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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Printing processes > Colour reproduction and new printing processes           Update: March 20th 2007
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Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
(November 2006)
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"

IV - Printing processes

IV - 3 - Colour reproduction and new printing processes

Lionel Chagas, Anne Blayo, Robert Catusse, Denis Curtil, Jean-Luc Tourron

The objectives of colour management are the preservation and the most accurate reproduction possible of colour information throughout the processing chain, which uses various types of equipment. It is therefore necessary to determine how colours are modified at each stage of the graphic chain in order to compensate for possible distortions. Each peripheral - digital scanner, screen, printers, proofing systems etc – must be characterised by the determination of the 3D space of the reproducible colours (“colour gamut”). These characterisations make it possible to establish the correlations between an "independent" colour space (defined by CIE), XYZ or L*a*b* scale and the peripherals' colour space (RVBscan for a scanner, RVBscreen for a screen, CMJNoffset for an offset press etc.). As a result, it is possible to achieve a colorimetric consistency between a colour ink jet print and the same print obtained in offset on another paper, for instance.

Colorimetric space
Figure 1 - Colorimetric space

All the factors which can influence the colorimetric appearance are considered in this study, notably:

For example, the characterisation of ink/substrate interactions of ink jet for the reproduction of works of art has been studied. Another very acute problem is the proofing of colours (to be reproduced in offset, gravure, flexography, or screen printing) by means of digital printing processes such as jet ink or electrophotography on printing media which can be very varied.

These themes require knowledge of different disciplines such as the properties of the materials used, their physical-chemical interactions, but also the algorithm to calculate the printing profiles as well as the study of colorimetric variations.

A real-time device measuring the colorimetric characteristics of printed matter at the end of the production chain can help the printer reduce costs and control production quality.

The originality of our approach is to use an intelligent camera having video communication and software allowing it to operate completely independently. This camera provides the press’s central unit with the information to modify its settings.

Logical inputs and outputs allow the camera to synchronise the taking of images with the rotation of the cylinder and flash release. An integrated arithmetic processor allows it to process the digital images stored in EPROM memory. This camera also has a VGA output which displays a moving image of the printed copy.

A single CCD sensor (8 bits) is used. RGB data is digitised and stored in mosaic format (Bayer filter). We have an access to this mosaic and not to the three reconstructed RGB signals. Demosaicing (stage of reconstruction of missing RGB information) and its influence on the digitisation of rastered images have been studied. A compromise has been found concerning the magnification chosen so that the printed halftone dot can be dealt with.

The influence of the RGB discretisation step on the measurement of colour has been highlighted and has shown the limits of data capture using only 8 bits per channel. The translation of colours in the CMYK (CMYN) colour space (conversions between colour spaces : RGB – acquisition/L*a*b* - (calculscomputation) -/CMYK – printing -) allows for example, optimisation of press adjustments, for both start up and production monitoring. From the perspective of use in real-time, it is necessary to adapt the colorimetric space change equations to simplify as much as possible the calculations : a conversion matrix (LUT) has been established. This work has been the subject of a doctorate under a CIFRE contact, with a small company specialised in the development of printing press peripherals. It shows the validity of such an approach and opens the perspective of an industrial development.

Our team is still working on these subjects.

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