Immaterials: Light painting WiFi from Timo on Vimeo.

Here’s one of the most creative examples of light painting and long-exposure photography we’ve seen — a few techie guys built a special 12-foot-long rod with 80 LED lights that light up depending on how strong a particular Wi-Fi signal is. By walking the stick around and capturing the lights in real time, they were able to photograph “light charts” showing how a particular Wi-Fi signal strength fluctuates in a particular area.
Source: PetaPixel
See also: nearfield.org and yourban.no