Abstract: The contrast of targets and backgrounds is a key feature detected by IR imager and a physical index reduced focally by smoke jamming. Combining the theory of radiance calibration with the radiation contrast, the apparent radiant brightness of the target and background was used to express the apparent radiation contrast, and as well connected with the gray values of the scene on IR images. Based on the change of contrast before and after screening with infrared smoke screen, the obscuring rate of infrared smoke screen was represented. It was proposed to determine the gray value of the background. The result of a certain jamming experiment was described with screening rate given by this method. Caculation results show that the gray values of background of three experiments are 91.902 4, 88.002 0, 33.285 6, respectively,and the visual distribution diagram of screening rate in the target region is given, which is superior to the traditional evaluation methods, and has a higher value in assessing the ability of infrared smoke.