The illumination methods related closely to detect random defects on strongly reflective and complex surfaces with CCD detection are discussed. A concept of defect expressivity is developed and the dependency of image quality on luminance is illustrated in the condition of scattered light. Based on the comparisons between parallel light and uniform scattered light, it is concludes that uniform scattered light is much better than parallel light in reducing the mirror reflection and good at revealing the various surface defects. The design principles under those particular conditions are presented.