Sensitivity of the human eye

The human eye is sensible to light wave which wavelength is roughly between 400 nm and 700 nm. When illumination is enough (in daylight) the maximum sensitivity is in the green region at 555 nm. This is called photopic vision. The eye response is visible in the two figures below in linear and logarithmic scale.

This standard sensitivity is also used to define a conversion between the radiated energy (in Watts) and the luminous flux (in Lumen) as shown in the table below:

Wavelength [nm]	Relative sensitivity [1/1]	Equivalent of 1W radiant power [lm]
380	0.00004	0.0273
400	0.0004	0.273
420	0.004	2.73
440	0.023	15.7
460	0.06	41.0
480	0.139	94.9
500	0.323	221
520	0.71	485
540	0.954	652
555	1.000	683
580	0.87	594
600	0.631	431
620	0.381	260
640	0.175	120
660	0.061	41.7
680	0.017	11.6
700	0.0041	2.80
720	0.00105	0.717
740	0.00025	0.171
760	0.00006	0.0410

In photopic vision the eye is able to distinguish colors because it's equipped with three kind of sensors sensible in the red-orange, green and blue regions. Peak sensitivities are 575 nm, 535 nm and 445 nm respectively. These three types of sensors allow the brain to "guess" the original color of the light.

When illumination is very low (in the dark) the sensitivity of the human eye is slightly different and is called scotopic vision. The eye response is "shifted" toward blue: the peak sensitivity is in the cyan region at 507 nm and sensitivity in the red region is much lower. In scotopic vision the colors sensors do not work and only a brightness image is perceived.