| 摘要 |
PURPOSE:To eliminate disturbance due to external light by arranging a light source and a photodetecting element opposite to each other across a passage for sheets and providing such a transparent plate that the angle of incidence of reflected light of external light from a sheet is larger than a Brewster angle on the front surface of the photodetecting element. CONSTITUTION:The light source 1 is arranged at a lower side across the passage P for sheets and the photodetecting element 2 is arranged at an upper side; and the front glass plate 3 of the photodetecting element is so arranged that its reverse surface faces the inner part of a device A and slants to a sheet surface. The external light is reflected by the sheet to travel toward the photodetecting element, but the angle of incidence of the external light is more than the Brewster angle, so almost none of the external light is incident on the photodetecting element 2. Therefore, the light from the light source 1 is incident on the photodetecting element 2 in the absence of the sheet, but when there is the sheet, neither the light from the light source 1 nor the external light is incident on the photodetecting element 2 and whether the sheet is present or not is detected.
|
