MAORI : New Apparatus to Measure the Optical Regularity of Printing

Abstract

Modern printing presses are more and more controled by automatic means such as printed paper on line optical controls for inking. These automatic control means will ask for new techniques for measuring paper properties based on an increased use of optical devices. At present, irrespective of the printing process, the printing quality analysis is made by visual inspection and judgment. Therefore, substituting an optical measuring device to the visual judgment would allow better, quicker and less expensive interpretation of the results. In a study of uniformity analysis on newsprint, we found out that solidcolour analysis could be solved with a micro-photometer, the signal of which, and more especially its variations, is well correlated with visual judgment. Further study of the analytical method induced us to design a new instrument fitted with a He-Ne laser source, and giving a measure of local variations of the light back seattering coefficient of a solid colour. This instrument called MAORI is described. The attention is focussed on two interesting points :-the moving holder of the printed sample, the motion of which is such that each point describes a circle, the sample staying in its own plane and remains parallel to itself-the signal processing which gives a voltage out put proportionnal to the RMS value of the backscattering coefficient.Implementing this method in the case of rotogravure and in the case of offset printing is also described. The extremely good correlation which exists between the MAORI quality index and the visual judgment, in terms of classifying the samples is shown.Commercialisation of the equipment is foreseen.