In this paper, we measure the difference of radiance between the surface and the standard sheet at the same real temperature. The study points out that with accurate infrared thermal imaging technology, we can obtain radiation temperature data on the surface and standard sheet. By calibrating the real temperature and ambient temperature of the measured object, we can calculate the difference between the surface emission rate of the measured object and the emission rate of the standard plate. When calculating the measurement error of emissivity difference, it is found that the selection of infrared thermal imager with certain accuracy, especially when measuring the surface of objects with high temperature and low radiation rate. This finding provides an important reference basis for practical engineering applications. In the practical engineering application, considering the particularity of the measured object and its storage environment, it is particularly important to select the appropriate parameter range, which will directly affect the accuracy of the radiability difference measurement. For large and complex object surfaces, this study also presents an innovative solution: to realize the accurate measurement of the full surface radiance difference through the image synthesis method of multiple measurements. The proposal of this method not only expands the application range of infrared thermal imager, but also provides a strong technical support for the research in related fields.