In view of the present disordered situation of using physics quantity and formula for infrared contrast calculation, the paper presents that the criterion of physics quantity used to evaluate the contrast should be infrared irradiance at the detector surface by analyzing the purpose and the actualized measures of infrared detection. When the object and background are in different positions, and both of them are far away from the detector, it is improper to calculate contrast by radiance or radiant emission. In order to make the contrast of infrared target and background represent effectively the infrared feature differences of the target-background, the infrared radiation relative contrast is defined by the ratio of the absolute value of the target and background difference in infrared irradiance at the detector surface to their mean value.