The iris (a word that comes from the Latin for rainbow) is one of the most readily observable parts of the eye. This entry is about different eye colours caused by the difference in iris structure. Also covered is some information about red eye viewed in the pupil (as in photographs), but not about the eye diseases which cause red eye. To find out about them you need to spend about £100 on a big book of ophthalmology.

As a side note this entry had a working title of 'Why my eyes are blue?' but as not everyone's eyes are blue it had to be called something a bit more practical.

Structure of the Iris

The iris is made up of four layers:

• The anterior border layer (the front layer facing out)
• The stroma
• Two layers of endothelium (at the back of the iris)

The double layer is responsible for dilating the pupil and absorbing any stray light that reaches the back of the iris. It is only the first two layers that determine iris colour.

The anterior border layer contains melanocytes. Everyone's body contains about the same number of melanocytes, but the amount of melanin in these cells is genetically determined. Melanin absorbs light and is the principle pigment in hair and skin. Differing levels of melanin account for the differences in skin and hair colour between races and individuals. People with dark skin and hair have a generally higher level of melanin than pale, blond people. As a result, people with darker skin and/or hair are more likely to have brown eyes. In the eye, low levels of melanin absorb less light and have a yellow appearance, while high levels look brown.

The stroma is a connective tissue layer which contains collagen, blood vessels and the iris sphincter. The iris sphincter is the muscle which constricts the pupil. White light entering the stroma is scattered by the collagen. The collagen absorbs most of the colours apart from blue or grey, these are reflected back by the collagen. The blood vessels and sphincter scatter the light in different ways giving different patterns of flecks. The ring that can sometimes be seen in the iris is the minor iridic circle, which is the artery ring supplying the iris with blood. Freckles and darker patches on the iris are caused by round groups of pigment and are called clump cells.

Whether the eye is blue or grey depends on the arrangement of the collagen fibres: fine arrangement causes blue eyes while a coarser arrangement causes grey ones.

Different Eye Colours

In a brown eye there is a lot of melanin in the anterior border layer. This absorbs the light and gives a brown velvety appearance.

In a blue eye there is not much melanin in the anterior border layer. The light passes into the stroma where the collagen fibres scatter the light back as blue.

In a green eye (or a hazel one) there is a variable level of melanin, so that some of the light is absorbed by the melanin and some is scattered by the collagen. The brown layer looks yellow as it is thinner, and so the yellow and blue mix to make green.

Red irides¹ are a result of albinism. Albinism is where there is no melanin in the melanocytes at all. Therefore all of the blood vessels (in the iris and retina) are seen and a redder appearance is given. In practice only very few albinos have red eyes, the blue reflections of the collagen show up stronger and so most have blue/grey or even brown. The mixing of red and blue reflections can also give rise to violet eyes.

Genetic Inheritance

Like many human characteristics, eye colour is determined at least partially by genetics. This is where your parents' characteristics are passed onto you and your siblings via your genes.

As with most genes, people have two copies of the gene controlling eye-colour (one from each parent). You only need one copy turned 'on' to make enough melanin to have brown eyes. Obviously the same applies to your parents, so a brown-eyed parent could have one or two 'on' copies, but a blue-eyed one must have two 'off' genes.

So, if your father has brown eyes, he might have both genes turned 'on', or just one. If he gives you an 'on' copy then you will have brown eyes, whatever colour eyes your mother has. The same applies to your mother. If she gives you an 'on' copy then your eyes will be brown. If you get an 'on' copy from each parent then your children's eyes will be brown too. You can still get blue-eyes from two brown-eyed parents, but only if they both have one 'on' and one 'off' copy and they both give their 'off' copy to you. If this were the case, you should have three brown-eyed siblings for every blue-eyed one (although you would need a lot a brothers and sisters to test this).

However it isn't as easy as that. There is more than one gene controlling eye colour, allowing for various degrees of pigmentation.

Heterochromia

Heterochromia iridium is the term used when the eyes are different colours, such as having one blue and one brown. There are several different causes of heterochromia: an inherited trait, a disease/disorder or a physical accident. As mentioned previously melanocytes contain melanin, which determines the eye's colour. Melanocytes need innervation (impulses) to survive. If there is an interruption to the impulses (such as damage to the nerves supplying the eye) then the eye colour can change. If this happens to only one eye then only this one will be affected. This is a very simple description so if you want to know more, buying the book mentioned above would be a good idea.

Heterochromia iridis is where different parts of a single iris vary in colour. This can be caused by areas with increased or decreased melanin due to tumours, clump cells or birthmarks.

Why the Pupil Usually Looks Black

The retina of a human eye looks red because it has lots of blood vessels supplying the cells with metabolites. One reason you don't see the red colour is because the retina absorbs nearly all of the light which enters the pupil. In normal circumstances very little light is reflected and so the pupil looks dark. When a very strong light is shone on the pupil, some of the light is reflected back and the pupil looks red (so you sometimes get red-eye in photographs).

Red Eye

You can normally only see the red in human eyes when a light source and the viewer are very close to the same line - the reflected light from the blood vessels of the retina is strongest in the direction the light came from (like cats-eyes in the road, or reflective safety material).

If you have a flashgun on a lead, and hold it a foot or two to the side of (or above) the camera when taking a portrait, your subject will get much less (or no) red-eye.

If you look at an animal with with reflective eyes at night, a cow or a cat, for example, illuminating it with a head mounted light, you can see a very strong reflection (often yellowish).

One reason you don't see red-eye during the day (from sunlight or strong artificial light) is that in strong light the pupil is tightly closed, so not only does less light get in but less light gets back out. In a human eye, where the pupil changes from roughly 2mm to 8mm in bright light, 16 times less light reaches the eye in strong light.

Additionally, the viewer is normally off-axis from the light source, minimising the reflection, and if the light source is diffuse, the face will be lit up from multiple sources. Only light rays that are on-axis with the viewer will cause significant reflection from the retina to be seen. Therefore the face will be relatively bright, compared to the light coming out of the pupil, which appears dark.

Also, dark colours are hard to distinguish from a distance. If you move your face closer to someone's eye to have a look, the angle between any light source that can get past your head into the eye in question, and back out into your eye will get larger the closer you get. Some cameras use a small pre-flash before taking a picture. This causes the eye to respond by constricting the pupil, and minimises red-eye.

Summary

Eyes are blue because of light scattering by collagen, except if there is too much melanin in the way, in which case they are brown. Other colours are somewhere in between to give a rainbow effect.