The gain of super second generation image intensifier, hereinafter referred to as image intensifier, is determined by photocathode response, gain of microchannel plate, screen efficiency and screen voltage, and is directly proportional to the product between them. The gain of microchannel plate has an exponential relationship with its operation voltage. Generally, when the operation voltage of the microchannel plate is increased by 50 V, the gain of the microchannel plate can be doubled, and the gain of the corresponding image intensifier can also be doubled. Because the photocathode response, screen efficiency and screen voltage are relatively invariant, the gain of image intensifier is mainly determined by the gain of microchannel plate, that is, by the operation voltage of microchannel plate. The higher the operation voltage of the microchannel plate, the higher the gain of the image intensifier. However, when the operation voltage of the microchannel plate is increased, under normal circumstances, only some scintillation points will appear on the fluorescent screen. The higher the voltage is, the brighter the scintillation point is. However, when the operation voltage reaches a certain value, stable output brightness will appear on the fluorescent screen, that is, the image intensifier will produce self-excited luminescence. The so-called self-excited luminescence refers to that the image intensifier can output a certain stable brightness under the condition of no input illumination. When the image intensifier produces self-excited luminescence, the contrast and resolution of the image disappear, and the enhancement of image brightness is meaningless. Therefore, the maximum brightness gain of image intensifier is limited by self-excited luminescence.The reason why the image intensifier produces self-excited luminescence is that the part of luminescence penetrates the aluminum film of the fluorescent screen and microchannel plate, reaches the photocathode, and excites the photocathode to emit photoelectrons. The photoelectron is multiplied by the microchannel plate, and finally the fluorescent screen is excited to emit light, so as to form light feedback. Therefore, light feedback is the direct cause of self-excited luminescence.The main factors affecting the light feedback are the transmittance of the aluminum film of the fluorescent screen and the photocathode response for the luminescence of the fluorescent screen. Light feedback is directly proportional to the photocathode response. The higher the sensitivity of photocathode, the easier it is to produce light feedback. For example, for Na₂KSb(Cs) photocathode and Cs₂Te photocathode, since the response of Cs₂Te photocathode to fluorescent screen luminescence is much lower than that of Na₂KSb(Cs) photocathode to fluorescent screen luminescence, the image intensifier of Cs₂Te photocathode is more difficult to produce light feedback than that of Na₂KSb(Cs) photocathode, so higher gain can be obtained. The light feedback is also directly proportional to the transmittance of the aluminum film of the fluorescent screen. The higher the transmittance of the aluminum film of the fluorescent screen, the easier it is to produce light feedback. Therefore, to suppress the light feedback, it is necessary to reduce the transmittance of the aluminum film of the fluorescent screen.In addition to light feedback, the signal-to-noise ratio is also the factor limiting the maximum gain of image intensifier. Because when the inherent gain of the microchannel plate is certain, increasing the operation voltage of the microchannel plate can not affect the resolution and equivalent background illumination of the image intensifier, but can reduce the signal-to-noise ratio of the image intensifier, which is the most important performance parameter of the image intensifier. Therefore, in order to further improve the maximum brightness, gain of the image intensifier, under the conditions of certain photocathode response, screen efficiency and screen voltage, it is necessary to further improve the inherent gain of the microchannel plate and reduce the transmittance of the aluminum film of the fluorescent screen.