| 摘要 |
<p>A method of and an apparatus for detecting a medium includes means or steps for providing a light beam and means or steps for detecting the light after it is reflected from the medium. The light providing means is positioned to direct a light beam onto a substantially planar surface at a predetermined angle of incidence. The presence of a sheet medium that may be laid on the planar surface is detected by detecting a portion of the light reflected by the sheet medium. Preferably, the detecting means is positioned so that it senses a portion of light scattered or diffused by the paper but is positioned outside of the path of the reflected light from the planar surface so that it does not detect the reflected light. In this manner, the apparatus is rendered to be capable of detecting the presence of sheet medium that diffuses the light but avoid detecting the reflected light from a planar surface. The ability to discriminate is enhanced by pulsing the light at a given repetition rate and by providing an amplifying arrangement in the photodetecting means that is tuned to the repetitive rate of the pulsed light. The discrimination capability is further enhanced by choosing a light beam that has an energy spectrum which is substantially transparent to the planar surface, i.e., is substantially absorbed by the planar surface but which is non-transparent to the sheet medium; i.e., is bounced back and diffused substantially by the sheet medium.</p> |
