In the past few years, inkjet printing has evolved into a successful competitor with electrophotographic methods. This success is due, among other things, to the fact that the pr int ing proces s i s comparatively simple and has high potential for development both economically and ecologically. Inkjet printing, especially in the production of coloured prints, has made inroads into high-grade applications that up to now have been reserved for laser printing.
Within the scope of research project a test procedure was developed for the accelerated light-induced ageing of inkjet prints based on studies conducted on the ageing behaviour of coloured inkjet prints under realistic environmental conditions. The first part of this article is devoted to discussing ageing phenomena under realistic environmental conditions which laid the foundations on which the test procedure was subsequently developed.
Ageing under realistic environmental conditions
The studies were based on prints made using four representative inkjet printing systems designed for home, office and archive use. Comparisons were based on two bubble inkjet printers (printer A with an aqueous inkjet ink; printer B with an aqueous alternative inkjet ink), a piezoelectric printer with solvent wax ink (printer C) as well as a piezoelectric printer with an aqueous inkjet ink (printer E). The prints made by a colour laser printer (printer D) served as the reference for comparison purposes. A wide selection of different papers was printed in order to cover different applications.
To simulate application-oriented conditions, the prints were subjected to the following ageing conditions for approximately one year:
• Climatised room 23°C / 50% rel. humidity, in darkness (in a closed folder),
• Office (artificial light, dry),
• Behind a window pane (with incident sunlight).
A testo 545 light meter with a measuring range from 0 to 100,000 Lux was used to determine the amount of radiation the print samples were exposed to while they were being stored in the office and behind the window pane. The colour changes in the prints were evaluated at predetermined time intervals.
Storage in darkness
The trials simulated the type of long-term storage that is most common both in archives and in the home, i.e. storage of documents in folders without incident light. The trials were intended to show the extent to which prints fade even under such storage conditions and how properties such as legibility or surface strength change.
The results showed that colour changes occurred in the prints in the visually barely noticeable range (change in chromaticity coordinates ÄE to 5) irrespective of the paper grade or printer. Even here, it became clear that findings regarding the stability of the prints are always dependent on the printerpaper- ink system and therefore cannot be generalised (see Figure 1.) Under these ageing conditions, the stability of the inkjet prints was equal to or better than the stability of the laser prints (see , laser printer = printer D). It is absolutely essential to systematically select papers that match the corresponding printer-ink system to obtain prints that are as resistant to ageing as possible.
When the prints stored in darkness were evaluated visually a year later, their photo quality showed no changes that would disturb the human eye (change in chromaticity co-ordinates of ÄE < 1). Even legibility, surface strength and resistance to agents did not change at all during the year of storage compared to the original state. Moreover, these results were totally independent of the printer or paper used. The only exceptions were the prints that had been made on recycling paper (papers 8 and 9) using printer E, as these prints were found to have suffered a slight deterioration in print contrast after a year of storage.
The results of these trials demonstrated that storage of inkjet prints in darkness under constant climatic conditions did not exhibit any disadvantages compared to prints produced by electrophotographic printing.
Office storage not in direct daylight
Prints frequently lie on desks or hang on wall charts for longer periods of time in offices and are thus exposed to artificial light in a day-night rhythm. The results of these trials demonstrated that inkjet printers achieved results comparable to those of laser printers even under such storage conditions. The changes in chromaticity co-ordinates in most of the printer-paper combinations were generally less than one ÄE of max. 5. The colours yellow and magenta showed the greatest reaction to the office light situation.
The storage of prints in the office, however, resulted in visible colour changes in those prints that had been printed using wax ink (printer C). All of the other prints produced by printers A and E (except for the yellow colour of printer E) and the laser prints 
(printer D) remained significantly less than ÄE = 3 in most cases (see Figure 4). In the case of the prints made by printer C (wax ink), the most significant changes were found in the colours magenta and cyan, e.g. on paper 3 (inkjet, standard matte) with as much as ÄE = 8, whereas only slight changes in chromaticity co-ordinates were observed in the case of black and yellow with a ÄE from 1.5 to 2. The results were highly dependent on the paper grade used.
It is evident from Figure 4 that the ageing stability of the prints can be optimised by selecting an appropriate grade of paper. The trial findings revealed that prints made on special inkjet printing paper (papers 1 to 4) did not necessarily achieve the best ageing stability. The prints made with printer A (bubble inkjet) on multipurpose papers 6, 10 and 12 proved to have much greater light stability than the prints made on special inkjet printing paper. Under the storage conditions described above, inkjet prints thus achieved virtually the same good
light stability as coloured laser prints when the printers and papers were selected appropriately.
Storage behind a window pane in direct daylight
Prints are often exposed to direct sunlight for long periods of time behind a window pane as notices or announcements, placards or posters or as documents mounted in glass picture frames. This ageing situation was simulated in another set of trials.
When prints were stored behind a window pane, much higher irradiation energy including part of the daylight UV radiation acted on the prints. The colour differences were accordingly greater after a year of storage. Significant differences again became evident depending on the printer-ink-paper combination that was used (see Figure 5).
The colour stability of the inkjet prints was significantly poorer in the case of yellow and magenta when compared with the stability of the laser prints (printer D). The results obtained with the colours black and cyan, on the other hand, were comparable to those produced by electrophotographic printing (see Figure 6).
Conclusions
When inkjet printing technology is used to produce coloured prints, both the fields of application and the resulting requirements and storage conditions must also be taken into consideration. Systematic selection of the three components of the printing system, i.e. the printer, printing ink and paper, is essential to achieve optimum ageing and light stability. PTS can provide effective support when a decision is pending for a specific print system using the requirements profile developed in the project for different fields of application and with the help of the test method developed for accelerated light-induced ageing of coloured inkjet prints.
