超二代与三代像增强器性能的比较研究

超二代和三代像增强器是两种不同技术的像增强器，其在光电阴极、减反膜、离子阻挡膜以及阴极电压方面存在区别。在极限分辨力方面，尽管三代像增强器GaAs光电阴极的电子初速小、出射角分布较窄以及阴极电压较高，但目前两种像增强器的极限分辨力均相同，三代像增强器GaAs光电阴极的优势在现有极限分辨力水平下并未得到发挥。在信噪比方面，尽管GaAs光电阴极具有更高的阴极灵敏度，但因为较高的阴极电压以及离子阻挡膜透过率的影响，使得两种像增强器的信噪比基本相同，三代像增强器GaAs光电阴极高灵敏度的优势也未得到发挥。在增益方面，尽管三代像增强器具有更高的阴极灵敏度以及较高的阴极电压，但超二代像增强器通过提高微通道板的工作电压来弥补阴极灵敏度以及阴极电压的不足，因此在现有像增强器增益的条件下，两种像增强器的增益完全相同。在等效背景照度方面，由于三代像增强器GaAs光电阴极的灵敏度更高，因此在相同光电阴极暗电流的条件下，三代像增强器可以获得更低的等效背景照度，所以三代像增强器较超二代像增强器具有更高的初始对比度。在光晕方面，由于三代像增强器光电阴极的灵敏度较高，同时具有离子阻挡膜，因此理论上讲，三代像增强器较超二代像增强器具有更高的光晕亮度，但实际的情况是两种像增强器的光晕亮度基本相同。在杂光方面，GaAs光电阴极具有减反膜，因此杂光较超二代像增强器低，所以三代像增强器的成像更清晰，层次感更好。在带外光谱响应方面，由于超二代像增强器Na₂KSb（Cs）光电阴极的带外光谱响应高于三代像增强器，因此在近红外波段进行辅助照明时，超二代像增强器较三代像增强器成像性能更好。在低照度分辨力方面，具有相同性能参数的超二代和三代像增强器具有相同的低照度分辨力。需要注意的是，这是在标准A光源测试条件下所得出的结论。当实际的环境发射光谱分布与标准A光源发射光谱分布不相同时，两种像增强器的低照度分辨力将会不同。

Performance Comparison Between Super Second Generation and Third Generation Image Intensifiers

Super-second-generation and third-generation image intensifiers are two types of image intensifiers that use different technologies. Super-second-generation image intensifiers employ a Na₂KSb(Cs) photocathode, whereas third-generation image intensifiers employ a GaAs photocathode. Third-generation image intensifiers employ higher cathode voltages than those employed by super-second-generation image intensifiers. In addition, third-generation image intensifiers employ an antireflection coating between the glass input window and GaAs photocathode; however, this is not employed in super second-generation image intensifiers. Furthermore, third-generation image intensifiers employ ion barriers on their MCP(microchannel plate), whereas super-second-generation image intensifiers do not. In terms of limiting resolution, despite the small initial electron velocity, narrow exit angle distribution, and high cathode voltage of the third-generation image intensifiers, the limiting resolutions of the two types of image intensifiers are the same; the advantages of the GaAs photocathode of the third-generation image intensifiers have not been introduced under the existing limiting resolution level. In terms of signal-to-noise ratio, the GaAs photocathode has a higher cathode sensitivity, normally more than twice that of the super-second-generation image intensifier. Thus, theoretically, the third-generation image intensifiers have signal-to-noise ratios that are 1.4 times those of the super-second-generation image intensifiers. However, the two types of image intensifiers are basically the same owing to the influence of higher cathode voltage and ion barrier transmittance and the advantage of not introducing the high sensitivity of the GaAs photocathode of the third-generation image intensifiers. In terms of gain, although the third-generation image intensifiers have higher cathode sensitivity and cathode voltage, the super-second-generation image intensifiers compensate for the shortcomings of cathode sensitivity and cathode voltage by increasing the working voltage of the microchannel plate. Therefore, in terms of the existing image intensifier gain, the gains of the two types of image intensifiers are identical. In terms of equivalent background illumination(EBI), owing to the higher sensitivity of the GaAs photocathode, the third-generation image intensifiers can obtain lower equivalent background illumination under the same photocathode dark current. Therefore, the third-generation image intensifiers have higher initial contrast than that of the super-second-generation image intensifiers. The higher the initial contrast of the input image, the higher the contrast of the output image. In terms of halo, because the photocathode of the third-generation image intensifiers has high sensitivity and an ion barrier film, theoretically, the third-generation image intensifiers have higher halo brightness than that of the super-second-generation image intensifiers. However, in actual situation, the halo brightness levels of the two types of image intensifiers are basically the same. In terms of stray light, the GaAs photocathode has an antireflection coating; thus, the stray light is lower than that of the super-second-generation image intensifier, so the imaging of the third-generation image intensifier is clearer and the sense of gradation is better. In terms of spectral response beyond the long-wavelength threshold, because the spectral responses beyond the long-wavelength threshold of the super-second-generation image intensifiers are higher than those of the third-generation image intensifiers, the super-second-generation image intensifiers have better imaging performance than that of the third-generation image intensifier under supplementary illumination using the near-infrared waveband. For example, without the presence of any light, the super-second-generation image intensifiers can obtain better images at a supplementary illumination of 980 nm wavelength, whereas the third-generation image intensifiers cannot. In terms of the resolution of low illumination, the super-second- and third-generation image intensifiers with similar performance parameters have the same low luminance resolution. It should be noted that this conclusion was obtained under the test conditions of a standard A light source. When the actual environmental emission spectrum distribution is different from that of a standard illuminant A, the low illumination resolutions of the two types of image intensifiers are different. Photocathode sensitivity is a parameter of the photocathode and not of the image intensifier. Thus, the performances of the two types of image intensifiers cannot be compared in terms of photocathode sensitivity. The difference between the super-second and third-generations cannot be understood using the meaning of "generation; " their differences do not lie in the meaning of "generation."