Details:
Summary:
Through the modern ages of printing, various optical instruments have been used to monitor the stability of print quality: densitometers transducers have employed a wide observation aperture, i.e. a rather large illumination spot, while the register measurement has been based on a small focus point giving a sharp detection of register control marks. Recently some novel methods have been developed at the Technical Research Centre of Finland which enable the use of a small illumination spot in an on-press densitometer without losing the noise suppression properties of a conventional light spot. In other words, the proper areal integration results from sequential reading of running sheets and the validity of on-press density readings is comparable with those obtained using established methods. On the other hand, the density sensors of new type can be used to measure also the print register errors. The analog signal of a densitometer channel is densely converted into digital form across normal register marks and data are stored in a static memory. The interesting edges of these printed marks can be accurately estimated using new computational algorithms. The relative positions of printed marks, i.e. the register error, can then be defined using the edge estimates. The combined optical transducer and estimation algorithms have been successfully tested in laboratory scale.