Gen is a commonly used abbreviation that refers to the power or “Generation” of image intensifier night vision tubes. The key difference between the night vision generations is the intensifier technology. The United States government recognizes 1st, 2nd, and 3rd Generations of tubes and has recently begun purchasing newly issued tubes, recognized as Gen 4 by certain companies. See 1st Generation, 2nd Generation, 3rd Generation, 4th Generation and the FAQ section for more information on how these tubes work.