The Physiology of Seeing Color
About Rods and Cone Cells: How to Distinguish Color Nuances
The eye is responsible for the creation of images. To put it simply: light enters the eye through the cornea, is bundled by the pupil, and lands on the retina. The retina is covered by two types of photoreceptors: rods and cones.
Rods Bring Light to Darkness
More than one hundred million rods are responsible for our perception of light. They cannot distinguish color; in fact, if we only had rods, we would only see the world in black and white.
Cone Cells: No Light, No Color
In the back of the retina is the macula through which the visual axis runs. There are three types of cones surrounding this spot that are responsible for distinguishing color: blue, green, and red receptors (S, M, and L cones). Similar to additive color mixture, different color images are sent to the brain with the help of cones; together with the brightness information of the rods, the world becomes a colorful place.
The darker the surroundings, the more the rods control perception. Color cannot be distinguished at night.
Bright Is Not Always Bright – The V(l) Curve
The spectral sensitivity of the human eye depends on the wavelength: We perceive some colors brighter and some darker than others. This dependence is represented by the empirically determined brightness sensitivity or V(l) curve. Accordingly, the highest sensitivity is at the wavelength of 555 nm (i.e., in the green color range). A green light source appears brighter than a red light source when both give off the same energy flux (see the eye’s brightness sensitivity curve on pg. 015).