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Something You Might Want to Know About the Iris
The human iris is 11-13 mm in diameter.
How much of this diameter is visible when viewed is determined by the clarity of the cornea at the limbus, the transitional tissue where the clear cornea meets the white opaque sclera. Although the iris is anatomically round, the visible iris is somewhat ovoid, with a rim covering the top and bottom portions. This ovoid appearance is more noticeable near the cornea's base and in older eyes (Warwick, 1976).
This "geriatric arc" is an opaque, gray ring around the cornea's periphery. The artist must keep in mind that the corneal rim and eyelids cast shadows on the iris. The iris is generally conical in shape anatomically. The lens that surrounds its central portion pushes it forward slightly. The illustration of this feature floating independently shows how widely it is misunderstood. The optical properties of an artificial cornea are typically used to create a conical artificial iris.
As a result, the way light strikes the surface of the iris changes. Because the illustration depicts light from the upper left, a painting or drawing of the eye would emphasize the upper right iris in the light. Under a biological microscope, the iris appears stereoscopically best at 40x magnification (Daughman, 1999).
The skin is thickest at the conjunctiva, while the pupil margin and iris root are thinnest. The iris's radial striations are caused by pupillary sphincter muscle contraction, while the circumferential folds in its peripheral portion are caused by dilator muscle action. There are no perfect or continuous circles in these peripheral folds.