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The irises of human eyes exhibit a wide spectrum of colours. Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris [1] [2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
The iris (pl.: irides or irises) is a thin, annular structure in the eye in most mammals and birds that is responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by the iris.
Central heterochromia is also an eye condition where there are two colors in the same iris; but the arrangement is concentric, rather than sectoral. The central (pupillary) zone of the iris is a different color than the mid-peripheral (ciliary) zone. Central heterochromia is more noticeable in irises containing low amounts of melanin. [32]
Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. [41] Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green.
Unlike other primates, all human beings have eyes with a distinct colour contrast between the white sclera, the coloured iris, and the black pupil. This is due to a lack of pigment in the sclera. Other primates mostly have pigmented sclerae that are brown or dark in colour. There is also a higher contrast between human skin, sclera, and irises.
The pupil is the central opening of the iris on the inside of the eye, which normally appears black. The grey/blue or brown area surrounding the pupil is the iris. The white outer area of the eye is the sclera. The central outermost transparent colorless part of the eye (through which we can see the iris and pupil) is the cornea.
Iris is an ambiguous color term, usually referring to shades ranging from blue-violet to violet. However, in certain applications, it has been applied to an even wider array of colors, including pale blue, mauve, pink, and even yellow (the color of the inner part of the iris flower).
A blue iris with some melanin. The color of blue eyes is due to the Tyndall scattering of light by a translucent layer of turbid media in the iris containing numerous small particles of about 0.6 micrometers in diameter. These particles are finely suspended within the fibrovascular structure of the stroma or front layer of the iris. [9]