How the Eye Pupil Works

One of the most significant parts of the eye is not its structure, its mainly open space. It is the pupil of the eye.

Pupil Definition
The pupil is the opening in the core of the iris (the structure the gives our eyes their color). The purpose of the pupil is to allow light to enter the eye so it can be converged on the retina to begin the process of sight.

The pupils appear perfectly round, equal in size and black. The black color is because the light that passes through the pupil is absorbed by the retina and is not reflected back.

If the pupil has a cloudy or pale color, typically this is due to the lens of the eye (which is positioned directly behind the pupil) has become opaque due to the formation of a cantata. When the clouded lens is replaced by a clear intraocular lens (IOL) during cataract surgery, the usual black look and color of the pupil are restored.

There’s another common situation when the pupil of the eye changes color – when someone takes your photo using the cameras flash function. Depending on your direction of your gaze or line of vision when the photo is taken, your pupils might appear bright red. This is due to the bright light from the flash being reflected by the red color of the retina.

Pupil FunctionTogether the iris and pupil regulate how much light enters the eye. Using the analogy of a camera, the pupil is the aperture of the eye, and the iris is the diaphragm that controls and size of the aperture.
The size of the pupil is guided by muscle within the iris —
– One muscle contracts the pupil opening (makes it smaller ( and another iris muscle dilates the pupil (makes it large). This dynamic process of muscle reaction within the iris controls how much light enters the eye through the pupil.

In low-light conditions, eye pupil dilates so more light can reach the retina to improve night vision. IN bright conditions, the pupil constructs to limit how much light enters the eye (too much light can cause glare and discomfort, and may even damage the lens and retina.

Pupil Size
The size of the pupil differs from person to person. Some people have large pupils, and some people have small pupils. Also, pupil size changes with age – children and young adults tend to have large pupils, and senior usually have small pupils.

Normal pupil size in adults ranges from 2 to 4 millimeters (mm) in diameters in bright light to 4-8mm in the dark.
In addition to being influenced by light, both pupils constrict when you focus on a near object. This is called the accommodative pupillary response.

Pupil TesyomhDuring a conventional eye exam, your eye doctor or an assistant will examine your pupils and perform testing for pupil function.
Typically, pull testing is conducted in a dimly lit room, While you are gazing at a distant object, the doctor will quickly direct the beam of a small flashlight at one of your eyes several times. While doing this, the response of the pupil of both eyes observed.

The observer typically will the alternate direct the light in each eye and again observe the pupil responses of both eyes. This is called Murs Gunn pupil testing, which is sometimes called the swinging flashlight test.
PUpils usually react both directly and indirectly to light stimulation. The effect of the pupil of the eye receiving direct illumination is described as the immediate response; the reaction of the other pupil is referred to as the consensual response.

The examiner may then turn up the room lights a bit and have your focus on a handheld one while moving that object closer to your nose. This is a test of the accommodative response of your pupils.
If your pupils look normal and respond normally, the clinician may note this common abbreviation in your medical chart PERRLA, which is an acronym for pupils are, round and reactive to light and accommodation.”
A pupil is abnormal if it fails to dilate dim light or fails to contract in response to light or accommodation.
Condition that Affects the Pupil

Understanding The Eye: Anatomy


Some conditions can affect the size, shape and .or function of the pupil of the eye. These include:

Adie’s tonic pupil. This is a pupil that has approximately no reaction to light direct or consensual ) and there is a postponed response to accommodation. Adie;s tonic pupil( also called Adie’s pupil, tonic pupil, or Adis’ syndrome) usually affects only one eye, with affected pupil being large than the pupil of the unaffected eye. The cause of Adi’s pupil usually is unknown; but it can be caused by trauma, surgery, lack of blood flow (ischemia) or infection.

Argyll Robertson Pupil. This is a pupil that is non-reactive to light (direct or consensual), but the reaction to accommodation is normal. Argyll Robertson pupil usually impacts both eyes, causing smaller-than -normal pupils that can react to light. The condition is rare, and the reason commonly is unknown, but it has been linked with syphilis and with diabetic neuropathy.

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Marcus Gunn pupil – Also defined as the relative afferent pupillary defect (RAPD) or afferent pupillary defect; this is an abnormal effect of the swinging-flashlight test where the patient’s pupils constrict less (hence seeming to dilate) when the light oscillates from the affected eye to the affected eye. The most frequent cause of Marcus Gunn pupil is damaged in the posterior region of the optic nerve or severe
retinal disease.
Trauma – Penetrating eye trauma that affects the iris is a common cause of abnormally shaped pupils; Comparable trauma can occur in complications of cataract surgery phakic IPL surgery or refractive lens exchange. Pupillary reactions to light and accommodation often remain normal or nearly normal.

Sexual arousal. Recent research has indicated that sexual arousal produces a pupil dilation response and that this response can be useful in sexuality research to evaluate sexual orientation.