There are two types of photoreceptors in the human retina, rods and cones.
Rods are responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity. The density of rod cells increases as you move away from the fovea, reaching a peak and declining again towards the periphery. Rod vision provides high sensitivity to light, but with relatively low spatial discrimination and no ability to distinguish different wavelengths of light. This is why they are not capable of detecting different colours.
Cones are active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity. The central fovea is populated exclusively by cones. There are 3 types of cones which we will refer to as the short-wavelength sensitive cones, the middle-wavelength sensitive cones and the long-wavelength sensitive cones or S-cone, M-cones, and L-cones for short. Cone outer segments are generally shorter than that of rods and, as their name implies, are often conical. As is the case of rod cells, the inner segment of the cone cells has an outer mitochondria-rich part and an inner part containing endoplasmic reticulum.
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