Photo-eye sensor
The safety sensors near the floor of a garage door that stop it from closing on something.
Photo-eye sensors are the small units mounted a few inches above the floor on each side of a garage-door opening. They shine an invisible infrared beam across the doorway; if anything breaks the beam while the door is closing, the opener reverses the door so it can't crush a child, pet, or car. Required on openers since the early 1990s, they're also the #1 reason a door won't close — if the lenses are dirty, knocked out of alignment, or the indicator lights are off, the opener refuses to close as a safety measure.