definition of Wikipedia
Purkinje images are reflections of objects from structure of the eye. They are also known as Purkinje reflexes and as Purkinje-Sanson images. There are at least four Purkinje images that are visible on looking at an eye. 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. The fourth Purkinje image (P4) is the reflection from the inner (posterior) surface of the lens. Unlike the others, P4 is an inverted image.
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 phenomena.
The first and fourth Purkinje images are used by some eye trackers, devices to measure the position of an eye.
The brightness of the Purkinje images can be determined using Fresnel's equation where the Purkinje image I is the brightest of the four then PII, PIII and PIV (III and IV have about the same brightness). The Purkinje images can be used to assess the curvatures and separations of the surfaces in the eye.
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