Abstract: The ellipsometer can measure the film thickness in a period and its refractive index. The article investigates that the farthest fathomable periodicities and the farthest film real thickness are existent when the variable incident angles are used to confirm the film real thickness. The graphics mode is proposed to describe the connection between the periodicities, the period, the film thickness in a period and the incident angle.

Key words: ellipsometer, film real thickness, farthest fathomable periodicity, graphics mode