APPARATUS FOR SCANNING COLORED REGISTRATION MARKS

An apparatus for scanning colored registration marks printed on a moving web by a plurality of printing cylinders of a printing press is provided with a photoemitter for generating a beam of light directed at the moving web, a detector for detecting the amount of light reflected from the moving web and generating a reflectance signal having a magnitude relating to the amount of light, an A/D converter for periodically sampling the reflectance signal to generate a number of scan values, a memory for storing the scan values, and a baseline generator for determining a baseline level of the reflectance signal. The baseline generator may include means for assigning each of the scan values to one of a plurality of predetermined ranges of values based on the magnitudes of the scan values, means for determining a baseline range from the plurality of the predetermined ranges based on the range having the largest number of the scan values, and means for determining the baseline level based upon the baseline range. The predetermined ranges may be overlapping ranges.