a benchmark such as a default value or an actual value obtained when that particular pen was qualified when the optical line sensor detects a pen failure or pen deficiency the plotter corrects the problem fig7 shows in more detail the processing of the output signal from the sensor through op amp 180 resistors 182 183 filter circuits 184 185 and a d converter 186 in the flow chart of fig1 it is important to first check the media such as paper at the actual spot where the sample vectors are to be drawn to be sure there are not a ready previous plots or other non white interference the first percentage measurement at 200 is based on the percentage drop in reflected signal intensity from total white to absolute dark total light absorbance thus if five of the proposed six points each show a print contrast ratio of less that 8 the it is ok to proceed to the next stage of actually drawing the sample plot as shown at 202 in order to establish the pcr threshold various procedures can be used in the preferred form of the invention the average intensity of the pcrs for five points is computed and then so long as the pen plots checked during the pen verification procedure have pcrs of not less than 40 of such average then the plots actually tested are deemed satisfactory if a user is using only black pens and does not need high quality plotting it is possible to forego the actual pen qualification procedure and just accept that any pen having a print contrast ratio of more than the default of 8 on the scale of percentage signal drop form all white to total darkness will be deemed a u201c good u201d pen.