optical densities vs. gamut volumes for image lighfastness evaluation

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 00 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 18909 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
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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 18909
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 Dmin. 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 450 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 18909, but reflects the specific issues inherently associated with
the ageing process of inkjet prints. Similarly to the original ISO standard, temperature and
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humidity must be controlled during testing. The loss of optical density at patches of primary and
secondary colours having Dmin+1,0 and Dmin+0,6 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 450 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 (E2000 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* = 50 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 ± 1,30 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 9.18
+ CMY step wedges were mounted into picture frames behind glass plates, which enclosed
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the prints and thus protected them from the interaction with air pollutants. The incident
daylight spectrum is depicted at figure 2.
UV /E, mW lm –1
E e
5
4
3
2
1
0
April
May
June
July August
April
March
February
January
December
November
October
September
May
June
July August
September
Figure 1: Average month illumination during the experiment
rel. irradiance
120
100
80
60
40
20
0
300 400 500 600 700 800
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 10 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-950 probe). The samples were regularly measured by automated reflection
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12
10
8
6
4
2
0
E, klx

spectrophotometer Gretag Macbeth Spectrolino. Measured spectral data was imported into
Gretag Macbeth ProfileMaker 5.0.5 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 (D0 = 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
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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 10 times exceeding the recommended average value (450 lx) and should
be therefore considered as „slightly accelerated“ experiment.
rel. intensity
1.0
0.8
0.6
0.4
0.2
0
400 450 500 550 600 650 700 750
vlnová délka, nm
Figure 4: Fuji Frontier, RA-4, Fujicolor Professional Paper
rel. intensity
1.0
0.8
0.6
0.4
0.2
C
M
Y
K
Status A
B
G
R
0
400 450 500 550 600 650 700 750
wavwlwngth, nm
C
M
Y
K
Status A
B
G
R
Figure 5: Epson Stylus Photo R220, 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 30 % of optical density is lost (the stipulated
endpoint). Column VG gives the number of equivalent days corresponding to a gamut
decrease of 30 %. Finally, column VG gives the gamut shrinkage at the time when the first
densitometric criterion failed.
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5
4
3
2
1
0
5
4
3
2
1
0
rel. intenzita, status A
rel. intensity, status A

Table 1: Printouts durability calculation according to ISO 18909 and corresponding
gamut volumes and gamut volumes decrease
Media
Durability, days
C M Y VG VG, %
Epson Premium Glossy *) 2707 858 2785 9
Ilford Galery Classic Gloss *) 1441 790 1217 21
Foma 1224 *) 1575 522 1049 16
Epson Archival Matt *) 466 209 489 12
Ilford Galery Smooth Gloss *) 2528 1015 3536 11
Ilford Galery Smooth Perl *) 2617 1093 3323 13
Chagall, ofsetový výpravný karton *) 3076 1486 1329 34
Noritsu, RA-4, Kodak Royal Digital Pap. 10260 10160 9253 34
Fuji Frontier, RA-4, Fuji Crystal Archive 12993 11733 34
ZBE Chromira, RA-4, Kodak Endura 11608 10010 11313 26
Epson Archival Matt **) 329 174 386 13
Ilford Galery Classic Gloss **) 580 2187 14
Ilford Galery Smooth Gloss **) 186 986 840 1838 3
Foma 1224 **) 617 280 710 13
*)
inkset ESC-R200-4, MIS Dyebase, MIS Associates, Inc., USA
**) in a frame without cover glass – with impact of pollutants, particularly of ozone
rel. optical density, %
100
80
60
40
20
Y
M
C
VG
Figure 6: Long-term ageing test, Foma 1224
0
0
0 1000 2000 3000 4000 5000
durability, day
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 30 %, the gamut shrinkage was in the range of 26–4 %. On the
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100
80
60
40
20
rel. gamut volume, %

other hand, in the case of inkjet prints, the dyes fade at very different rates. Therefore, the
ISO 18909 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, %
100
80
60
40
20
Y
M
C
VG
0
0
0 1000 2000 3000 4000 5000 6000
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 18909 Photography – Processed photographic colour films and paper prints and
paper prints – Methods for measuring image stability, 2006, 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 2002, p. 32–37;
ISBN 0-89208-237-2
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100
80
60
40
20
rel. gamut volume, %

3 http://www.hp.com/products1/printpermanence/pdfs/light_fade_testing_methods_Aug2
42004_final.pdf [cit. 11. 2. 2010]
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. 25–30. Las
Vegas 2004. ISBN 0-89208-249-6.
Acknowledgements
Authors thank to Ministry of Education, Youth and Sports of Czech Republic for support by
project OC09069.
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