PURPOSE: To discriminate a defective through hole precisely by arranging prescribed shieldig masks on the image formation surfaces of the surface opening and back opening of an electric member on which the through hole is formed and detecting only defective leakage light from the inwall of the hole.
CONSTITUTION: Optical systems forming images are arranged on the surface opening 12c of a printed board 10 on which a through hole 12 is formed and the back opening 12a on the illumination side and the 1st and 2nd shielding masks 23, 24 having a prescribed shape are arranged on the image formation surfaces. Irradiated light 15 is multiply reflected in a gap between the shielding mask 14 and the printed plate 10 and separated into masked leakage light P121, defective leakage light P2 20 and substrate transmitted light P3. The substrate transmitted light P3 is interrupted by the 1st mask 23, the mask leakage light P1 is interrupted by the 2nd mask 24 and only the defective leakage light P220 reflected by a defective hole part is guided to a photodetector 19. Therefore, a fine defective hole can be also detected easily and precisely.
MITA KIKUO
KAKIGI GIICHI
