They aren't coming to an airport near you, at least in New Zealand, but international travellers are becoming more and more familiar with the full body scanners now commonly used to detect concealed items at security checkpoints. Their ability to see right through clothes has sparked controversy over privacy, and the way they work has sparked controversy over health risks. But how do they work, anyway?
There are two main kinds of full body scanner, one using X-rays, the other millimetre waves. We are all familiar with the matter-probing abilities of X-rays. But what are millimetre waves and how come they can see through clothes? The same principle lets both X-rays and millimetre waves see through to the real you, and its not as unfamiliar as it might seem.
We usually think of clothing as being (ideally) opaque, but opacity and transparency are not clear-cut concepts. We usually think of a green filter as being transparent, because under ordinary conditions we can see through it, even if everything has an eerie tinge. But the reason the filter is green is that it absorbs other colours shining through it - try to shine a red light through a green filter and nothing comes out the other side. In other words it is opaque to red light, but transparent to green.
This is all very interesting, but you can't see through clothing no matter what colour light you shine on it. Or can you?
The colour of light corresponds to the wavelength of its waves. The wavelength of red light is around 650 nanometres and green light around 550 nanometres, for example. But visible light is only a small chunk of the entire electromagnetic spectrum. Heading to shorter wavelengths you pass right by blue and violet into the ultraviolet, X-rays and gamma rays. Heading to longer wavelengths you run past red into infrared, millimetre waves, microwaves and radio waves. All these other waves are invisible to our eyes, but not to our technology. And it turns out that fabrics that are quite opaque to visible light are translucent or transparent to millimetre waves and X-rays.
Most of your body is transparent to X-rays as well, of course; but just as a window reveals itself by reflecting a little light, so the body reveals its contours by reflecting (or backscattering) a fraction of any X-rays falling on it. Some of the X-rays still pass through, but because X-ray body scanners probe only the surface they can get away with using much less radiation than in medical imaging. Millimetre waves don't really penetrate the body, so scanners based on them have no choice but to rely on either reflection or detecting waves given off naturally by the body.
That's the principle, though of course lots of engineering is needed to bring the principle to life and lots of clever computer software is needed to help interpret the results. Look past all that, though, and at the heart of these machines is a nifty piece of physics. It's easy to miss - you literally can't see it - but once you understand it, much becomes clear.
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