Humans have two types of photoreceptor layers depending on their shape. Rod cells detect dark light, and cone cells detect bright light, so rod cells are more responsible for night vision, and cone cells are responsible for color vision. Human retina have approximately 92 million rod cells and 4.6 million cone cells. Among these cells, cone cells have important role in color vision, and are concentrated toward the macula. Cone cells are consisted of 3 types; S-cones, M-cones, and L-cons. Each cone has a sensitive wavelengths region of light, so the disfunction of these cons induce difficulty in color recognition.
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