A light curtain is an infrared safety edge fitted to the car door that projects a grid of beams across the door opening and reverses the doors if any beam is broken. Modern units use 30 to 200 beams covering the full door height from about 25 millimetres above the sill to 1800 millimetres, so a child or a walking stick is detected as reliably as an adult. Light curtains have largely replaced mechanical safety edges because they act before physical contact occurs.