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 light that spans the entire height of the door opening. The light receiver contains an array of photodetectors, each of which is aligned with one of the light beams from the emitter. During normal operation, each photodetector receives the light beam from its corresponding emitter. The receiver sends a signal to the elevator controller indicating that the light curtain is clear and the doors can close safely. If an object or person interrupts one or more of the light beams, the corresponding photodetector(s) in the receiver will no longer receive the light. This interruption signals the elevator controller that an obstruction is present in the doorway. Upon detecting an obstruction, the elevator controller immediately stops the doors from closing or, if they are already closing, causes them to reopen fully. This prevents the doors from striking the obstruction and potentially causing injury or damage. The light curtain system is designed to be highly sensitive and responsive, detecting even small obstructions and reacting quickly to prevent door closure. The infrared light beams used in the light curtain are invisible to the human eye, so passengers are not aware of the system's operation unless it is activated by an obstruction. Some light curtain systems use a pulsed or modulated light beam to improve their immunity to ambient light and electrical noise. This helps to prevent false detections and ensures reliable operation in a variety of lighting conditions. The light curtain is a crucial safety feature in modern elevators, providing reliable protection against door-related accidents.