A method for judging the performance of image intensifiers is described and used for night vision devices. Based on the operating distance model， some factors affecting the detection ability are analyzed. The difference between test conditions for integral sensitivity， ultimate resolution and actual night vision environment is studied. The influences of night sky radiation spectral characteristics， spectral attenuation characteristics of atmospheric transmission， and spectral difference of background reflection characteristics on detection distance are analyzed. The shortcomings of traditional methods for judging the quality of the image intensifiers are sorted out. Figure of Capability （FOC） is proposed， which consists of signal-to-noise ratio， resolution tested in low illumination and low contrast， and technical features. The performance echelon and intergenerational of two major technology routes （including GaAs tubes and multi-alkali tubes） are analyzed with FOC， and one kind of Gen.3rd development route is designed. Experimental results show that FOC can accurately reflect the developing law of the technical route and grade of image intensifiers.