This invention relates generally to optical scanners, and has particular reference to a novel two channel optical scanning device for reading bar codes of high data density.
Existing hand held scanning devices read the varying amounts of light reflected from the various different elements of a bar code with a photodetector which coacts with an optical system that provides the required resolution to detect the smallest element of the bar code. The information that is encoded in the succession of bar code elements appears at the photodetector output as an electric signal of changing amplitude and variable width as the scanner is moved across the bar code.
The width of a light or dark code element is expressed in the time period between the mean value of the detector output change and the mean value of the following output change. Since the slope and amplitude of the detector output change are functions of the scanner characteristics and bar code print contrast, it is impossible to determine ahead of the following output change the mean value of said change. It is therefore virtually impossible to produce an electrical square wave signal that accurately expresses the width of a given bar code element with previously developed hand held scanners in a real time process.
Various electronic circuits such as slope detectors and controlled threshold comparators have been developed to process the signal of prior art scanners reducing the errors caused by scanner and code characteristics. However, with the exception of some relatively complex and expensive systems not operating in a real time mode, the prior art hand held scanning devices are generally not suitable for high data density bar code reading to the best of the applicant's knowledge.