Glossary term
light curtain
An array of infra-red beams across the door opening that holds the doors open while anything breaks a beam.
What it means
A light curtain is a row of invisible beams across the doorway. While something interrupts a beam, the doors stay open. Unlike a safety edge, nothing has to be touched: the doors reverse before contact, which is why light curtains have largely replaced simple photocells and mechanical edges on passenger lifts.
Where you see it
Down both sides of the car door opening, usually built into the door edges as a strip of small lenses. Older lifts may have a single photocell beam at about waist height instead.
How it works
Transmitters on one side send beams to receivers on the other. The detector reports a clean or blocked state to the door operator many times a second. Any blocked beam holds the doors, and the doors only close when every beam is clear for a set time.
Dirt is the enemy. A film of dust, a smear or a sticker over a lens looks the same to the receiver as a passenger standing in the doorway, so the lift holds the doors open and eventually parks with a door fault.
Why it matters
Because a light curtain fails safe — a blocked or faulty beam means open doors, not closing doors — the usual symptom is a lift that will not leave the floor. That is inconvenient, but it is the behaviour you want.
Commonly confused with
A light curtain is confused with the safety edge. The curtain detects before contact, using invisible beams across the opening; the edge only reacts on touch. Many modern lifts have both, which is why doors can reopen with nothing visibly touching them.
Wear, service and lifespan
The lenses at each side of the door collect dust, fingerprints and cleaning film, and a dirty lens gives false detections. Service work cleans the lenses, checks alignment and tests detection across the full height of the opening. Damaged edges caused by trolleys are a common cause of curtain faults on busy goods routes.
Signs to watch for
- Doors stay open at one floor with nothing in the doorway.
- Doors close only when someone stands to one side.
- The lift parks open and shows a door fault.
- Lenses look dusty, smeared or damaged.
- The behaviour started after decorating or cleaning work.
Troubleshooting light curtain
Doors keep reopening with the doorway clear
What it usually means: Dirty or misaligned curtain lenses
Safe check: Look along both door edges for dust, tape, stickers or damage and report what you see.
Doors close on people without reopening
What it usually means: The curtain may not be detecting, which is a stop-use condition
Safe check: Stop using the lift and report it urgently. Do not test it with a hand or foot.
Detection fails only near the floor
What it usually means: Lower beams blocked or misaligned
Safe check: Note the height at which it stops working, roughly.
How to report it
Say whether the doors reopen too much or too little. Too much is a nuisance fault; too little is a safety fault and should be reported as urgent with the lift taken out of use.
Ask us about light curtain on your lift
Tick anything you can see or hear from where you normally stand, add a note in your own words, and we will put it into the message below for you. Never open, force or enter any part of the lift to check.
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Related terms
Definitions that usually come up alongside light curtain when you are describing a fault.
- safety edge
A sprung or sensitive strip on the leading edge of a door that reverses it the moment it touches an obstruction.
Safety devices
- door operator
The motor and gear on top of the car that opens and closes the doors and reports their position to the controller.
Doors and entrances
- car door
The powered door on the lift car itself, driven by the door operator and responsible for opening the landing door as well.
Doors and entrances
- fault code
A number, letter sequence or lamp pattern the controller shows to identify why it stopped.
Controls and electrics
- interlock
A lock and switch combined, so a door cannot be opened unless the car is there and the lift cannot move unless the door is locked.
Safety devices