Gen1+ night vision optics (that's Gen for Generation) were developed during the sixties and were the first to require only "passive infrared" light to function, meaning that they didn't require an infrared light source to function (though it always helps). On cloudless nights under a full moon they do an adequate job of converting the invisible infrared light that always surrounds us into visible monochrome green images. In the timeline of technology, think of them as the "black and white TVs" of night vision.
Moving forward to the 80's, Gen2+ night vision devices came on board. Adding a micro-channel plate doubled their brightness by amplifying the visible image produced from the invisible infrared light. Even better, Gen2+ image intensifiers increased image sharpness by 50%. When you hear night vision wonks talk about "resolution" this is what they're talking about - how sharply defined the image is. Once again, using our TV comparison, Gen2+ tubes would be the "color TVs" of the night vision world.
Now to the most current tubed technology - Gen3. In addition to the micro channel plates, Gen3 upped the ante with the addition of a Gallinium Arsenide (GaAs) photo cathode. By using this exotic element engineers were truly able to turn night into day. You'll also find that Gen3 night vision equipment offers the greatest depth of features, including intuitive operating controls, autogated power supplies and numerous on-screen indicators. What kind of TV would we compare Gen3s to? Think "high definition 4K HDR" with all the bells and whistles. It really is that good.