Abstract: The absolute spatial distance of the widths of individual bars and spaces of a bar code are precisely measured by a bar code scanning apparatus having two photodetector circuit elements positioned a fixed and predetermined distance from each other for sensing light reflected from the bars and spaces of the bar code at a fixed differential time. The photodetectors are part of an integral unit and are parallel in independent circuits connected to a digital processing unit and storage memory for clocking the passage of the two photodetectors over the bars and spaces against an absolute time determined from a constant frequency pulse generator or digital clock. The precise measurement is compared with references and a readout is provided.

Abstract: Indicia having a requirement for a high degree of precision is printed onto a sheet and is scanned or read automatically shortly thereafter. Electronic control signals result from a comparison of the actually printed indicia with a pre-defined quality standard, and are used selectively to adjust command control signals to the printer to correct indicia printed thereafter. The leading edge of sheets to which the indicia is to be printed is precisely located by a light absorption detection or sensing mechanism which controls a sheet advance motor. The indicia printed, preferably bar code, is read by an optical bar code scanner traversing the printed code.