Iris

The iris is the coloured, circular tissue in your eye, visible through the transparent cornea. It lies in front of the lens and behind the cornea, and surrounds the pupil, the black opening in the centre. The iris consists of muscle tissue and connective tissue containing pigment, and the amount and type of pigment determine your eye colour: blue, green, brown, grey or shades in between.

The main function of the iris is to regulate the amount of light entering your eye. In bright light, the sphincter muscle contracts and the pupil becomes smaller; in dim light, the dilator muscle relaxes and the pupil becomes larger. In this way, your eye adapts to changing light conditions and the retina remains protected from excessive light.

The iris is unique to every individual, even more so than a fingerprint. That is why the pattern of the iris is sometimes used for biometric identification, for example in security systems.

The iris and contact lenses

The iris plays a role in contact lens wear. Soft lenses cover the iris completely and rest on the cornea; rigid lenses are smaller and float on the tear film above the iris. The diameter of the iris varies from person to person, and this affects which lens diameter fits properly.

Coloured or cosmetic contact lenses feature a printed iris pattern that covers your own iris. They can enhance or completely change your eye colour. Make sure the pupil area of the lens is large enough so that your vision isn’t restricted when your pupil dilates in the dark.

Changes to the iris

The colour of your iris may change slightly over the course of your life, particularly during the early years. In adults, iris colour is usually stable, but certain eye conditions or medicines can affect the colour or appearance of the iris. If you notice any changes in the colour, shape or pattern of your iris, have this assessed by your optician or ophthalmologist.

The iris and light sensitivity

People with a lighter iris colour, such as blue or green, have less pigment in the iris. This can lead to greater sensitivity to light, as more light can penetrate the tissue. A darker iris, with more pigment, filters out more light and can therefore offer slightly more protection against bright light.