Thermal emission from Infrared Radiation (IR) instrument is an important background source

which will reduce Signal-to-Noise Ratio (SNR) of the system. It is necessary to estimate background radiation due to instrument's thermal emission in opt-mechanical design phase. An analytic model about thermal emission of reflect/refract combined optical system is established here

which is used to calculate background of space IR systems. Object signal is compared with background to show how background affects SNR. Finally

methods about removing background are discussed.