I've had a look at the Naneye you linked - I couldn't find the reason why the camera would not be optical. Optical in the sense of "has an optical system with a

I've had a look at the Naneye you linked - I couldn't find the reason why the camera would not be optical. Optical in the sense of "has an optical system with a detector that registers photons close to the visible spectrum" (in contrast to microwaves). I can imagine it's got a pinhole and a CCD, which is why there have to be four powerful LEDs illuminating the vicinity for it to reach the 44fps framerate without everything being just noise.