Hutt, it is reflected light that makes them glow, but it's infrared light, in wavelengths that the human (and critter's) eyes cannot see. You can only see it if you are using a night vision device of some type, especially Gen 3. Even a weak IR illuminator will light them up at ranges that would require a pretty strong spotlight/red filetr, if you were using a visible light. IR wavelengths in the low end of the IR spectrum, from 730-808 nanometers, will have a slight dull red visible glow, but past 830 nM, it is completely invisible to the naked eye.