When the target is a grey body, especially when its temperature is nearly equal to that of circumferential bodies, the target's self-radiation and the circumferential radiation reflected by the target must be considered. Thus, the sensibility equations of various infrared systems have to be modified. According to these modified equations, we can explain some experimental phenomena that-can not be interpreted by the unmodified equations in which only self-radiation is taken into account.Here we discuss the thermal imaging systems in detail and the effects of following factors on them. These factors are: the temperature differential emissivity differential and reflectivity differential of adjacent detecting elements in the scene and the scanning efficiency scanning superposition ratio series or parallel arrangement of detector arrays additional field lens or immersion lens and so on. Then, we obtain the more complete sensibility equations and the formula of the noise equivalent temperature differential NETD for thermal imaging systems. The calculation methods of XT in NBTD are described and the numerical tables of XT are given.