It’s commonplace in caper movies: Light barriers as a security measure to detect intruders. Frequently (in films, such as the incredibly cool “laser dance” sequence in Ocean’s Eleven), there’s a number of laser beams sweeping seemingly randomly across the room, scanning the floor and walls.
But are there actually light barriers which work this way? As I understand, the barrier consists of a light source (not necessarily a laser) on one side and a photoelectric sensor on the other. As soon as an object blocks the light beam, the sensor detects this and sets off the alarm. This should make it impossible to have the beam sweep freely across the room, since there would be no receiver on the other side.
Maybe the barrier is based on reflection: The light ray is reflected by the surface back to the light source; if you include the sensor there, this should work as well. But, again, free sweeps would not be possible: If the ray does not hit the surface at a right angle, it will be reflected elsewhere, not back to the light source.
So is this just for dramatization, or are there actually devices which can do the cool sweeps?
(And, yes, I understand that there is another thing which most definitely is wrong about such movie sequences: In reality, you wouldn’t see the laser beam going across the room unless there was dust or fog to be illuminated.)
You could have a LIDAR setup, with either a mechanically-scanned or a phased-array emitter. LIDAR doesn’t need a detector at the other end of the beam; the detector is at the emitter. You’d set these up, have them establish a baseline for the room, and then when anything tried to walk in, the travel time of the return pulses on the detectors would change, tripping the alarm.
Of course in real life, you wouldn’t just have one detection modality. MythBusters did an episode a while ago where they tried to defeat a number of real life security systems, including a proper IR laser detector and a sonar detector. The latter worked on the same principle as the LIDAR setup I outlined, but with ultrasonic pulses instead of light.
In real life, by the way, a security system in one building I know uses a laser ‘tripwire’. It has a single emitter/detector with a reflector on the other side of the corridor. Trying to block that with a mirror won’t help you one bit - what the detector is measuring is the round-trip time, and a mirror will set it off just as much as someone tripping the beam would. The beam is IR, of course, and completely invisible. Don’t bother trying unless you have some magical metamaterial that’ll re-emit the laser pulse in the direction it came from at exactly the same time that it would have passed through that point if it had reflected from the opposite wall.
That’s a trivial problem to solve, actually. Whether it’s practical or not is another matter, but all you’d need to do is “fold up” the length of the path the beam would take with multiple mirrors or prisms and a bit of retroreflective material at the the halfway point. Placement timing and alignment accuracy would be the practical stumbling blocks here but not, I think, unsolvable. If one wanted to bother, I mean.
I don’t know if they exist but you can measure distance with a laser so it seems easy to program one to sweep a room and keep a record of the distance it measures at certain intervals. If someone walks through it, the measurement would be off.
It’s not necessarily a security system, but my parents’ garage doors are set up with a similar system to turn the lights on when someone walks through them (when they’re open, of course).
And what is the principle behind this LIDAR system? Is it simply to detect the refraction of the laser beam after hitting a surface? The amount of light which arrives back at the detector must be miniscule, given that the beam is refracted into all directions and a part of it is absorbed by the surface.
I also remember reading that one of the Apollo missions set up a mirror on the moon for experiments with laser rays. Why was this done if LIDAR can do totally without reflectors?
It’s our old friend the inverse square law. It’s one thing detecting the light reflected from a wall - or a person - a couple of metres away, quite another detecting the reflection from the moon 400000 km away!
Because bouncing a laser beam off a poorly reflecting object gives you a very low-power return. At short distances, with a reasonably powerful beam and a reasonably sensitive detector, this isn’t a problem, but when you’re talking about having a beam be emitted on Earth, go through the atmosphere (power loss), reach the moon (power loss), bounce off the lunar surface (power loss), go back to Earth (power loss), pass through the atmosphere again (power loss), and be detected, you want all the help you can get.
The reflector helps in two ways. First, it minimizes absorption and maximizes reflection. Second, it’s a retroreflector; that means that the direction of reflection is as close to antiparallel to the incident beam as possible. This helps because it maximizes the amount of power in the specific place you build your detector on Earth.
Even under optimum conditions, the power you receive back on Earth is tiny. If the reflectors weren’t there, your initial beam power would have to be truly monumentally huge. This isn’t a problem for short range applications, as anyone who has gotten a ticket due to police speed LIDAR found out. For long range applications, however, that r^2 falloff is a killer.
Just for further edification, the Mythbusters demonstrated that the old dust trick doesn’t work. The whole scene you’re thinking of (regardless of the movie) is stupidly, stupidly implausible.
Kills police LIDAR, too. That’s why they aim for license plates, marker/tail lights and headlights, which are all effective retroreflectors. In the absence of those, the range is markedly reduced, except in the special cases where a large reflective surface (such as a window) are angled so as to reflect the beam straight back to the receiver.
They use them in movies because they look cool, and give the hero or villain the opportunity to loook impressive dodging the brightly-colored laser beams. In practice, they’re about as reasonable as those detonators that give you the nifty LED readout. In real life, infrared lasers would be harder to see, and motion detectors of various kinds would give an alarm even if your contortionist thief was trying with all his might to avoid the beams.
But then it wouldn’t look anywhere hear as impressive. And that’s all the movie needs.
Even with the retroreflectors, the power loss from all the other factors is still ridiculous. The figures I’ve heard from various people who do these lunar-ranging experiments is that they get one photon back for every quadrillion (!) they send.
Yeah, I have the habit of starting a post, getting interrupted for a few minutes, and finishing it later after others have covered what I was going to say, and better than I was going to say it.
