Patent ID: 7978381

Claim:
An automatic document feeder for feeding a document, said automatic document feeder comprising: an input tray for placing said document thereon; a transfer path severing as a passageway of said document; a roller assembly for transporting said document through said transfer path; an ejecting tray for supporting said document ejected from said transfer path; a light emitter for successively emitting multiple light beams of different intensities to said document; a light receiver electrically connected to said light emitter for receiving said multiple light beams penetrating through said document, and outputting corresponding voltage signals, wherein said voltage signals have a first voltage level and a second voltage level; and a discriminating unit electrically connected to said light receiver for discriminating document types, said discriminating unit having been previously stored therein a look-up table concerning the relationships between a high voltage reference value, a low voltage reference value and said document types, wherein said document is discriminated as a first document type by said discriminating unit if said first voltage level is greater than said high voltage reference value and said second voltage level is smaller than said low voltage reference value, said document is discriminated as a second document type by said discriminating unit if said first voltage level and said second voltage level are both greater than said high voltage reference value, and said document is discriminated as a third document type by said discriminating unit if said first voltage level and said second voltage level are both smaller than said low voltage reference value.