- Home
- Knowledge centre
- Eye glossary A–Z
- Iris (Eye): What It Is, What It Does and Which Conditions Affect It
Iris (Eye): What It Is, What It Does and Which Conditions Affect It
Medically reviewed by {{surgeon}}, {{surgeon's title and specialty}}, on {{review date}}. Last updated {{date}}.
Iris (noun; plural irises or irides): the iris is the coloured, ring-shaped part of the eye that sits behind the clear cornea and around the pupil. Its two muscles open and close the pupil to control how much light enters the eye, and the amount of pigment (melanin) in it sets your eye colour. The iris is part of the uvea, the eye's vascular middle layer, and it is one of the few parts of the eye you can see in the mirror.
Iris of the eye: key facts
- Where it is
- In the front of the eye, behind the cornea and the anterior chamber, with the pupil as the dark opening in its centre
- What it is made of
- A front layer of loose connective tissue with blood vessels, pigment cells (melanocytes) and fibroblasts, plus two smooth muscles and a heavily pigmented back layer
- Main job
- Controls pupil size, and so the amount of light reaching the retina
- Muscles
- Sphincter pupillae (narrows the pupil) and dilator pupillae (widens it)
- What sets its colour
- Mainly the amount of melanin: brown eyes have a lot, blue eyes much less
- Part of
- The uvea (iris, ciliary body, choroid)
- Common problems
- Inflammation (iritis), coloboma, aniridia, injury, cysts, moles and, rarely, melanoma; the iris is also involved in angle-closure glaucoma
- Specialist
- Ophthalmologist; at Laser Istanbul Clinic, Doç. Dr. Muhammet Derda Özer (Associate Professor) leads uveitis, cornea and ocular oncology
What is the iris of the eye?
The iris is the circular, coloured disc in the front of your eye. In its centre is the pupil, the opening through which light enters. According to the American Academy of Ophthalmology (AAO), the iris is the coloured part of the eye, and it controls the size of the pupil to let light into the eye. When people search "iris eye" they usually mean exactly this structure, not the whole eyeball.
Is the iris the coloured part of the eye?
Yes. The iris is the coloured part of the eye. The clear dome over it is the cornea, the white around it is the sclera, and the black-looking opening in the middle is the pupil, which is a hole, not a separate structure.
What part of the eye contains the iris?
The iris lies in the front of the eye. The AAO describes it as sitting behind the anterior chamber (the fluid-filled space behind the cornea), with the pupil as the dark opening in its middle. Behind the iris is the natural lens. The iris belongs to the uvea, the eye's vascular layer, which also includes the ciliary body and the choroid.
What is the iris made of?
Under the microscope, the iris is built from the front to the back like this:
| Layer | What it is | What it does |
|---|---|---|
| Front surface and stroma | Loose connective tissue, blood vessels, pigment cells (melanocytes) and fibroblasts. There is no skin-like covering layer (epithelium) on the front surface | Gives the iris its visible texture and most of its colour; melanocytes here are central to eye colour |
| Sphincter pupillae muscle | A circular band of smooth muscle around the edge of the pupil, supplied by the parasympathetic nervous system | Makes the pupil smaller |
| Dilator pupillae muscle | Smooth muscle formed from the front of the two cell layers at the back of the iris, supplied by the sympathetic nervous system | Makes the pupil wider |
| Posterior pigmented epithelium | A heavily pigmented layer at the very back, continuous with the ciliary body | Forms the rear surface of the iris |
So when someone asks what the iris is made of: pigmented connective tissue, blood vessels, two tiny muscles and a pigment-rich back layer (EyeWiki, AAO).
Iris eye function: what does the iris do?
The iris works like the adjustable aperture of a camera. Its function is to control how much light reaches the retina by changing the size of the pupil. In bright light the pupil narrows; in dim light it widens.
The iris does more than that. A StatPearls review on iris injuries notes that the iris helps vision quality by reducing the aberrations from the lens and cornea, limiting glare and improving the eye's depth of focus. This is why a damaged iris can cause glare, light sensitivity and reduced contrast, not just a change in appearance.
Iris eye muscle: how the two muscles work
The iris has two opposing smooth muscles that work automatically; you cannot control them directly.
- Sphincter pupillae encircles the pupil. According to StatPearls, it constricts the pupil in bright light through the pupillary light reflex, and during accommodation (focusing on something near). It receives parasympathetic nerve supply.
- Dilator pupillae opens the pupil wider. It receives sympathetic nerve supply (EyeWiki, AAO).
Eye doctors check how the pupils react to light as part of an examination, because the reflex depends on the iris, its nerves and the eye working together.
What decides iris colour? Brown eyes and the iris
Eye colour depends on how much melanin the iris contains. MedlinePlus (US National Library of Medicine) explains that people with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less of this pigment.
Genetics decides the amount. MedlinePlus names two neighbouring genes on chromosome 15, OCA2 (which influences melanin production in the iris) and HERC2 (which controls how active OCA2 is), as central, with at least eight other genes also contributing. Inheritance is more complex than the old "brown beats blue" school rule: parents with blue eyes can have a child with brown eyes.
Iris eye brown: what makes a brown iris
A brown iris contains a large amount of melanin, which is why a brown eye in a brown eye iris close up photograph looks dense and deep. Pigment also matters medically: the AAO lists light-coloured eyes among the risk factors for ocular melanoma, a very rare cancer, which is one reason regular eye checks are worthwhile whatever your eye colour.
Want to see how eye colour is passed on in your own family? Try our tool.
Eye colour chart and inheritance calculatorDifferent coloured irises (heterochromia)
Heterochromia means a person's irises are different colours. The AAO describes three types: complete (one entire iris differs from the other), partial (part of one iris differs) and central (an inner ring differs from the outer area). It can be present from birth, or acquired after eye injury, bleeding in the eye, inflammation (iritis or uveitis), eye surgery or the use of some glaucoma medicines. The AAO advises that an infant with heterochromia should be examined by an ophthalmologist, and an adult who develops it should also see an eye professional.
Iris coloboma: when part of the iris is missing
An iris coloboma is a gap in the iris, typically giving the pupil a "keyhole" shape. EyeWiki (AAO) explains that it results from failed or incomplete closure of the embryonic fissure, the seam in the developing eye, in the second month of development. The defect is characteristically in the lower inner quadrant of the iris, and the cornea, ciliary body and the zonules that hold the lens can be involved too.
How common is it? EyeWiki quotes 0.5 to 2.2 cases of ocular coloboma per 10,000 live births. It can occur on its own or run in families.
What does it mean for vision? That depends on what else is involved. Iris colobomas frequently come with other findings such as a small eye (microphthalmia), cataract, glaucoma and refractive error, and EyeWiki reports a retinal detachment risk of 23–42% where the coloboma involves the retina. The gap itself cannot be closed, so management focuses on correcting refractive error and monitoring for complications, with regular dilated examinations every 6–12 months (EyeWiki).
Related: aniridia. In aniridia, which EyeWiki describes as a rare bilateral congenital disorder, the iris is partly or almost completely missing; a small rim of iris is almost always present. Most cases are linked to the PAX6 gene, and it occurs in about 1.8 per 100,000 live births. It is associated with reduced vision from underdevelopment of the central retina (the fovea), with nystagmus, and, over time, with cataract, glaucoma and corneal problems, so it needs lifelong specialist follow-up.
Iris conditions and symptoms
| Condition | What happens | Typical signs |
|---|---|---|
| Iritis (anterior uveitis) | Inflammation of the iris, the front part of the uvea | Eye pain (sometimes worse when reading), redness, light sensitivity, blurred vision, watery eyes, floaters |
| Iris coloboma / aniridia | Part or most of the iris is missing from birth | Keyhole-shaped pupil, glare, light sensitivity, reduced vision |
| Iris injury (trauma) | Sphincter tears, iridodialysis (iris torn from its root), transillumination defects | Irregular pupil, glare, light sensitivity, a change in appearance |
| Iris cyst | A fluid-filled sac in the iris; usually benign | Often none; sometimes visual disturbance, raised eye pressure or discomfort |
| Iris nevus (mole) and tumours | A benign pigmented spot or, rarely, a melanoma | A dark spot on the iris; early melanoma often causes no symptoms |
| Narrow-angle (angle-closure) glaucoma | The drainage angle between the iris and the cornea becomes blocked | Sudden severe eye pain, red eye, halos around lights, blurred vision, nausea: an emergency |
| Acquired heterochromia | Iris colour changes after injury, inflammation, surgery or medicines | A visible colour change |
Iritis (anterior uveitis)
Iritis is inflammation of the iris, also called anterior uveitis, the most common type of uveitis. The National Eye Institute (NEI) lists blurry vision, floaters, eye pain, red eyes and light sensitivity as the early warning signs. According to the NHS, in most cases the cause is unknown, but it can be triggered by autoimmune conditions, infections such as herpes simplex, herpes zoster and toxoplasmosis, or eye injury. The AAO adds that systemic inflammatory diseases such as rheumatoid arthritis and lupus, and smoking, are risk factors. Left untreated, uveitis can permanently damage vision.
Iris injury
Blunt or penetrating injury can tear the iris sphincter, tear the iris from its root (iridodialysis) or leave an irregular pupil. StatPearls notes that traumatic iris damage can cause glare, photophobia, reduced contrast sensitivity and reduced cosmetic appearance, and that both non-surgical and surgical approaches to iris reconstruction exist.
Iris cysts, moles and tumours
Iris cysts are usually benign and found during routine examination; StatPearls says asymptomatic ones may simply be monitored, and that a careful differential diagnosis is needed to tell them from melanoma. An iris nevus is a benign mole. The AAO notes that a melanoma differs from a nevus in appearance and behaviour, and that eye melanoma typically causes no early signs or symptoms, so a new or changing dark spot on the iris deserves an examination. According to StatPearls, melanoma is the most common malignant iris tumour and has significant metastatic potential.
Angle-closure glaucoma and the iris
In narrow-angle glaucoma the drainage angle between the iris and the cornea is blocked and eye pressure can rise quickly. The NHS lists sudden severe eye pain, a red eye, rainbow-coloured halos around lights, blurred vision and nausea as signs of acute glaucoma and says to seek emergency care straight away (999 or A&E in the UK). A laser peripheral iridotomy, which the AAO describes for angle-closure glaucoma, creates a full-thickness opening in the iris with a laser.
How is the iris examined and treated?
Examination. An ophthalmologist examines the iris with a slit lamp, a microscope with a bright, thin beam of light that shows its surface, colour, pupil shape and any cysts, spots or tears, and also checks how the pupil reacts to light. Imaging such as ultrasound biomicroscopy or optical coherence tomography (OCT) can characterise iris cysts, as StatPearls notes. A dilated examination is used to look at the retina, for example in coloboma (EyeWiki).
Treatment depends on the condition:
| Condition | Typical approach |
|---|---|
| Iritis / uveitis | Corticosteroid eye drops to reduce inflammation and dilating drops to ease pain (AAO); injections, tablets or immune-suppressing medicines for severe or recurrent cases, and a search for the underlying cause (NHS, NEI) |
| Coloboma | Correction of refractive error and regular dilated checks for retinal problems (EyeWiki) |
| Aniridia | Lifelong monitoring and treatment of glaucoma, cataract and corneal disease (EyeWiki) |
| Iris injury | Observation, or non-surgical and surgical iris reconstruction for significant damage (StatPearls) |
| Iris cyst | Monitoring if it causes no symptoms; medication, laser, aspiration or surgery if it does (StatPearls) |
| Iris tumours | Monitoring of benign lesions; for melanoma, radiation (such as plaque brachytherapy) or surgical removal depending on size and location (AAO, StatPearls) |
| Angle-closure glaucoma | Urgent pressure-lowering treatment; laser peripheral iridotomy (AAO) |
A note on cosmetic eye colour change. Procedures that tattoo pigment into the cornea or implant an artificial iris to change eye colour are cosmetic, not a medical treatment. The American Academy of Ophthalmology warned against them in 2024, because of the risk to vision, and we do not offer them. If you are simply curious what colour your children are likely to have, use the calculator linked above.
Treatment with us: uveitis, corneal and ocular tumour care is led by Doç. Dr. Muhammet Derda Özer (Doç. means Associate Professor). Prof. Dr. Ahmet Satıcı and Op. Dr. Aysan Bayram look after cataract and laser patients, and the iris and pupil are assessed as part of every pre-operative examination in Istanbul.
Iris eye meaning, plural and "big iris eye"
What does "iris" mean, and what is the plural of iris?
The word comes from the Greek iris, meaning rainbow, which is also the name of the Greek goddess of the rainbow, a fitting name for a structure that comes in so many colours. The plural of iris in anatomy is irises or irides. In everyday English "irises" is perfectly acceptable; "irides" is the more clinical form. The same word also names the iris flower, which has nothing to do with the eye.
Why does my iris look big?
How large the iris looks depends on how wide your eyelids sit, how much white (sclera) shows around it, how strong the contrast is between a dark iris and a white eye, and on pupil size. A "big iris eye" look is usually just a matter of appearance and eyelid position. Coloured cosmetic lenses can also enlarge the apparent iris, but any contact lens should be fitted by an eye professional.
See a doctor if the size or shape of one iris or pupil changes, particularly after an injury, with pain, or with any change in vision.
Iris eye drawing: what to include
If you are drawing an iris, these are the features an ophthalmologist sees: a round, dark pupil in the centre; fine fibres radiating outwards like spokes; a ring partway out called the collarette, which marks the thickest part of the iris; small pits and flecks (crypts) in the surface; and the outer edge where the iris meets the sclera. Seen from the front, the iris is a ring around the pupil; in side view, it is a thin diaphragm in front of the lens.
When to see a doctor about your iris
Seek same-day care from an eye doctor or eye emergency service if you have:
- sudden severe eye pain with a red eye, halos around lights, nausea or blurred vision (possible acute glaucoma, an emergency per the NHS)
- a painful red eye with sensitivity to light, which can be iritis
- an eye injury, especially with an irregular pupil, bleeding in the eye or a sudden drop in vision
- a sudden change in the size or shape of a pupil
Book a routine appointment if you notice:
- a new or growing dark spot on the iris, or an iris colour that changes
- different iris colours in a baby
- glare and light sensitivity that are getting worse
- a keyhole-shaped pupil or missing section of the iris that has never been assessed
If you already have a diagnosis, such as uveitis, coloboma or a suspicious spot on the iris, and want a second opinion, you can send your reports and scans for review before you travel.
Frequently asked questions
The iris is the coloured, ring-shaped tissue behind the cornea that surrounds the pupil. It controls how much light enters the eye and gives the eye its colour.
Yes. The iris is the coloured part of the eye. The white part is the sclera, the clear front is the cornea and the dark centre is the pupil.
The iris lies in the front of the eye, behind the cornea and in front of the natural lens. It is part of the uvea, together with the ciliary body and the choroid.
Pigmented connective tissue and blood vessels at the front, two smooth muscles (the sphincter and dilator pupillae) and a heavily pigmented layer at the back.
It acts as the eye's aperture: it narrows or widens the pupil to control how much light reaches the retina, and it helps reduce glare and improve depth of focus.
There are two: the sphincter pupillae, which makes the pupil smaller, and the dilator pupillae, which makes it wider. Both work automatically.
Because they contain a large amount of melanin; blue irises contain much less. The amount is governed by several genes, chiefly OCA2 and HERC2, so eye-colour inheritance is not as simple as "brown is dominant".
It is a gap in the iris, present from birth, that usually makes the pupil keyhole-shaped. It happens when the embryonic fissure does not close fully, and it can come with other eye findings, so an eye examination is advised.
Irises or irides. Both are correct for the eye; irides is more usual in medical writing.
Some iris damage can be treated with surgical or non-surgical reconstruction, depending on the injury (StatPearls), but the iris cannot be regrown. Cosmetic eye colour change surgery is not recommended: the American Academy of Ophthalmology warned against it in 2024.
Sources for the facts on this page
- American Academy of Ophthalmology. Iris (eye anatomy). https://www.aao.org/eye-health/anatomy/iris-list
- American Academy of Ophthalmology. Parts of the eye. https://www.aao.org/eye-health/anatomy/parts-of-eye
- American Academy of Ophthalmology. Eye anatomy overview. https://www.aao.org/eye-health/anatomy/eye-anatomy-overview
- Bloom J, Motlagh M, Czyz CN. Anatomy, Head and Neck: Eye Iris Sphincter Muscle. StatPearls, 2023. PubMed: https://pubmed.ncbi.nlm.nih.gov/30335285/
- Gurnani B, Kaur K. Traumatic Iris Reconstruction. StatPearls, 2023. PubMed: https://pubmed.ncbi.nlm.nih.gov/35201728/
- Moshirfar M, Rageh A, Ronquillo Y. Benign and Malignant Iris Tumors. StatPearls, 2024. PubMed: https://pubmed.ncbi.nlm.nih.gov/38261685/
- Han LS, Tripathy K, Meyer JJ. Iris Cyst. StatPearls, 2024. PubMed: https://pubmed.ncbi.nlm.nih.gov/34662087/
- EyeWiki (AAO). Basic Histology of the Eye and Accessory Structures. https://eyewiki.org/Basic_Histology_of_the_Eye_and_Accessory_Structures
- EyeWiki (AAO). Coloboma. https://eyewiki.org/Coloboma
- EyeWiki (AAO). Aniridia. https://eyewiki.org/Aniridia
- MedlinePlus Genetics. Is eye color determined by genetics? https://medlineplus.gov/genetics/understanding/traits/eyecolor/
- American Academy of Ophthalmology. What Is Heterochromia? https://www.aao.org/eye-health/diseases/what-is-heterochromia
- American Academy of Ophthalmology. What Is Uveitis? https://www.aao.org/eye-health/diseases/what-is-uveitis
- National Eye Institute. Uveitis. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/uveitis
- NHS. Uveitis. https://www.nhs.uk/conditions/uveitis/
- NHS. Glaucoma. https://www.nhs.uk/conditions/glaucoma/
- American Academy of Ophthalmology. What Is Ocular Melanoma? https://www.aao.org/eye-health/diseases/what-is-ocular-melanoma
- American Academy of Ophthalmology. Laser peripheral iridotomy. https://www.aao.org/eye-health/treatments/laser-peripheral-iridotomy
- Wikipedia. Iris (anatomy): word origin, plural forms, collarette (used only for these points). https://en.wikipedia.org/wiki/Iris_(anatomy)
Worried about your iris, or want a second opinion on an eye condition?
Send us your eye report or scans. Doç. Dr. Muhammet Derda Özer will review them and tell you which treatments may suit your eyes, with no obligation.
3 eye surgeons
Your case is reviewed by a named surgeon, not a salesperson. Reply within {{response time}}.
Prof. Dr. Ahmet Satıcı
Doç. Dr. Muhammet Derda Özer
Op. Dr. Aysan Bayram