Basing on the space distribution characteristics of the long term light image in the turbulent atmosphere the background signal of a CCD imaging system can be determined accurately, and then we can measure the image size accuratly with the CCD imaging system. The maximal radius and space filling ratio related to engineering applications are introduced for the description of the deformation characteristics of the light image degraded in the turbulent atmosphere by analyzing both simulation data and experimental data. Finally we mentioned the phenomenon of the enhancement of light intensity in certain regions in the simulated images and its potential importance.