The brain blends these colors to obtain the large variety of colors we perceive. The retina is composed of 10 layers with nearly 2 million nerve fibers. When stimulated by light, they transmit electrical impulses from our eyes to the brain where the signals are interpreted to give vision. Receptors, called rods, function well in dimly lit situations. Rods perceive only black, white and shades of grey.
Cones are the second type of receptor.
This type of receptor functions to provide daytime vision, but also are responsible for color vision. There are about 3 times more rods than cones. The fovea, a tiny pit in the retina, has the highest concentration of cones. The fovea gives us our finest central detailed vision. The purpose of color vision testing is to identify color defects in your vision.