Every human has a unique iris pattern, which is why many modern security systems use iris recognition technology.
Two sets of muscles in the iris contract and relax to control the amount of light entering the eye through the hole in its centre – the pupil. Circular muscles contract in bright light, making the pupil smaller to prevent a dazzling effect. In dim light, radial muscles (like spokes on a wheel) contract to make the pupil bigger so it allows in more light. Different amounts of melanin pigment inside the iris give eyes their different colours. Brown is the most common colour, found in more than half the world’s population.
The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism,[1] or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue and green-eyed people.
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