Abstract: In view of the CCD vertical target application in outdoor natural environment and with active light source, a method to evaluate the validity of light source based on background image analysis was presented. On the condition same with pratical measurement, certain line background images without object were captured to determine the gray scale range f and gray mean value fmean of each pixel in these line background images. To each pixel, if the value of fmean was always bigger than two times of f, the object could be recognized after image analysis. The effectiveness and reliability of the criterion were all demonstrated by CCD vertical target application experiments in laboratory with active light and outdoors with natural light.