Method and apparatus for sensing and maintaining color registration

Abstract

A method and apparatus for automatically sensing and maintaining registration of each of the color cylinders utilized in a multi-color printing press is disclosed. Predetermined register marks are printed in the non-image areas on the sheet to be printed. Typically, in a four-color printing process, the black image is used as a reference, and registration of the other three image colors is achieved relative to the black image. Accordingly, a register mark for each process color is printed in the same location as a corresponding black reference register mark. The reference register marks are configured such that the average reflectance of the printed overlay will change as the registration of the color image changes with respect to the black image. In general, the configuration of the register marks for the black and color images differs. The reflectance of each printed overlay is measured on the printing press, while the sheet is moving. One type of sensor that can be used for this measurement is an on-press densitometer, or more generally, a reflectometer. If desired, the reflectance information obtained thereby can be converted to a registration error signal and used to achieve automatic, closed-loop control of the position of each of the color cylinders.