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The retina is the main organ of sight. It lines the back of the eyeball and is mainly made up of photoreceptor cells, called cones and rods. The retina analyses the light that has been focused and filtered by the cornea and pupil and then sends the visual information on to the brain through the optic nerve, which extends from the retina to the brain.
Each of the two types of photoreceptors cells has a different specialised function. Rods, named for their cylindrical shape, are highly sensitive to the amount of light. Cones allow the perception of colour. The combined action of the two defines the image that the brain interprets. The retina also has a very dense network of blood vessels. Its pattern is unique for each person, which makes it an interesting element for biometrics. Retina-based identification systems count among the most reliable because the blood-vessel pattern is impossible to reproduce. They are also among the most costly and complex to set up and use.