Electric eye

A man and woman sat before an electric eye in a London laboratory tonight and a group of people in a darkened room in this village outside New York watched them turn their heads move from side to side.

Another frequently cited possibility is a system of electric eyes reading beams of light that are aimed along tracks so that any displacement of the rails would disrupt the beam.

The doors have electric eyes: "If you're carrying large specimens or carts full of plants, the electric eye beam is broken and the door opens up."