PURPOSE: To absorb the light scattered by the defect near the rear surface of a transparent body during passing of the transparent body and prevent the defect from being detected near the rear surface by using an incident ray of the absorption wavelength region of the transparent body.
CONSTITUTION: When an incident ray is the ray of the absorption wavelength region of a transparent body 1 to be detected of defects, the incident ray is gradually absorbed and weakened by the body 1 while it is passing through the body 1. Hence, the ray projected to a defect 4 existing near (b) the rear surface of the body 1 is gradually absorbed and weakened by the body 1 while it is passing through the transparent body before it is projected, and further the scattered and reflected abnormal light by this defect 4 is also absorbed while it is passing through the transparent body; therefore at the time when the reflected abnormal light toward a detector 3 emerges from the body 1, it is substantially absorbed by the body 1 and is difficult to be detected by the detector 3.
TAKAMINE EIJI
TOMIOKA DAISAKU
