Print MonitorThis invention relates to the monitoring of printed matter in a printing press.
In accordance with this invention, there is provided a method of monitoring printed matter in a printing press, in which the press prints a pattern of dots on successive copies and a video camera forms an enlarged video image of the successive dot patterns and writes the successive images into a video frame store, and an analyser is used to analyse the successive video images.
Also in accordance with this invention, there is provided a print monitoring apparatus, comprising a video camera for directing at a pattern of dots on successive printed copies to form an enlarged video image of the successive dot patterns, a frame store into which the successive images are written, and an analyser for analysing the successive video images.
The analyser can be arranged to measure or determine various parameters of the dot pattern, for example one or more of the following: dot gain, spread, shape, skip, slur, ink density, and registration of the component colours. These measurements can be interpreted by the analyser to control or make adjustments to the printing press, in a closed-loop control system.
Also, the analyser can compare each newly-read dot pattern with a stored dot pattern from a previous reading, to determine any significant changes with time, which would indicate that the printing plate is suffering wear.
The pattern of dots is preferably very small, e.g.
occupying an area of 1 square millimetre, and is formed on the printing plate during pre-print preparation.
An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawing, the single figure of which is a schematic diagram of a print monitoring system in accordance with the invention.
Referring to the drawing, there is shown a print monitoring system used with a moving web W in a printing press.
The web W carries successive printed copies, each with a small pattern of dots (e.g. along an edge of each copy). the system comprises a strobe flash tube 1 to give an instantaneous illumination of the web at timed intervals synchronised with the movement of the web. The system further comprises a video camera 2 with microscope directed at the illuminated area of the web, and a control unit 4 controls the arrangement such that the camera 2 is focused on the pattern of dots carried by each copy, and an enlarged image of the dot pattern is formed on a video monitor 3 and captured in a video frame store 5.
A microprocessor-based analyser 6 is arranged to analyse the image stored in the frame store 5, to measure or determine various parameters of the dot pattern, e.g. dot gain, spread, shape, skip, slur, ink density, and registration.
These measurements are interpreted by the analyser 6 to control the printing press, over control lines 7. Also the analyser 6 compares each newly-read dot pattern with a stored dot pattern from a previous reading, to determine any significant changes with time, which would indicate that the printing plate is suffering wear.
It will be appreciated that the monitoring system which has been described provides considerable benefit to the operator of the printing press. The system can automatically adjust various controls of the press, and the printer will also know when the printing plate is becoming worn.