Performance does not solely depend on one factor alone. This is the degree to which the system increases the brightness of the scene being viewed. For example if a particular system is said to have 60,000 times light amplification factor, this means that the existing level of light is amplified 60,000 times to your eye. This refers to the light threshold at which the tube will operate. No systems of night vision will operate in total darkness.
Different systems have different sensitivity to the available light.
On moonless nights a large proportion of the available light is in the invisible IR part of the spectrum. A tube which is hightly sensitive in this region will outperform a tube with higher sensitivity in the visible spectrum. Generally GEN III tubes are highly sensitive to the IR light which makes them superb performers on dark moonless nights. GEN I tubes are highly sensitive to the visible spectrum and insensitive to IR light. This refers to the amount of detail you can see with the system.