Eye tracking white sclera (limbus tracking). It presents another challenge for vertical eye movements due to obstruction of eyelids. Eye-tracking setups vary greatly: some are head-mounted, some require the head to be stable (for example, with a chin rest), and some function remotely and automatically track the head during motion. Most use a sampling rate of at least 30 Hz. Although 50/60 Hz is more common, today many video-based eye trackers run at 240, 350 or even 1000/1250 Hz, speeds needed in order to capture fixational eye movements or correctly measure saccade dynamics. Eye movements are typically divided into fixations and saccades
The Eye of the Beholder covered by bandages, dub the single line spoken by Tyler when she is revealed, portrayed by the actress Donna Douglas. However, Douglas had been listening to Stuart's voice as she recorded her part, and was able to imitate her so successfully that she was allowed to speak the line on camera herself. The original title for this episode was "Eye of the Beholder." Stuart Reynolds, a television producer, threatened to sue Serling for the use of the name because at the time he was selling an educational film of the same name to public schools. Reruns following the initial broadcast