The third major characteristic of light is intensity which is related to the sensation of brightness. Essentially, with an increase in intensity, there is a uniform increase in all the wave lengths reflected, causing the hue to become brighter. Under normal viewing conditions, the wavelength composition of light reflected from a given surface varies continuously depending on the illuminant used. Although the wavelength composition reflected back in each condition is unique, the color of the apple remains red, even though the shade of red is subject to change. The only physical constant available to the nervous system that allows it to construct the constant hue of a surface from the continually changing illuminant conditions is reflectance.
Reflectance refers to the percent of light of any given wave length that a surface reflects.
Land's experiments suggest that the nervous system accomplishes this independence by comparing the intensity of the reflected light of one wavelength from one surface with the intensity of the reflected light of the same wavelength from the surrounding surfaces. This process occurs independently for each wavelength and are followed by a comparison of these comparisons. Before examining the neural pathways involved in the construction of color, it is necessary to discuss other theories of color perception. More specifically, there are three types of photopigments that are segregated in three types of cones, and each of these photopigments absorb light of several different wavelengths but absorbs more of one specific wavelength than the others. The more the photopigment of a cone absorbs a given wavelength, the more sensitive that cone is to light of that wavelength.