As to visualize the leaking gas cloud which is not visible to the naked eyes, three categories of techniques have emerged,
Backscatter Absorption Gas Imaging, Passive Thermal Imaging, and Imaging Spectrometer. Among these systems,
Signal to Noise Ratio (SNR) is generally used to deduce gas leakage detection limit and leads to several performance
evaluation parameters, such as Noise-Equivalent Spectral Radiance and Noise-Equivalent Concentration-Path Length.
However, in most cases, measuring the SNR accurately is not accessible and usually needs auxiliary instruments.
Therefore, we focus on researching a gas leakage detection model according to the general parameter of a thermal imager,
Noise Equivalent Temperature Difference (NETD). Firstly, the Gas Equivalent Blackbody Temperature Difference
(GEBTD) is obtained by calculating the attenuated radiation of the On-plume path and that of the Off-plume path
respectively. A simplified form of GEBTD was derived by our previous paper, assuming that the work range was short
and the affection of atmospheric transmission was omitted. But in this paper, more factors are considered to establish a
more realistic and accurate detectivity model. The radiation of the gas cloud and the attenuation of the atmosphere are
taken into account as well as the size of the leakage spot which inevitably affects the concentration path length. Secondly,
the NETD and the GEBTD are compared to determine the detection capability. At last, an experiment is designed to
verify the accuracy and reliability of this model on the basis of the gas cloud concentration cone distribution model.