comparative study on the lightfastness of traditional and modern ...

COMPARATIVE STUDY ON THE LIGHTFASTNESS OF
TRADITIONAL AND MODERN INKJET PHOTOGRAPHIC
OUTPUT MEDIA
Petr Dzik, Michal Veselý, Eva Štěpánková
Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republic
ABSTRACT
Photography as a part of the cultural heritage is an ever-lasting subject of various stability
experiments and analyses. In this work, samples produced by classic chromogenic and
modern digital printing processes were compared in an accelerated light-fastness test. Color
fading was monitored in a novel way. Measured spectral data were converted into Lab
values and corresponding ICC profiles were calculated. Further, gamut volumes of these ICC
profiles were calculated and relative gamut volume changes plotted as a function of
exposure dose. These plots were then used to determine formal rate constants, which
express the degradation rate of each sample.
Key words: image fading, color gamut, accelerated aging
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1 INTRODUCTION
Up to a quite recent history, all colour photograps were produced by optical or electronic
exposure of photosensitive silver halide based paper or film. During the color photography
evolution, many photographic processes have been developed, but all shared the same
fundamental principle of image forming based on the changes of crystal stucture of AgX
upon irradiation. These processes include, but are not limited to the chromogenic (i.e. dye
creating) processes C-41, RA-4, E-6, K-14 and R-3, the dye bleaching process known as
Cibachrome/Ilfochrome and the dye diffusion process utilised in Polaroid, Fuji and Kodak
instant films.
The boom of digital photography brought major changes into the imaging industry and most
of the traditional processes were discontinued or shrinked to just a fraction of the market
share. However, there are two important exceptions to this development – the RA-4 and P-3
processes.
RA-4 is Kodak's proprietary name for the chemical process most commonly used to make
color photographic prints 1 . It is used for both digital printers of the types most common today
in photo labs and drug stores, and for prints made with older-type optical enlargers and
manual processing. More specifically, common color photographic paper is carefully exposed
to form a latent image of the picture, and then the paper is run through the series of
chemicals that together comprise the RA-4 process to convert the latent image into the final
print, the colors of which are dyes in the paper.
RA-4 is a standardized chromogenic process used worldwide to make prints with a variety of
equipment, photographic paper, and chemicals. Kodak created the RA-4 process for its color
negative photographic papers. Fuji, Agfa, and other present and past photographic supply
companies also make or have made both papers that are compatible with the Kodak
chemicals, and chemicals that are compatible with the Kodak papers. These other
companies typically call their equivalent processes by other names, but to most
photographers, RA-4 is used as a generic term.
Nowadays, most consumer and professional photographs of small and medium formats are
produced on this type of paper. Digital image files are optically exposed by LCD, LED or
laser recorders and the digitally exposed film film goes through the standard processing
sequence.
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P-3 process is used to develop the Ilfochrome (formerly known as Cibachrome) materials Error!
Reference source not found. . It is a dye destruction positive-to-positive photographic process used for
the reproduction of slides on photographic paper. The prints are made on a dimensionally
stable polyester base, essentially a plastic base opposed to traditional paper base. Since it
uses azo dyes on a polyester base, the print will not fade, discolor, or deteriorate for a long
time. Characteristics of Ilfochrome prints are image clarity, color purity, as well as being an
archival process able to produce critical accuracy to the original slide.
Similarly to the RA-4, also P-3 processed material can be digitally exposed, ensuring the
viability of this process in the new digital imaging era.
However, as the number of digitally recorded images has been increasing, so has been the
demand for high-quality digital printing techniques which would enable the digital
photographer leave the dark room entirely and use purely digital photographic workflow.
Unobjectionably, inkjet printing has become such solution.
Inkjet is a digital printing process where the ink is ejected directly onto a substrate from a jet
device driven by an electronic signal. The majority of printers used for colour printing in
offices and homes today are inkjet printers. Due to its ability to print on a wide variety of
substrates, inkjet technology is also increasingly being used in industrial printing and in the
package printing industry. As the quality of the printers has improved and high-end
photographic papers have become available, this printing technique is becoming increasingly
popular among amateur and professional photographers. Actually, nowadays we witness a
massive development of fine-art inkjet printing as a peculiar way of artistic expression.
As the traditional silver halide photographic media have been around for the past 60 years,
the consumers are well aware of their properties. They usually have some idea or at least
personal experience of the the image permanence at various conditions. Therefore, the
informed users know how to handle, store and display their valuable images. However, there
is no such general knowledge nor experience with the new inkjet prints. Therefore, it is
important to provide the community with sufficient information on this topic so that the new
technology can be used optimally. So, the goal oif the presented <strong>study</strong> was to compare the
image permanence of traditional photographic papers with new inkjet printers.
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2 THEORETICAL PART
At present, there are several standard practices and ISO standards, which deal with the lightfastness
testing of colour photography and prints. Those papers include information about
test conditions, methodology and endpoint criteria. Of these, the ISO 18909 standard is
especially worth mentioning. This standard sets the methodology for testing image
permanence of traditional silver halide materials of different types (as mentioned in the
Introduction part).
The problem is that this standard is not suitable for testing images produced by new digital
printing techniques. There are multiple reasons for this and therefore a new standard is
necessary. One of the reasons is that the ISO 18909 uses densitometric measurements with
status A filters. While the spectral characteristics of this filter set are well suited for the silverhalide
photographic materials, they might not match the absorption maxima of dyes and
pigments used in digital printing. The problem has been clearly demonstrated at Wilhelm
Imaging Research by inequal density readings on a visually neutral composite grey patch 3 .
Further reason originates from the limited amount of test patches given in ISO 18909.
Modern inkjet printers utilize more than the 4 traditional primary colors. Printers with 8 to 12
inks are widely used for demanding tasks such as proto-realistic printing or certified digital
proofs nowadays. In such imaging system, e.g. green color is reproduced by direct printing of
green ink rather than oveprinting cyan and yellow. If the green ink suffers from low
permanence, we will not be able to notice it when the ISO 18909 methodology is applied,
because only CMY patches are monitored in this case. It is obvious that we need much more
test patches in order to get reliable data about these sophisticated imaging systems.
Also, the problem of catalytic fading of inkjet prints bring further complications to the process
of permanence determination. Catalytic fading commonly refers to the phenomenon when a
dye fades faster in the presence of another dye than when it is present on its own 4 . Strictly
speaking, the process is not catalytic, but rather photocatalytic and the role of hydrogen
donors and singlet oxigen sensitizers is crucial. Anyway, the process results in uneven fading
across the density scale and therefore preferably the whole gray scale should be monitored.
The problem has been adressed by several reaserch teams and many test new test methods
have been proposed based on densitometric measurements of extended test targets 5,6 ,
colorimetry 7 or image analysis 8 . However, none of these received a general acceptance
articulated in the form of a new ISO standard for evaluation of digital prints permanence.
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3 EXPERIMENTAL PART
Standard calibration target RGB 9.18 (see figure 1) was printed by different processes on
various photo-papers. The papers, inks and printers are summarized in Table 1.
Table 1 Sample composition
Sample
No.
Target group Machine Reproduction process Paper
2 Consumer inkjet Epson R220
3 Epson 9600
Professional inkjet
6
HP 500PS
Inkjet
Refill dye ink set MIS Dyebase
Inkjet
OEM pigment set Ultrachrome K3
Inkjet
OEM dye ink set
Ilford Smooth Gloss
Epson Profesional Paper
HP Glossy Photopaper
7 Konica R3 RA-4 Konica Long Life
Consumer minilab
10
Fuji Frontier RA-4 Fuji Crystal Archive
11 Professional photofinishing OCE Lightjet RA-4 Kodak Endura Glossy
Figure 2: Sample 2
Accelerated ageing tests of prints were performed in a xenon light test chamber. The
machine (Q-Sun-Xe-1B by Q-lab Corporation) was set according to ASTM F 2366-05: The
irradiation intesity was 0.9 W m –2 nm –1 at 420 nm and black panel temperature was
maintained at 63 °C. This irradiation intesity corresponds to illumination intensity of 64 klx, as
measured by Gigahertz-Optik X-11 optometer with XD-950 probe. Each sample was given a
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series of 5 exposure doses, each lasting 10 hours. Thus each sample recieved a total
exposure dose of 3200 klx hour, consisting of five 640 klx hour steps.
The samples were measured in the beginning of the experiment and after each exposure
step. Automated reflection spectrophotometer GretagMacbeth Spectrolino was used with
polarising filter and in absolute calibration mode. Measured reflectance spectra were
converted into L*a*b* values (D50 luminant, 2° observer). Converted L*a*b* values were then
used to calculate corresponding ICC profiles (for each sample and each exposure step).
Profiles were then visualized in the Gamutvision software and gamut volume was calculated
for each measurement. These values were normalised by the gamut volume of starting
unexposed sample and normalised data (as percent of unexposed sample) was plotted
against the exposure dose. Also, the equivalent sample age A expressed in days was
calculated according to equation (1) as the total exposure dose H to average daily dose ratio
and co-plotted as an alternative horizontal scale. Average daily dose was accepted to be 450
lx for 12 hrs, as the de-facto standard in the industry 9 .
H
A �
(1)
450�12
Quite surprisingly, the relative gamut volume followed a linear trend within the observed
range of exposure doses. Therefore, a simple linear fitting model was adopted to find the
slope of the plot (eg. 2).
y � 100 � ax
Parameter a corresponds to the rate constant of the gamut shrinking process. It was found
by the least-square method (eq. 2). Also, parameter S was found according to eq. 3 as the
relative quadratic deviation of fitted points from the regression line. The S parameter is thus a
quality measure of the linear model fitting to the experimental data.
N
� x �
i i �i
2
� xi
100
a � N
S �
1
N
N
�1 �1
i�1
( 100
N
i
� 2
i�1 yi
x
i
y
2
� ax � y )
i
i
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(2)
(3)

4 RESULTS AND DISCUSSION
The following figures depict the results for all studied samples:
Gamut volume [%]
100
90
80
70
60
50
40
0 100 200 300 400 500 600
y = 100 - 4.24x10 -3
S = 0.086
Equivalent age [days]
0 500 1000 1500 2000 2500 3000
Exposure dose [klux.h]
Gamut volume [%]
100
90
80
70
60
50
40
0 100 200 300 400 500 600
y = 100 - 3.57x10 -4
S = 0.004
Figure 2: Sample 2 Figure 3: Sample 3
Gamut volume [%]
100
90
80
70
60
50
40
0 100 200 300 400 500 600
y = 100 - 1.30x10 -2
S = 0.053
Equivalent age [days]
0 500 1000 1500 2000 2500 3000
Exposure dose [klux.h]
Gamut volume [%]
100
90
80
70
60
50
40
Equivalent age [days]
0 500 1000 1500 2000 2500 3000
Exposure dose [klux.h]
0 100 200 300 400 500 600
y = 100 - 1.04x10 -3
S = 0.005
0 500 1000 1500 2000 2500 3000
Figure 4: Sample 6 Figure 5: Sample 7
Equivalent age [days]
Exposure dose [klux.h]
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Gamut volume [%]
100
90
80
70
60
50
40
0 100 200 300 400 500 600
y = 100 - 1.31x10 -3
S = 0.004
Equivalent age [days]
0 500 1000 1500 2000 2500 3000
Exposure dose [klux.h]
Gamut volume [%]
100
90
80
70
60
50
40
0 100 200 300 400 500 600
y = 100 - 1.07x10 -3
S = 0.008
Equivalent age [days]
0 500 1000 1500 2000 2500 3000
Exposure dose [klux.h]
Figure 6: Sample 10 Figure 7: Sample 11
All gamut shrinking rates are summarized in Table 2. The table gives both the gamut
shrinking rate constants as well as a life time estimate based on extrapolation of the
experimental data. The end point criterion for this extrapolation was adopted to be 30 %
gamut loss. This corresponds to the density endpoint used in the ISO standard 18909, but
obviously these two quantities are totally different in their nature. The problem of acceptable
gamut loss certainly requires further <strong>study</strong>.
Speaking about table 2, several interesting facts are worth discussing. First of all, samples 7,
10 and 11, i.e. those produced by RA-4 process, show essetially very similar values of gamut
shrinking rate. It seems that the image permanence of this media is not influenced by the
manufacturer, but is rather determined by the common chemical nature of chromogenic dyes
which is shared by all the manufacturers in order to ensure their materials' compatibility with
the RA-4 process.
On the other hand, samples 2, 3 and 6 show a great variability of measured shrinking rates.
This well illustrates the wide range of dyes and pigments used in inkjet formulations.
Therefore no general conclusion can be drawn with respect to the image permanence of
inkjet prints, all cases have to be treated individially according to experimental data.
Moreover, the inkjet samples selected for this paper clearly illustrate the improvements in the
inkjet technology during its development in the past 20 years. Of these, sample 6 is the worst
one in therms of print permanence. This printer (HP 500 PS) dates back to mid 1990's a
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utilizes dye based inks which have very poor lightfasness. Sample 2, printed by MIS
Associates Dyebase inkset, shows marginally better lightfastness. However, sample 3
printed by Epson K3 pigment inkset has by far the best lightfastness of all the tested
materials and outperforms the silver halide papers as well.
Table 2 Experimental results summary
Sample
Rate constant
[klx -1 hour -1 ]
Accelerated ageing
Time for 30% gamut
volume loss [years]
2 4,24·10 –3 1,9
3 3,57·10 –4 22
6 1,30·10 –2 0,6
7 1,04·10 –3 7,7
10 1,31·10 –3 6,1
11 1,07·10 –3 7,5
5 CONCLUSION
The presented results bring valuable <strong>comparative</strong> information on the lightfastness of
traditional silver halide based and modern inkjet photographic media. Although the this paper
presents only a limited number of selected samples, some trends can be identified. First of
all, the inkjet printing technology utilises a great variety of dyes, pigments a other
components which have very varied light fastness. Therefore no general assessment of
inkjet printing can be made, the materials have to be carefully evaluated on individual bases.
However, it seems that recent pigment based inksets superseed the traditional silver halide
papers in terms of fade resistance.
6 REFERENCES
1 Kodak Processing (Z) Manuals: Z-130
2 http://www.ilford.com/en/products/ilfochrome/index.asp
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3 Mark McCormick-Goodhart, Henry Wilhelm: Progress Towards a New Test Method
Based on CIELAB Colorimetry for Evaluating the Image Stability of Photographs.
http://www.wilhelm-research.com/ist/ist_2004_2t.html
4 P. Doll, F. Shi, S. Kelly and W. Wnek: The Problem of Catalytic Fadingwith Ink-Jet
Inks. IS&Ts NIP 14: 1998 International Conference on Digital Printing Technologies, 118-
121.
5 JEITA CP-3901. Digital color phtoto Print stability evaluation.
6 Henry Wilhelm, Kabenla Armah, Dmitriy Shklyarov, and Barbara Stahl: Improved Test
Methods for Evaluating the Permanence of Digitally-Printed Photographs.
http://www.wilhelm-research.com/japan/WIR_Imaging_Japan_2009_HW.pdf
7 Mark McCormick-Goodhart and Henry Wilhelm: A New Test Method Based on
CIELAB Colorimetry for Evaluating the Permanence of Pictorial Images.
http://www.wilhelm-research.com/pdf/WIR_CIELAB_TEST_2003_07_25.pdf
8 Mark McCormick-Goodhart, Henry Wilhelm, and Dmitriy Shklyarov: A Retained Image
Appearance Metric For Full Tonal Scale, Colorimetric Evaluation Of Photographic Image
Stability.
http://www.wilhelm-research.com/ist/WIR_IST_2004_11_MMG_HW_DS.pdf
9 Epson print permanence white paper.
http://www.epson.com/cgi-bin/Store/Landing/PrintPermanence.jsp
Acknowledgements
Authors thank to Ministry of Education, Youth and Sports of Czech Republic for support by
project OC09069.
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