Codprawn - "gen1/2"?? It's hybrid?
Admittedly it's been 17 years since I last played with these, so technology may have moved on a long way since then.
The difference between gen 1 & 2 was huge, they work on totally different principles. The gen 1 units had a low gain, so the image intensifier units were stacked up in the housing to increase the total gain (the one's I saw had 3 working together). The image from them was very clear, however the downside was the physical bulkiness of the unit - about the size of a large lens. Worked well as a gun sight, or as a handheld unit, but certainly not capable of being head mounted.
The Gen 2 units were about the same size as a single stage of the Gen 1 units - therefore total unit size was about 1/3rd. They work on a totally different principle to the Gen 1's, and image quality is much lower. They produce a very grainy image - look at nighttime TV footage from iraq at the overall graininess. This is how the Gen 2's I used looked.
The Gen 1 units use very very high voltage to energise the tubes. As a result, they're very susceptible to moisture.
Afraid I know nothing about Gen 3's.
If you're looking 2nd hand or military surplus, make sure it's a genuine image intensifier and not just an infra-red unit. Physically they can look identical, so it's easy to get them mismatched. An image intensifier can work with infra-red light, but an infra red unit won't work as an image intensifier.