three different cone receptor types produce an input from which the brain create perception of color.

In general, eye sensitivity to light increase exponentially with the decrease in light intensity, with the wavelength of peak sensitivity shifting from 550 nm in day-light conditions, to 507 nm in darkness. As light intensity decreases, cone function transforms toward more effective light collection. Below certain light intensity, cone function enters dormancy, buy cones are still ready to respond to a sufficiently intense light.

On the other hand, decreasing light intensity stimulates accumulation of rhodopsin in rods, which was washed out at higher light intensity. It activates rods, enabling the eye to respond to light stimuli of much lower intensity.

The spectral sensitivity function of the human eye under photopic vision is defined by the CIE spectral luminous efficiency function V(λ). Only in very rare cases, the spectral sensitivity of the human eye under scotopic vision, defined by the spectral luminous efficiency function V’(ë), becomes technically relevant. By convention, these sensitivity functions are normalized to a value of 1 in their maximum.

Sources : Gigahertz-Optic : http://www.light-measurement.com/spectral-sensitivity-of-eye/ Wikipedia : http://en.wikipedia.org/wiki/Spectral_sensitivity Naver : http://endic.naver.com/enkrEntry.nhn?entryId=59b4cdfcbb25434ba38c5c79f0d3c82f Naver : http://terms.naver.com/search.nhn?query=spectral+sensitivity&where=&x=0&y=0