Switching of Threshold Value for Document Scan
Original Publication Date: 1984-Sep-01
Included in the Prior Art Database: 2005-Feb-05
Preceding document scan, a CCD (charge-coupled device) sensor unit views a white-colored reference zone. A control unit responds to the output signal of the CCD sensor unit for generating a set of threshold values, one of which is selectively fetched and supplied to a comparator which compares the threshold value and the output signal of the CCD sensor unit for successive pel (picture element) positions on the document. In Fig. 1, the white-colored reference zone 1 is provided on table 2 on which the document 3 is placed. The optical system, including light source 4, lens 5 and the CCD sensor unit 6, and the table are moved relative to each other to sequentially scan the white-colored reference zone 1 and the document 3. Initially, the white-colored reference zone 1 is scanned. Referring to Fig.