The operational effectiveness of optic － electronic weapon system changes significantly with the atmosphere /aerosol mode and observation geometry． Aiming at this problem， the effect of atmospheric transmission and observation geometry on contrast was analyzed and the apparent contrast transfer model was presented． Then，by combining with the TTP criteria， the effect model of observation geometry on target detection performance was established． Based on basic database using Modtran software computed offline，a fast calculation method of the performance model was proposed． Using the method， the target detection performance of imaging system，which changing with time location， time， observation geometry and distance， can be calculated fast． The effect of the atmosphere /aerosol model and observation geometry on apparent contrast，detection probability and range was computed and analyzed by using the fast calculation method． The results show that: the observation geometry effects on target detection perform significantly． The detection range when observing following sunlight is 1. 38 to 1. 7 times the detection distance when observing against sunlight． The significance of the effect of observation geometry on detection distance is different under different atmospheric /aerosol mode． Under marine aerosols mode， the effect is most significant． Under rural aerosol model， on the contrary， is much less． In this paper， an effect model of observation geometry on target detection performance and fast calculation method was put forward． This method provides theoretical and technical support for the evaluation of task planning and operational effectiveness of optic － electronic weapon system．
