Reflections of outside objects in the eye. Do you remember how annoying anime characters always have one or more white blotches in their eyes? Those are the Purkinje reflections (also known as Purkinje images) of light sources. After all, your corneas are little transparent domes, and it stands to reason that they should reflect the world.

Since the eye is not exactly a reflecting ball of chrome, we get more than one reflection. In fact, four Purkinje images form:

  1. Front surface of the cornea (the one facing outside). Very bright.
  2. Back surface of the cornea.
  3. Front surface of the lens
  4. Back surface of the lens (very weak)

Purkinje images are used in eye tracking technology to locate orientation of the eye, by comparing the position of the first reflection of a known, fixed, infrared light source with the position of the pupil (easy to track, as a black blob).
Some systems are also able to track the fourth image, which allows additional measurements of eye position.

The phenomenon is named after the Czech physiologist Johannes Purkinje (nineteenth century), who also did interesting work about optical illusions. See also Purkinje's cells.

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