Abstract: A method of monitoring and controlling the quality of pen markings on a plotting medium by qualifying each pen based on optically sensing across a sample line drawn on an actual medium. During each subsequent plotting task of a particular pen which has been qualified, an actual line plot is optically sensed across a selected point to make a comparison with the sample line. If the actual line plot is unsatisfactory, the deficient pen is replaced and the plot is restarted from the beginning, or retraced from the last good verification, or is stopped to allow the user to select an appropriate corrective procedure.

Abstract: An optical sensor for monitoring plotter pen performance by sensing the quality of lines drawn on a medium. An LED emitting green light beam is angularly directed toward an underlying line so as to reflect into an optical sensor which measures the print contrast ratio of a point on the line. Circuit means amplifies and filters the signal generated by the optical sensor.