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2013-08-01T07:23:39Z

203.106.67.96: /* Recognizing impossible evaluations */

'''Purkinje images''' are [[Reflection (physics)|reflection]]s of objects from the structure of the [[human eye|eye]]. They are also known as Purkinje reflexes and as Purkinje-Sanson images. At least four Purkinje images are usually visible. The [[first Purkinje image]] (P1) is the reflection from the outer surface of the [[cornea]]. The [[second Purkinje image]] (P2) is the reflection from the inner surface of the cornea. The [[third Purkinje image]] (P3) is the reflection from the outer (anterior) surface of the [[lens (anatomy)|lens]]. The [[fourth Purkinje image]] (P4) is the reflection from the inner (posterior) surface of the lens. Unlike the others, P4 is an inverted image.

Purkinje-Sanson images are named after [[Czech people|Czech]] [[anatomist]] [[Jan Evangelista Purkyně]] (1787–1869) and after [[French people|French]] [[physician]] [[Louis Joseph Sanson]] (1790–1841).

The third and fourth Purkinje images can be visible from within the eye itself. Light reflected away from the surfaces of the lens can in turn reflect back into the eye from the rear surface of the cornea. These images are, therefore, [[entoptic phenomenon|entoptic phenomena]].

The first and fourth Purkinje images are used by some [[eye tracker]]s, devices to measure the position of an eye.

== Brightness of Purkinje Images ==

The brightness of the Purkinje images can be determined using [[Fresnel equations|Fresnel's equation]]:
<math>\text{Intensity (brightness)} = (n'-n)^2/(n'+n)^2,</math>
where <math>n</math> and <math>n'</math> are the refractive indices before and after the reflecting surface.
Purkinje image P1 is the brightest of the four then P2, P3 and P4 (P3 and P4 have about the same brightness).
The Purkinje images can be used to assess the curvatures and separations of the surfaces in the eye.

==See also==
*[[Red-eye effect]]

==External links==
*[http://www.diku.dk/~panic/eyegaze/node9.html Explanation of using purkinje images for eye tracking.]

{{Visual phenomena}}

[[Category:Optics]]

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