electric eye

plural electric eyes

1. A sensor which measures the intensity of light, often used to start and stop electrical equipment (motorized doors, etc.).

   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."

   2005 May 10, Gary Shapiro, “Botanical Garden Will Unveil Eight New Greenhouses”, in New York Sun, retrieved 14 Nov 2013

   2 more examples

   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.

   1928 February 10, “Span Atlantic by Electric Eye”, in Spokesman-Review, Spokane, USA, retrieved 14 Nov 2013, page 1

   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.

   1996 November 29, Richard Perez-Pena, “Rail Accident Stirs Debate About Sensors”, in New York Times, retrieved 14 Nov 2013

   Synonyms: photocell, photoelectric cell