A simple coil of wire could act as a receiver. And attached to a light emitter, the strength of the signal would translate to a brighter light.
Just a guess though.
They have a video you can watch
The video doesn't really explain what they're doing though. Can anyone explain how they're visualizing the EM fields here?
It looks like they have a metering device that glows different colors based on the strength of the field detected. They then appear to be systematically moving the meter across a grid with time lapse photography. The lights photographed from different positions are blended together to create a voxel-like view of the field.
Thanks for explaining that makes sense. What kind of metering device will glow based on these kinds of EM fields?