[dcroley]

If a sheet of paper is roughly .1 mm, then this must be running over 300GHz, or what is more commonly called infrared light, no?

[mediocrejoker]

I think it runs around 60Ghz and uses phased arrays / doppler to make up the rest of the resolution.

[DoctorOetker]

good catch, probably it's not actually resolving the individual card layers, but using the Lambert-Beer law of attenuation, after callibrating on a deck of known number of cards?