Patent US 6,433,353

A beam of light from a light source is irradiated toward a surface of a transparent plate at an angle of incidence ranging from 86 to 89 degrees or at an angle of incidence ranging from 60 to 89 degrees after being polarized as a P-polarized light beam or S-polarized light beam by a polarizing element disposed between the light source and the transparent plate. This enables a reflected image from a front surface of the transparent plate to be projected on a screen without being influenced by reflection from a rear surface of the transparent plate. By visually inspecting the reflected image on the screen, or by picking up the reflected image by a camera and visually inspecting a picture on a monitor display, or by picking up the reflected image by a camera to obtain density signals representative of the reflected image and calculating the irregularities of the surface of the transparent plate on the basis of the density signals by an image processor.