Mahovic, S. Agic, D, Gojo M.


Abstract :

"Computer to Plate" (CtP) technology has developed in the last ten years and it is today the subject of investigations of a series of manufacturers. On the graphic product market, there are considerable number of the producers of the output devices and the printing plates, but their products have not yet been standardized. Only a few of them are compatible with each other, and the two basic technologies have been defined: the thermal technology and the “visible laser” one. In this work, the most represented CtP technologies of the world known manufacturers have been investigated and compared with the conventional production process of the printing plates. The work defines the advantages and the disadvantages of the each observed technology. The analysis of the investigation results determines the differences in the print quality obtained by the usage of the observed technologies by measuring the relevant parameters for monitoring the reproduction.

Key words: conventional and CtP platemaking systems, offset printing, print quality.


1 Introduction


The incursion of the technical revolution into all the areas of the communication industry pushed the traditional reproduction processes into the history. The boundaries among the prepress departments, photographic laboratories, photographs processing and printing departments disappear gradually. The simultaneous development of the devices, computer systems and program applications influenced numerous novelties in the processes of the graphic industry. The tendency is to minimalize the devices and to eliminate the greatest possible number of intermediate processors as well as the human factor, an important problem in the need of the production standardization.

The conventional process“Computer to Film to Plate” (CtFtP) enables the production of the printing forms in a way which is mostly used today. From the computer, by means of the imagesetter the films of the primary colours have been obtained (CMYK). After the developing, the films are mounted and copied on the printing plates. After exposing and processing the printing plate, its surface gets characteristic physical-chemical properties. Some surface parts are of hydrophilic character and in the reproduction process they attract water. Some parts of the surface are of oleophillic character and they attract ink. By using the CtP system for the platemaking, the printing plates have been produced directly from the computer by means of the platesetter. The surface of the CtP printing plate has the same physical-chemical properties as the conventional printing plate. By eliminating the phase of the film production, the time necessary for the platemaking is decreased and the possibility of the error appearance (deformations) decreases, which appear during the conventional platemaking process (eg. in size and shape of the printing elements on the printing plates) (Agic et al., 1999). Because of that it is supposed that the usage of the different production processes of the printing plate will cause the appearance of the different deformations of the screen elements on the reproductions in spite of the same conditions of their realization (in regard to the printing substrate, ink, damping fluid and printing process). For investigation of such possible deformations the original has been reproduced with the different platemaking systems and has been printed in offset. The aim is to define the boundaries of the possibilities of the optimal reproduction in regard to the observed systems (the conventional and the CtP systems) in the same conditions of realization.


2 Background


The foreseen investigations are based upon the fact that the conventional platemaking process prevails today in graphic industry. Independent from the choice and from the integrated platemaking system (CtFtP or CtP), the aim of each reproduction process is the same – obtaining the reproduction true to original. Because the different platemaking processes can influence the variety of the printing elements on the printing plates (Mahović et al., 2003), the purpose of this investigation is the observing of these eventual differences and the defining of



their influence on the quality of the realized reproductions.

The original with all the necessary measuring fields and the motives with the characteristic dominating tones for visual evaluation of the prints were reproduced. The original was reproduced in two different ways: by using the conventional platemaking process and the by the application of three different CtP digital platemaking processes. The possible changes in dimensions and plates of the printing elements on the printing plates, from the place defined on the original, and the appearance of the obtained deformations on the realized reproductions were assumed during the production of the printing plates by different processes. In the conventional system, the printing plates with the diazo copying layers (on diagrams with the marks “1”) were used. From the CtP systems, the CtP device with the internal drum, violet laser and the silver halogenide copying layer (the mark “2”), the CtP device with the internal drum, IR laser and thermal printing plates (mark “3”) and the CtP device with the external drum, IR laser and thermal printing plates (mark “4”) were used. The spectrophotometric measurements were done on the samples. The obtained spectral data have been transferred into LAB values and the lightness, hue and saturation of the observed prints have been defined (Wiszecky, Stiles 1982).


3 Results


Because the colours differ in two directions (lightness and saturation) the parallel monitoring of both parameters is most often performed to get the more precise investigation results (Hunt, 1991). The measurements were performed on the full tone patches of the primary colours (cyan, magenta, yellow) and on the full tone patches of the secondary colours (red, green blue) of the subtractive synthesis. From the measuring results of lightness, it is visible that it decreases on each observed sample in regard to the original (Fig. 1). Only on the prints with CtP system (3) there is the considerable increase. The measuring results of colour saturation point at the increase of value in all the systems (except cyan and yellow) (Fig. 2). Because of more precise colour description the hue angle has been defined as the attribute for defining the difference between the two colours. By the determination of the hue angle on the samples, the decrease on all the colours is visible (Fig. 3). Only in cyan the hue is uniform for all the


observed systems; magenta is evidently decreased and there is the increase of the hue angle in blue.



Fig. 1. Measuring results of lightness on samples.



Fig. 2. Measuring results of saturation on samples.



Fig. 3. Measuring results of hue angle on samples.


4 Discussion


The quality of reproduction can be influenced by a series of parameters such as the system of input units, printing process, conditions of reproduction, kinds inks and damping fluids, printing substrate, separation parameters and similar. The mentioned parameters in this work were strictly defined and it was believed that the conditions for reproduction were standardized for all the observed systems. The variations were


done in the intermediate process i.e. in the process of platemaking. Because of that the investigated parameters could point at the quality of the print reproduction in regard to the application of a particular system for platemaking.

Lightness, saturation and hue are some of the relevant characteristics for monitoring the reproduction quality. It is visible from the observed diagrams that defined changes appear on all prints in the sense of decreasing the measured values. Such investigation results have shown to be identical in presenting gamut in CIE LAB colour system (Fig.4). The values a* and b*, determined by measuring the prints, are evidently smaller than the values on the original, for the observed reproduction systems. In this way the range of colour reproduction in all the reproduction systems is decreased, most on the prints realized in CTP system (3).



Fig. 4. Colour gamut in CIEL*a*b*diagram.


5 Conclusion


The integration of the new digital technologies demands the settling in and adaptation of the whole reproduction process. The conventional reproduction system has been used for years and the need for the production adaptation has developed to the level which enables the obtaining of high quality prints. The application of the CtP processes in platemaking accelerates the reproduction process, but the adaptation of the reproduction process has not been completely solved. This can be concluded from the measuring results where the lightness, saturation and hue differ in all the observed systems in regard to the original but the values of the difference of the conventional print are the smallest ones. In visual observation of the


reproductions with the characteristic prevailing tones, the results were uniform. The conventional system for platemaking has optimally reproduced the motives with the prevailing light and dark tones, shadows and details. In CtP systems, the results of the visual evaluation were not uniform. CtP system marked by (3) has been stressed, where the light tones and the details on prints were lost, as well as the shades on the dark motives. Because the digital platemaking process gives more regular form of the printing elements (Mahovic et al., 2003) simpler standardization of the production in the digital surroundings is evident. For reproduction standardization the application of the colour management is suggested, which will ensure the optimal reproduction in regard to the used input and output reproduction devices and in regard to the used materials in the production.


6 References


1 Agic, D.; Mandic, L. & Gojo, M. (1999). Influence of Some Parameters on the Change of Printing Elements Dimension, Proceedings of 10th International Daaam Symposium, Katalinic, B., pp. 5-6, ISBN 3-901509-10-0, Vienna, October 1999, DAAAM International Vienna, Austria

2 Mahovic S.; Agic D. & Gojo M. (2003). Mechanical and Optical Differences in Long Run Printing in Conventional and CtP Offset Systems, Proceedings of IARIGAI, Lovrecek, M. (Ed.), pp. 213-221, ISBN 953-96276-8-0, Dubrovnik-Cavtat, September 2003, Acta Graphica Publishers, Croatia

3 Wyszecki G., Stiles W.S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae-2nd Edition, John Wiley & Sons, ISBN 0-471-02106-7, Canada

4   Hunt R. W. G. (1991). Measuring Colour, Ellis Horwood Limited, ISBN 0-13-567686-X, England