Purkinje images

Purkinje images are reflection

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

s of objects from structure of the eye

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

. 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

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

. 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

Lens (anatomy)

The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

. 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

Czech people

Czechs, or Czech people are a western Slavic people of Central Europe, living predominantly in the Czech Republic. Small populations of Czechs also live in Slovakia, Austria, the United States, the United Kingdom, Chile, Argentina, Canada, Germany, Russia and other countries...

 anatomist Jan Evangelista Purkyně

Jan Evangelista Purkyne

Jan Evangelista Purkyně was a Czech anatomist and physiologist. He was one of the best known scientists of his time. His son was the painter Karel Purkyně...

 (1787-1869) and after French

French people

The French are a nation that share a common French culture and speak the French language as a mother tongue. Historically, the French population are descended from peoples of Celtic, Latin and Germanic origin, and are today a mixture of several ethnic groups...

 physician

Physician

A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

 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 phenomena

Entoptic phenomenon

Entoptic phenomena are visual effects whose source is within the eye itself. In Helmholtz's words:...

.

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

Brightness of Purkinje Images

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.

External links

• Explanation of using purkinje images for eye tracking.