Purkinje images
Also called: Purkinje-Sanson images · Catoptric images
A candle held near someone's eye produces four tiny reflections, one from each surface of the cornea and lens. Eye trackers and eye doctors use them. In the right conditions you can see two of them inside your own eye.
How common
Does this happen to you?
How it works
Light entering the eye crosses four curved surfaces: the front of the cornea, the back of the cornea, the front of the lens, and the back of the lens. Each surface reflects a small amount of light back out. So a single bright light held near the eye produces four separate reflections. These are the Purkinje images, numbered one to four from front to back1.
The first image, from the front of the cornea, is the bright dot you see on someone’s eye in a photograph. It is by far the strongest. The second is the dimmest. The third and fourth are about equally bright1. The fourth image, from the back of the lens, is special. The back of the lens curves the other way, so it acts like a concave mirror4 and the fourth image is upside down1.
The images move in different ways when the eye rotates, because the surfaces have different centres of curvature. Video eye trackers use the first image together with the centre of the pupil to work out where a person is looking2. Laboratory dual-Purkinje trackers use the first and fourth images together2. The position of one image relative to the other changes little when the head shifts slightly5.
The third image changes size when the lens changes shape to focus on a near object. In the 19th century this was the evidence that the lens really does change shape during focusing4.
The third and fourth images can also be seen from inside the eye. Light that reflects forward off the lens reflects again off the back of the cornea and goes back into the eye1.
Is it a problem?
No. Purkinje images are a normal optical effect of light bouncing off the cornea and lens, present in every normal eye1. Noticing the faint reflections is not a sign of eye disease. Eye doctors and eye-tracking devices use them routinely to check the eye’s shape and movement12. There is nothing about them to mention to a doctor unless your vision itself has changed.
History and culture
Jan Evangelista Purkyně described the reflections in 182334. Louis Joseph Sanson used them in the 1830s to study the lens, which is why they are also called Purkinje-Sanson images14.
Sources and further material
- Wikipedia Purkinje images
- Wikipedia Jan Evangelista Purkyně
- NCBI The quest for the human ocular accommodation mechanism Acta Ophthalmologica. History of the candle reflections, from Purkinje (1823) and Sanson (1838) to Cramer and Helmholtz.
- NCBI High-resolution eye-tracking via digital imaging of Purkinje reflections Journal of Vision. Dual-Purkinje tracking, and how little eye translation affects it.
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Related oddities
Purkinje tree
A branching shadow of your own retinal blood vessels. It is always on your retina, but you only see it when light enters the eye from an unusual angle and the shadow moves.
Blue arcs of the retina
Look at a small red light in a dark room and two faint blue arcs curve away from it towards your blind spot. The arcs follow the paths of the nerve fibres in your own retina.
Purkinje effect
As the light fades at dusk, red things go dark first and blue things stay bright. Your eyes switch from colour-sensing cones to blue-favouring rods, and the whole world shifts towards blue.
Accommodation reflex
When you look at something close, three things happen at once. The eyes turn inward, the pupils shrink, and the lens in each eye gets fatter. You do not decide any of it.