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 infra-red smoke screen ,the obscuring rate of infrared smoke screen was represented .It was pro-posed to determine the gray value of the background .The result of a certain jamming experi-ment 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 ,respective-ly ,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 .%目标与背景的辐射对比度是红外系统探测目标的主要依据，也是烟幕干扰重点降低的一个参量。结合辐射对比度的定义和红外定标原理，由目标和背景的视在辐射亮度引入其视在辐射对比度，并与目标和背景热像图的灰度值关联起来。利用烟幕遮蔽前后与目标和背景视在辐射对比度变化相对应的热像灰度对比度的变化，对烟幕遮蔽率进行表征。用提出的背景灰度方法对干扰实验结果进行了表征，计算得到3组试验的背景灰度值分别为91．9024、88．0020、33．2856，并获得了目标区域各像元遮蔽率的直观分布图，克服了以往以点代面的评价方法。
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