How we perceive the light

Light produced by a light source is generally a mixture (amount) of electromagnetic radiation of different wavelengths and intensities, ie, equivalent monochromatic radiation overlap. A complete characterization of light can only be done by expressing the power radiated per wavelength (or, equivalently, for each frequency). This characterization is given by a function of SPD Spectral Power Distribution.
Note that, in fact, radiated power just over a specific wavelength is zero, which specifies the power spectral distribution function is the power radiated within a wavelength or frequency, relative to the width of the interval. As such, the power spectral distribution of light can be given in two forms, according to the wavelength, or by frequency.
Color perception by the eye – in fact, the brain – is complicated by the fact that the visual analyzer compares the color of light reflected from an object with light colors in the environment.
Thus, a white sheet (which reflects all the colors of the spectrum equally) is white and is illuminated with yellow light (from an incandescent bulb), and is illuminated white light (from Sun), although the spectral distribution power of reflected light in the two cases is different. This happens because the brain “compensates” the eye color of the light received from the paper, “calibrating the” after ambient light colors.
Conversely, the same color seen under different conditions is perceived differently by the same process of compensation.