Look at DICK'S SPORTING GOODS, BASS PRO SHOPS,SPORTS-MANS GUIDE CATALOGUE. Also, Wal-Mart & Meijer both carry Night-Vision Monoculars. Oh yeah, try Surplus Shed or any ARMY/NAVY Surplus store. All these places carry either binocular or monocular night vision devices. Some carry both. They can cost anywhere from just under $200.00, to over $3,000.00 for the binoculars with the 3rd gen. light gathering tryphium tubes. The higher quality items will be found at the ARMY/NAVY Surplus stores, 'cause this stuff is usually state-of-the-art military equipment. A lot of it comes from the now defunct soviet union, & carries the RED STAR/soviet union symbol.
It usually depends on which generation tryphium tubes it has in it. 1st gen. tubes are oldest less advanced. 3rd gen. tubes Are the newest & most advanced, & (as far as I know, there is NO 4th gen.)
I don't know the exact science behind it, but it has something to do with the maximum amount of amplification of any & all available light. This has to do with the military version, don't know about civilian models. When you turn on one of the military models though, you can barely hear it but there is a VERY high pitched hum coming from the tubes as they warm up. rob