How do Airport body scanners work?
July 28, 2010 7:52 PM   Subscribe

At one airport, the scanner always beeps if I pass through without removing my shoes. The same scanner lets my colleague pass without removing shoes. At another airport, our roles are reversed. The colleague has to remove shoes because the scanner beeps, while I pass through without any problems. Both scanners (at different airports within Australia) are the gated type, not the enclosed-chamber sorts. What is going on?
posted by vidur to Science & Nature (8 answers total) 1 user marked this as a favorite
 
Best answer: An airport Walk-Through Metal Detector (WTMD) is also known as a magnetometer, which begins to explain how it works: it detects changes in magnetic fields.

Even if identical WTMDs are used at each airport, any fixed metal objects in the vicinity of the WTMD (e.g. desks, rebar, etc) will slightly alter the shape of the magnetic field, and hence the detection characteristics. If the WTMDs are different models, or even just calibrated differently, then that would also contribute to differences in the detection pattern.
posted by Dimpy at 8:54 PM on July 28, 2010


Best answer: A little but about the technology used in metal detecting scanners here. My personal experience is that some detectors seem to be mis-calibrated to the point where they are causing lots of people to have to be searched for metal objects that they do not have (I am guessing that equally some detectors are set at too low a sensitivity - but I would not notice that). I suppose the way I would design a test for the calibration setting would be to make sure that it did not trigger with a test object of some minimum size and make sure that it did with something above it. The details of the test would have to be kept pretty secret.
posted by rongorongo at 1:59 AM on July 29, 2010


..but apparently not so secret that it is not possible to buy a test kit!
posted by rongorongo at 2:01 AM on July 29, 2010


I have a pair of shoes that has a metal plate under the insole that maintains the arch support. (That's how they came.) More than likely, the sensors in the detectors are just of a different design or different calibration.
posted by gjc at 5:44 AM on July 29, 2010


Oddly enough, I was just reading a thriller where it was mentioned that men's shoes of better quality will often have a metal plate in the sole. Apparently as a result many airport sensors have a dead zone at the bottom so they are not constantly triggered. Now the metal plate does explain why shoes set some sensors off, and some sensors having a dead zone while others do not explains why some might go off and some might not, but I have no explanation as to why the role reversal!
posted by biffa at 6:41 AM on July 29, 2010


In addition to general arch support, some work boots and shoes have a steel shank specifically so that the wearer can stand comfortably on a narrow peg and is common in footwear worn by people who have to climb poles.

My combat boots have this and set off nearly every metal detector I go through, but not all of them. I've always attributed this to randomness in the calibration.
posted by quin at 8:14 AM on July 29, 2010


My understanding is that how you walk through the detector also makes a difference. If you're closer to the edge of the detector vs. more in the center of it, for example. You're not going to be able to slide through the center with a gun, but being near the edge might give you a false positive. At least that's the explanation I was given when San Francisco's metal detectors kept pinging on me despite the fact that I was not wearing any metal anywhere on my body, not even so much as the hooks on a bra -- I'd worn an underwire free sports bra just for the occasion, because I've always had trouble with the SFO detectors.
posted by jacquilynne at 11:20 AM on July 29, 2010


Response by poster: I am not sure if this came out clearly in my question, but my puzzlement is over the role reversal. I understand that these things (WTMDs, thanks Dimpy!) can have different sensitivities and different calibrations. But whatever those values are, it appears to me that the role reversal is a bit of an anomaly.

Lets assume that my shoes contain 10 amount of metals (not that I can see any), and my colleague's have 5. The first WTMD beeps at, say, 8. So, it beeps me out while my colleague breezes through. The second one beeps at some mysterious X, so that 10 passes through while 5 has to stop?? This is a bit hard to understand.

Of course, if there were other things (say, Metals and Stinkiness) triggering the WTMD beeps, and our shoes contained different amounts of each (one higher, one lower) and the WTMDs were calibrated just so, that would explain things. This is what I want to understand.
posted by vidur at 7:18 PM on July 29, 2010


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