影响超二代像增强器最高增益的因数分析

Analysis on Factors Affecting the Maximum Gain of Super Second Generation Image Intensifier

在阴极灵敏度、微通道板固有增益、屏效以及屏压一定的条件下,超二代像增强器的最高亮度增益由微通道板增益所决定。实验中发现微通道板增益存在一个最高值,当微通道板增益超过最高值时,像增强器会产生自激发光。像增强器产生自激发光时,图像的对比度、分辨力消失,图像亮度的增强作用失去意义。因此超二代像增强器的最高亮度增益受自激发光的影响。超二代像增强器产生自激发光的原因是像增强器内部微通道板电子倍增形成了正反馈。微通道板的最高增益与光电阴极灵敏度、荧光屏铝膜的透过率成反比,所以在阴极灵敏度一定的条件下,影响微通道板最高增益的因数是荧光屏铝膜的透过率。在微通道板固有增益一定的条件下,提高微通道板的板压不会影响像增强器的分辨力和等效背景照度,但会降低像增强器的信噪比,因此信噪比又是影响超二代像增强器最高亮度增益的一个因数。要进一步提高超二代像增强器的最高亮度增益,在阴极灵敏度、屏效以及屏压一定的条件下,需要提高微通道板的固有增益,同时减小荧光屏铝膜的透过率。

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 Na_(2)KSb(Cs)photocathode and Cs_(2)Te photocathode,since the response of Cs_(2)Te photocathode to fluorescent screen luminescence is much lower than that of Na_(2)KSb(Cs)photocathode to fluorescent screen luminescence,the image intensifier of Cs_(2)Te photocathode is more difficult to produce light feedback than that of Na_(2)KSb(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-tonoise 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-tonoise 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.