Each sensor has its own lens and photosensitive cell ( s ).
2.
The retina contains two types of photosensitive cells, the rod cells and the cones.
3.
When, via random mutation across the population, the photosensitive cells happened to have developed on a small depression, it endowed the organism with a better sense of the light's source.
4.
In some animals, the retina contains a reflective layer ( the tapetum lucidum ) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.
5.
Historically, photometry in the near-infrared through long-wavelength ultra-violet was done with a photoelectric photometer, an instrument that measured the light intensity of a single object by directing its light onto a photosensitive cell.
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