An apparatus includes a controller coupled to a stereoscopic image viewer. The stereoscopic image viewer includes a viewing lens, an image capture unit, and a plurality of light sources. The image capture unit is configured to capture image frames. The controller receives the captured image frames. The controller is configured to switch light output from one of the plurality of light sources to another one of the plurality of light sources if a corneal reflection in a captured image frame intersects a fixed reflection in the captured image frame.