为了精确测量三代微光像增强器的信噪比，采用光子计数法研制了瞬态类针孔弱光照度计来实现微光像增强器阴极面上Ф0.2 mm类针孔成像光斑的1.08×10-4 lx照度测量。首先, 分析了色温为2 856 K的光源形成的类针孔弱光光斑的光子分布概率及光子数涨落偏差等光子统计特性，计算了平均光子通量。然后, 针对类针孔弱光光斑的光子通量特点及成像特性,研究了光子计数法测量弱光照度原理。最后,介绍了瞬态类针孔弱光照度计的组成及测量方法。对三代微光像增强器的信噪比进行了测量实验，结果显示：三代微光像增强器信噪比测量装置的照度测量偏差为0.4%，像增强器信噪比测量不确定度为5.0%，表明基于光子计数法研制的瞬态类针孔弱光照度计有效提高了类针孔光斑极弱照度测量的准确度。

To improve the measuring accuracy of Signal to Noise Ratio(SNR) of the third-generation low level image intensifiers, a transient low light level illuminometer was designed based on photon counting principles to measure the illuminance value of 1.08×10-4 lx from a circular area with diameter of 0.2 mm for the quasi-point imaging source on a photocathode. Firstly, the photon distributing probability and fluctuated deviation were analyzed for the quasi-point light source at a color temperature of 2 856 K, and the average photon flux was calculated. Then, the low light level illuminance measuring principle by the photon counting method was studied according to the photon flux and imaging characteristics. Finally, the composition of the transient quasi-point low light level illuminometer was introduced, and the SNR of a third-generation low light level image intensifier was measured. Experimental results indicate that the illuminance deviation and uncertainty of the SNR are 0.4% and 5.0%,respectively.It concludes that the proposed low light level illuminometer based on photon counting can effectively improve the accuracy of illuminance measurement for quasi-point low level light sources.