PRINTING APPARATUS
This invention relates to a printer for printing an image on an image receiving material provided on a backing material, said backing material having regularly spaced markings provided on the back thereof, said printer comprising means for detecting said markings, means for determining at least one of a spacing between two markings and a width of a marking, comparing the determined marking width and/or spacing with a respective reference value and for causing printing to be stopped if at least one of the determined spacing and/or width differs from the respective reference value by more than a predetermined amount.