Automatic gravure print feature determination at production speeds

This paper describes the development of a non-contact system for measuring the colour of printed material at web speeds, in addition to gravure-printed dot feature recognition. The system proposed uses a non-contact spectrophotometer based on a holographic grating, in conjunction with a conventional monochrome area scan camera, from which colour spectral data are extracted, while a xenon flash is used to illuminate colour samples. Dot features are captured by a magnification lens, in conjunction with a progressive scan camera. Software and hardware details of the system are given, together with the underlying mathematics for colour space conversion and measurement. Conversion equations from X, Y, Z chromaticity coordinates to the RGB system are presented, and also equations to convert from the L*a*b* colour space to X, Y, Z chromaticity coordinates. Experimental results are presented whereby the non-contact spectral system is shown to perform to a colour tolerance exceeding that of conventional colour video systems, and where performance of the dot feature system is comparable with traditional static examination methods using a microscope.