Dual-Purkinje-Image Concept

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Light rays striking the eye produce four reflections, called Purkinje images, from the front and rear surfaces of the cornea and lens. 

The first (virtual) and the fourth (real) Purkinje images lie in the same focusing plane. 

These two images move similarly under translation but differentially under rotation. The change in their separation is used to determine eye rotation free of error induced by translation. 

 Translation output is available independently.