• Question: Are the security lasers in spy movies actually used in real life? How do they work? Thanks!

    Asked by pegasister to Mike, Pip, Tianfu, Tim, Tom on 28 Jun 2012. This question was also asked by izlanexox.
    • Photo: Tom Lister

      Tom Lister answered on 28 Jun 2012:

      They are used in some smoke alarms. There is a light detector (photodiode) at one end that registers when the beam is blocked by smoke/burglers and triggers an alarm. They are also used on lift doors, so the lift doesn’t close with you standing in the way.

    • Photo: Tim Stephens

      Tim Stephens answered on 28 Jun 2012:

      Most smoke alarms actually use radioactive Americium 241 (look at the detector when you change the battery, and you’ll see the little radioactive warning sticker). The radioactive material ionises the air, which allows an electrical current to flow between two electrodes. When the smoke gets into the detector, it stops the current from flowing by removing the ionised air and that causes the alarm to go off.
      Lift doors don’t use lasers, but it’s a beam of infrared light from an LED. Same principle as if they used a laser though.

      As Tom says, the alarms would work by detecting when a burglar broke the beam. Most burglar alarms don’t use that technique – they use heat detectors which look for warm bodies moving around the room.

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