Explain how the light curtain in an elevator prevents door closing when an obstruction is detected, including the typical sensor arrangement.

A light curtain in an elevator is a safety device designed to prevent the elevator doors from closing on passengers or objects obstructing the doorway. It creates an invisible barrier of light beams across the elevator entrance, and if any of these beams are interrupted, the doors will automatically stop closing or reopen. The typical sensor arrangement consists of two main components: a light emitter and a light receiver. The light emitter, typically located on one side of the elevator door opening, projects an array of infrared light beams across the doorway to the light receiver, located on the opposite side. These light beams are arranged vertically, creating a curtain of ligh....