The iris is a diaphragm that regulates the amount of light that strikes the retina. It is composed mainly of smooth muscle, but it also possesses a deep layer called the pigment epithelium. This layer produces a pigment called melanin (i.e., eumelanin). This brown pigment is the same pigment produced in skin and hair to make them dark. In rare cases, a related pigment called pheomelanin is produced, giving the eyes a yellow/red color.
Melanin is the only pigment produced in the iris. The more melanin you produce, the darker your eyes will be. Those who produce a lot of melanin have dark brown eyes. Those that produce essentially no melanin have light blue eyes. Intermediate amounts of melanin produce lighter brown, hazel, and green eyes.
It's easy to see how producing a brown pigment produces brown eyes, but what makes blue eyes blue?
How to Make My Brown Eyes Blue
Water is clear, not blue. Likewise, air is clear, not blue. And yet both sky and seas appear blue. This is because both water and sky absorb, scatter, and reflect light. Longer wavelengths (red) are absorbed more whereas shorter wavelengths (blue) are scattered and reflected more. These reflected wavelengths make it into our eyes and so the sky and sea appear blue.
In the same way, the smooth muscle layer of the iris absorbs, scatters, and reflects light. Like the sky and sea, blue light is scattered and reflected out in higher amounts than red light, making the eyes appear blue. The amount of scattering is determined by the size and number of particles suspended in the cells of the iris. This light scattering, in combination with melanin, determines the final color of your eyes.
Eye Color Genetics
The genetics of eye color is rather complicated; at least 15 genes have been identified that affect eye color. However, the genes OCA2 and HERC2 appear to be the most important. OCA2 plays an essential role in melanosome synthesis. (Melanosomes are melanin-filled, intracellular compartments found in melanocytes). HERC2 is important in the expression of OCA2. Due to the number of genes involved and the complicated manner of eye color gene expression, it is virtually impossible to precisely predict a child's eye color from their parents. Worldwide, brown is by far the most common eye color; approximately 79% of the world's population has brown eyes. Approximately 10% of the population has blue eyes, 5% have hazel eyes, and only 2% have green eyes.
Heterochromia is a condition in which the two eyes of an individual are different colors (Figure 2D). Sectoral (or partial) heterochromia is when a single iris exhibits more than one color (e.g., Figure 2E).
True heterochromia is present in just 6 out of 1,000 live births, and even in most of those cases the condition is barely noticeable. However, it is not unusual (indeed, it is usual) for the central portion of the iris to differ slightly in color from the outer portion. The iris is composed of two layers of muscle; the internal sphincter muscle that constricts the pupil and the outer radial muscles that dilate the pupil. It is quit common for these two layers to differ slightly in color (Figure 3).
Photographic Red Eye
Before the advent of double-flash cameras, red eye was a frequent problem in photographs. What causes red eye in photographs and how do double-flash cameras get rid of them?
Red eye is caused by single-flash cameras. The flash produces an extremely bright light that illuminates the subject in order to obtain and fast-action photograph. The light is so bright and the photograph taken so quickly, that some of the light reflects off the choroid and back through the pupil to be captured in the photograph. The choroid is red because it is highly vascular. In other words, red eye is a photograph of the blood vessels behind your retina!
Double-flash cameras solve the problem by exposing the eye to a first flash of bright light and then pausing to give the pupils time to constrict. The second flash of light illuminates the subject so the picture can be taken. The smaller pupils reduce the amount of light escaping from the inside of the eye and thus the red appearance of the pupil.