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.