I've always wondered if it's possible to build a video camera that films on the microwave spectrum instead of the visible light spectrum. Would such a tool would allow you to "see" wi-fi access points ? I guess they would look like bright, pulsating sources of light, and they would be visible through walls.<p>I did a bit of research on the subject, and they don't seem to really exist. It's possible that the longer wavelength poses technical difficulties. The individual receptors are too big and you can't cram them together at a sufficient density. Also, the lensing and shielding needs to be completely re-thought.<p>There's a 2012 paper called "Portable Real-Time Microwave Camera at 24 GHz" but i couldn't access it (not in academia).
I wonder how much of the variation is temporal instead of spatial. There are a few shots in there that were done twice with different results, showing that the signal strength in a single spot varies over time. You can't use light painting to capture that usefully; you can only make pretty artistic impressions.
This reminds me of a project, that a group of us talked about, but never did.<p>I used to work on a radio system with a very large and powerful phased array transmitter, the ground mat being hundreds of metres on each side. We though it would be fun to stick a bunch of fluorescent tubes into the ground. In theory we should have been able to see the antenna beam sweeping across the ground mat, as it was steered in difference directions and the tubes lit up. It would have been a very cool demonstration, but like so many other projects we only talked about it.