Electric eye

An electric eye was a sensor mounted to the door frames of 23rd century Federation starships that was used to sense when a person passed within its visual range, triggering the door to open.

After a shrunken Lieutenant Sulu broke his leg falling from the helm console, Captain Kirk and Lt. Arex were hindered from taking him to sickbay, as they had become too small for the electric eye to detect them. Kirk solved this problem by waving a pin he had found under his chair in front of the sensor, opening the door.