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Comment by tsaoutourpants | Hacker News Reader

These devices, as best I can tell, actually don't come with a radiation risk. They're passive scanners, meaning they don't emit their own radiation.

TSA scanners on the other hand... ;)

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The distinction you are trying to make is not between emitting and not emitting radiation - most of these scanners emit some form of electromagnetic radiation.

The distinction you're trying to make is between ionizing radiation (x-rays; causes cancer) and non-ionizing radiation (all other sorts of electromagnetic radiation including visible light, infrared, microwaves, and so on; doesn't cause cancer).

The TSA currently has some scanners that scan using x-rays and some that scan with non-ionizing radiation.

A regular boring old security camera is also a scanner that emits non-ionizing radiation - most of them use infrared LEDs to illuminate at night. If you could see into the infrared it would look like every security camera has a big flashlight mounted next to it, shining on you. So the target, you, gets irradiated with non-ionizing infrared rays, which bounce back to the camera and are recorded. Using different wavelengths of illumination, you find that some of them penetrate clothes easily to see what's underneath. That's what the NYPD/security apparatus are interested in.

No, that's not the distinction I'm trying to make. The NYPD scanners, as best I can tell, are PASSIVE scanners which means they emit NO radiation (any moreso than your digital camera "emits radiation"), ionizing or non-ioninzing. They simply measure the terahertz emissions (and reflections thereof) of other objects -- they do not introduce their own terahertz waves.

I'm not defending the scanners (obviously)... I just want the case against them to be clear and sound.

I'm pretty sure you're mistaken. Terahertz waves are absorbed by the atmosphere, so those emitted by the sun don't reach the Earth's surface. So a terahertz scanner has to emit its own radiation.

Infrared radiation (e.g., from body heat) is in the terahertz range[1], and might be what these sensors are picking up. And there are definitely passive sensors for infrared, e.g., night vision goggles.

[1] https://en.wikipedia.org/wiki/Infrared (300 GHz to 430 THz)

Not quite as simple as that. Terahertz != Short Wave Infrared != Midwave Infrared != Longwave Infrared ("thermal"). These all require different photosensitive elements in your detector. They also have extremely different atmospheric absorptions. [1]

These particular sensors might indeed be passive, but it would limit their range and SNR. Be wary of manufacturers simply selling the emitter as a second SKU to garner the label passive on their detectors.

[1] I work at a company making SWIR/MWIR imagers.

+1. These things are basically fancy infrared goggles, detecting heat emitted from your body.

Oh. Well, in that case it seems to me that the objection to them can't be that they're invasive. We have a well-established principle that says it's okay to take photographs or video of people in public places where there's no expectation of privacy. I'm not sure I see how the fact that this uses non-visible wavelengths removes it from the scope of that principle. Can you explain that?

(I'm sympathetic to your goals here, but I think the tough questions need to be asked. They certainly will be in court.)

The simple answer is that most people expect clothing to provide privacy for what is kept under them (such as your wallet, cell phone, genitals, etc). Now when the day comes that everyone normally wears augmented reality glasses that can see terahertz / infrared radiation, and that one would normally check themselves out in a mirror with such glasses before going out, then I guess there would be no expectation of privacy for what is under you clothes.

What makes you think they're passive scanners?

Check the mfgr Web site :)

Personally I see the potential for false positives as rendering them mostly useless in a crowd setting.

IDK if a crowd would make them less effective, but there is certainly ample opportunity for false positives -- or worse, cops pretending to get a false positive to justify a manual intrusion.

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