The fundamental conception about visual range of low light level (LLL) imaging system is described. Based on the common apparent distance detecting equation, a more practicable and reliable detecting equation is established by using the factors, such as atmospheric transmissivity, spectral matching factor, etc. The relations of parameters, which have an influence on performances of system, are analyzed. The visual range of the night vision goggle under the specific circumstance is estimated which proves the revised apparent distance detecting equation is effective and the design of the system is feasible. Some approaches to improve the LLL imager′s performance are presented.