There is a video showing one in use here: http://www.spyops.net (Camera Finder video demonstration).
Monopole is spot on. A ring of LEDs are mounted around the eyepiece. A bandpass filter lets through the LED colour, but no other. Lenses appear as a bright spot against a dull background.
There is a video showing one in use here: http://www.spyops.net (Camera Finder video demonstration).
Monopole is spot on. A ring of LEDs are mounted around the eyepiece. A bandpass filter lets through the LED colour, but no other. Lenses appear as a bright spot against a dull background.