Automatic Threshold Design for a Bound Document Scanner.
Abstract
Research was carried out on an electro-optical bound document scanner using a charge-coupled device (CCD) as a sensing element. The goal was to develop a means whereby the voltage threshold level of the video analog-to-digital converter could be set automatically to provide optimum hard-copy output over a range of lighting conditions and document background colors and qualities. To determine an acceptable component of the analog video signal as a thresholding reference, an extensive study of the signal behavior was conducted over a variety of conditions. An Automatic Threshold Control (ATC) was designed that exploited the modulation transfer function of the CCD's analog signal. A CALIBRATION PATTERN is superimposed at the left-hand margin of the page being scanned. This pattern contains various discrete spatial frequencies. The threshold voltage is varied automatically until the number of black/white (zero/one) transitions is maximized for the CALIBRATION PATTERN. The threshold voltage producing this maximum number of transitions is equivalent to the threshold required to produce optimum resolution in the scanner hard-copy output. This threshold value is then locked in for the duration of the page being scanned.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 1982
- Accession Number
- ADA125316
Entities
People
- Bill James Stanton Jr
Organizations
- Air Force Institute of Technology