Correction techniques for a multilevel reflection scanner

This paper discusses the fundamental problems and design solutions for a multilevel reflection scanner. The scanner consists of a computer, a cathode ray tube light source, a system for imaging the light on the scanned document, two photomultiplier tubes, and a system for digitizing the PMT signals. One PMT measures cathode ray tube radiance while the other intercepts reflected light from the document. The computer controls the intensity focus and position of the spot and processes the digitized photomultiplier tube signals. The three fundamental problems associated with this type of scanner are tonal distortion due to the placement of the PMTs, the differing reflection properties of papers, and PMT drift. Procedures for reducing the effects of these problems, utilizing a computer correction table, are discussed. In addition, a detailed analysis is made of the most serious problem of tonal distortion due to the geometry of the system, and the results of this analysis are compared with experimental results. Finally, a noncomputer associated scanner is described.