The iris is the colored part of the eye, located behind the cornea and in front of the lens. It controls the pupil’s size, regulating the amount of light entering the eye. Each person’s iris possesses a unique appearance, akin to a fingerprint.
The Spectrum of Iris Colors
Iris colors, including browns, blues, greens, grays, and hazels, are primarily determined by the amount and type of melanin present. Melanin is a natural pigment also responsible for skin and hair color. Brown eyes, the most common globally, have high concentrations of melanin, which absorbs most light.
In contrast, blue eyes contain very little melanin. Their blue appearance results from the scattering of light within the iris’s stromal layer, a phenomenon similar to how the sky appears blue. Green and hazel eyes contain moderate amounts of melanin, with green eyes often resulting from a combination of light brown pigmentation and this light scattering. Occasionally, individuals may have heterochromia, a condition where their eyes are different colors, or different colors within the same iris.
Unique Patterns and Textures
Beyond color, the iris displays patterns and textures. These features include crypts, small, pit-like depressions where the iris tissue is thinner. Furrows, or contraction furrows, appear as concentric lines that form due to the iris’s constant movement.
Another feature is the collarette, a zig-zag ridge marking the division between the inner pupillary and outer ciliary zones. These features, including pigmented spots, are so unique that their arrangement and density are used in biometric identification systems.
How the Iris Appears to Change
The iris’s appearance can seem to shift based on factors like pupil size and ambient light. When the pupil constricts in bright light, the iris expands, and its textures, particularly the inner regions, may appear more compressed. Conversely, in dim light, the pupil dilates, causing the iris to contract and its features to become more spread out.
Light intensity influences the perceived color of the iris. Different lighting conditions can make iris colors appear lighter or darker, or bring out various undertones as light reflects off the iris. For instance, eyes with less melanin may seem more sensitive to subtle color changes, while darker eyes might exhibit greater vibrancy in well-lit environments.