OPTICAL DENSITIES VS. GAMUT VOLUMES FOR IMAGE LIGHFASTNESS EVALUATION - AN EXPERIMENTAL STUDY

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1 OPTICAL DENSITIES VS. GAMUT VOLUMES FOR IMAGE LIGHFASTNESS EVALUATION - AN EXPERIMENTAL STUDY Michal Veselý, Petr Dzik, Silvia Káčerová Brno University of Technology, Purkyňova 118, 612 Brno, Czech Republic ABSTRACT Digital photographs printed on various inkjet papers and digital photographs exposed on classic chromogenic papers (RA-4 process) undergone long-term ageing, illuminated with average light intensity 4.5 klx of natural light. Optical density, E*ab, ICC profiles, gamut volume and ink concentrations were calculated from measured spectral data. The lightfastness was evaluated according to ISO 1899 and by the means of gamut volume changes. The differences in both type of materials and methods for light ageing of inkjet and classic photographs are discussed. Keywords: gamut, inkjet, ageing 27 Senj, listopad 21. Senj, 6 th 9 th October 21

2 1 INTRODUCTION Digital photographs are usually printed with inkjet printers or exposed on classic silver halide paper followed by chemical processing. A consumer gets two types of prints, but a majority of people is not able to distinguish them. These prints have very different structure causing their different lightfastness. While the traditional silver halide based material contains the primary dyes deposited selectively in several isolated layers, in the case of inkjet papers the image receiving layer absorbs all the printed ink and the primary dyes thus get mixed. This mixing results in mutual interaction of the dyes during print storage and display which further enhances the usual process of image ageing manifested as dye bleaching and paper yellowing. Moreover, there are further major differences between silver halide materials, such as the spectral purity of inkjet dyes, uneven rate of degradation and poor fastness in humid and warm environment. The properties of traditional photographic image is specifically described by ISO 1899 standard. It deals especially with photographic image ageing both in dark storage and display conditions, sets the testing conditions (illumination intensity, temperature, humidity, recommends measuring devices and defines the end-point criteria. The standard specifies conditions for several tests. Image fastness is evaluated according to changes in optical density at test patches having optical density 1 (primary colours cyan, magenta, yellow and composite grey) and D min. During the fading experiment, the loss (or gain) of optical density is monitored as a function of exposure dose or equivalent image age. However, no explicit value for the average daily exposure is set, and therefore this information must always accompany any prediction of image life. Based on the common practice among the imaging industry, daily exposure of 45 lx for 12 hours is usually adopted for this purpose. Status A filter set is stipulated for the densitometric measurements of classic photographs. The transmission maxima of this filter set are identical with the absorption maxima of dye molecules present in the image forming layers. The situation becomes more complex when measuring inkjet prints, because the maxima might be shifted to certain extend. This results into unequal density reading on visually neutral composite grey patches. Moreover, the ageing process of inkjet prints tends to be uneven, i.e. the dyes fade at different rates resulting into shift of colour balance. Most of leading world inkjet media manufacturers accepted the fade-testing method developed by WIR. This method originated in the classical photography standard ISO 1899, but reflects the specific issues inherently associated with the ageing process of inkjet prints. Similarly to the original ISO standard, temperature and 28 Senj, listopad 21. Senj, 6 th 9 th October 21

3 humidity must be controlled during testing. The loss of optical density at patches of primary and secondary colours having D min +1, and D min +,6 is monitored. The endpoint is established as an acceptable loss of optical density at these patches and the value varies for each colour or colour combination. After reaching the endpoint criteria for any density and colour patch, the test is terminated and the exposure dose is calculated. The dose can be further expressed in terms of equivalent print age calculated on the basis of an average display conditions. This is generally accepted to be 45 lx for 12 hours a day, i.e. 5.4 klx hour day 1. With respect to the different properties of inkjet prints and classic silver halide photographic papers and to the absence of a new ISO standard, new ways of evaluating the image permanence of digital prints are investigated. It seems that the colorimetric measurements based approaches are more suitable for this purpose. However, once we adopt colorimetry for measuring the image colour changes, we need to define the acceptable fade and corresponding endpoint criteria in colorimetric terms. Moreover, various ways of colour difference expression can be derived from the measured CIE L*a*b values, such as total E, separated chromatic E* ab and lightness L, or any of the newly adopted ways of colour difference expression ( E 2 etc.). Also, it seems that gamut volume and its shrinking during fading can provide useful information for the print permanence evaluation. The colour gamut is a set of colours which can be reproduced by a certain device. In the case of inkjet printing, gamut is device-, ink- and paper-dependent. It is usually visualised in 2D as a subset of the xyy colorimetric diagram or in the CIELab L* = 5 plane. If we find the convex hull of gamut in the Lab colour space, we can determine its volume. During the fading process, this volume inevitably decreases and we can use the rate of this decrease as a measure of the lightfasness. 2 EXPERIMENTAL PART A long-term print ageing test was conducted on selected samples of inkjet prints a classic silver halide colour prints. The test lasted 534 days from April to next year October and took place on a brightly indirectly illuminated corridor having average illumination of 4,73 ± 1,3 klx. The ratio of UV-A irradiation to illumination was yet below 4,5 µw lm 1 (Figure 1). Prints of the test images and patches (standard RGB calibration target RGB CMY step wedges were mounted into picture frames behind glass plates, which enclosed 29 Senj, listopad 21. Senj, 6 th 9 th October 21

4 the prints and thus protected them from the interaction with air pollutants. The incident daylight spectrum is depicted at figure E e UV /E, mw lm May June July August September April March February January December November October May June July August September April E, klx Figure 1: Average month illumination during the experiment 12 1 rel. irradiance wavelength, nm Figure 2: Spectrum of natural light falling to samples An electronic light-to-frequency converter Taos TSL235 was mounted into one of the picture frames. This device converts the incident radiation intensity into frequency of output signal linearly across a five orders of magnitude range. The converter was wired to a digital multi-meter, from which the frequency values were saved to a PC at 1 minutes intervals. The sensor calibration was performed by measuring the absolute magnitude of incident radiation by certified radiometers and opto-meters (Gigahertz-optik X97 + X9-7 probe and X11 + XD-95 probe). The samples were regularly measured by automated reflection 3 Senj, listopad 21. Senj, 6 th 9 th October 21

spectrophotometer Gretag Macbeth Spectrolino. Measured spectral data was imported into Gretag Macbeth ProfileMaker 5..5 and corresponding ICC profiles were calculated.

5 spectrophotometer Gretag Macbeth Spectrolino. Measured spectral data was imported into Gretag Macbeth ProfileMaker 5..5 and corresponding ICC profiles were calculated. These were then visualised by Imatest LLC Gamutvision and gamut volumes were calculated (Figure 3). Optical density (status A) of C, M, Y (D = 1) patches were calculated from the spectral data and expressed as relative optical density with respect to the original value before fading. Figure 3: Visualisation of gamuts 3 RESULTS AND DISCUSSION Densitometric status A was designed according to typical spectral properties of chromogenic dyes in positive images both on reflective and transparent base. Modern inks are very variable in their composition and spectral characteristics. The difference is depicted in figure 4 (classic silver halide colour photo, process RA-4) and figure 5 (an after-market dye inset). In the case of inkjet, a visually neutral composite patch will not give equal readings of status A optical density. The so called catalytic fading is greatly enhanced by overlapping the tiny ink droplets and their merging in a single ink receiving layer. Since modern inksets contain more then just the 4 conventional process colours, the composition of a greyscale varies across the grey level. Thus, we can easily observe uneven fading in just one part of the printed grey scale and therefore the lightfastness cannot be reliably evaluated on the basis of one or two measurements, but the whole scale needs to be assessed. Moreover, as the colour 31 Senj, listopad 21. Senj, 6 th 9 th October 21

6 management solutions get wider acceptance, it would be logical to adopt them for the purpose of light fastness evaluation as well. The long term experiment was conducted at an illumination level 1 times exceeding the recommended average value (45 lx) and should be therefore considered as slightly accelerated experiment. rel. intensity C M Y K Status A B G R rel. intenzita, status A vlnová délka, nm Figure 4: Fuji Frontier, RA-4, Fujicolor Professional Paper rel. intensity C M Y K Status A B G R rel. intensity, status A wavwlwngth, nm Figure 5: Epson Stylus Photo R22, inkset MIS Dyebase, paper Ilford Smooth Gloss Table 1 summarizes the permanence of test images. Columns C, M, Y give the equivalent print life time (1 day = 5,4 klx h), after which 3 % of optical density is lost (the stipulated endpoint). Column V G gives the number of equivalent days corresponding to a gamut decrease of 3 %. Finally, column V G gives the gamut shrinkage at the time when the first densitometric criterion failed. 32 Senj, listopad 21. Senj, 6 th 9 th October 21

7 Table 1: Printouts durability calculation according to ISO 1899 and corresponding gamut volumes and gamut volumes decrease Durability, days Media C M Y V G V G, % Epson Premium Glossy *) Ilford Galery Classic Gloss *) Foma 1224 *) Epson Archival Matt *) Ilford Galery Smooth Gloss *) Ilford Galery Smooth Perl *) Chagall, ofsetový výpravný karton *) Noritsu, RA-4, Kodak Royal Digital Pap Fuji Frontier, RA-4, Fuji Crystal Archive ZBE Chromira, RA-4, Kodak Endura Epson Archival Matt **) Ilford Galery Classic Gloss **) Ilford Galery Smooth Gloss **) Foma 1224 **) *) inkset ESC-R2-4, MIS Dyebase, MIS Associates, Inc., USA **) in a frame without cover glass with impact of pollutants, particularly of ozone rel. optical density, % Y M C VG rel. gamut volume, % durability, day Figure 6: Long-term ageing test, Foma 1224 Our experiment confirmed that the density loss of monitored C, M, Y patches of starting density 1 does not correlate with the values of gamut shrinkage. In the case of classic silver halide media, all the dyes fade at approximately the same rate and when the density of the least stable dye dropped by 3 %, the gamut shrinkage was in the range of 26 4 %. On the 33 Senj, listopad 21. Senj, 6 th 9 th October 21

8 other hand, in the case of inkjet prints, the dyes fade at very different rates. Therefore, the ISO 1899 endpoint was reached very soon by the least stable dye and the corresponding gamut volume was very variable, with the average value of 14 % rel. optical density, % Y M C VG rel. gamut volume, % durability, day Figure 7: Long-term ageing test, RA-4, Fuji Crystal Archive The gamut volume gives a good scope about the total integral colour change of the print. Therefore, it can hardly be in a simple relation of optical density of the primary process colour. On the other hand, the relative gamut volume change during ageing gives a single number quantifying the process of dye fading. Moreover, it gives the consumer the option to simulate the effect of ageing by the means of the ICC profiles and also to soft-proof the fading process on the computer screen. 4 REFERENCES 1 ISO 1899 Photography Processed photographic colour films and paper prints and paper prints Methods for measuring image stability, 26, Switzerland. 2 Wilhelm, H.: How Long Will They Last? An Overview of the Light Fading Stability of Inkjet Prints and Traditional Color Photographs. IS&T s Twelfth International Symposium on Photofinishing Technologies Fort Lauderale, Florida 22, p ; ISBN Senj, listopad 21. Senj, 6 th 9 th October 21

9 _final.pdf [cit ] 4 McCormick-Goodhart M., Wilhelm H.: Progress Towards a New Test Method Based on CIELAB Colorimetry for Evaluating the Image Stability of Photographs. Proceedings of IS&T s 13 th International Symposium on Photofinishing Technologies, pp Las Vegas 24. ISBN Acknowledgements Authors thank to Ministry of Education, Youth and Sports of Czech Republic for support by project OC Senj, listopad 21. Senj, 6 th 9 th October 21

The longevity of ink on paper for fine art prints. Carinna Parraman, Centre for Fine Print Research, University of the West of England, Bristol, UK

The longevity of ink on paper for fine art prints Carinna Parraman, Centre for Fine Print Research, University of the West of England, Bristol, UK Fine art papers http://www.nasheditions.com http://www.wilhelm-research.com