A lot of computational methods have been proposed to decide the detection range of the thermal infrared imager. Analysis on the detection range equation is given,with consideration of the temperature difference between the target and the background—ΔT0 and the noise equivalent temperature difference—NETD. Novel analysis on the attenuation effect of ΔT0 by atmospheric transmittance—τa is presented,after which a correction is given to the ΔT0-and-NETD based range equation. Study on the factors that affect the NETD parameters is also provided. Range equation of the thermal infrared imager for targets with multiple infrared radiation sources and different temperature is deduced in conclusion.