The problem about target discretion is very important in many space applications. It was obtained that the relation between the natural radiance of targets and the targetbackground contrast at 0.4～14 μm. And detailed research about the effects of the radiance and the reflection of the target was also carried out. As the result, the contrast is increasing with the radiance and the reflection increase. And the spectral radiance and the spectral reflectivity affect, the spectral distribution of the contrast