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Comparing color image qualityof four digital presses
Jon Y. Hardeberg and Sven E. SkarsbGjvik University College, Gjvik, Norway
[email protected], [email protected]
Abstract
Color image quality is a very important deciding factor for
buyers of color imaging devices. It is therefore of utmost
importance for manufacturers of imaging equipment such
as digital presses, to pay special attention to this factor.
We have carried out a study in which we have eval-
uated the color image quality of four different commer-
cially available digital color presses, one of which is us-
ing a magnetographic technique for image formation; the
other three use a more conventional electrophotographic
technique. To evaluate color image quality we use a com-
bination of several different techniques, based on colori-
metric and spatial measurements, visual expert evalua-
tions, and panel tests.
We have found that there are significant quality differ-
ences between the devices under test. Note, however, that
we have tested specific total systems, including printer,
RIP, parameters, ICC profiles, paper, etc., and also thatthere are many important factors that are not in the scope
of this test, such as price, printing speed, long term sta-
bility/expected lifespan, etc. Nevertheless, we believe that
the results of this study is of significance to both manufac-
turers and customers of digital press equipment.
1. Introduction
After several years of market hesitation, digital presses
have now become common. In todays printing mar-
ket, where flexibility, variable content, shorter lead times,
and on demand publishing, are being demanded, digitalpresses represent an attractive supplement to conventional
offset presses.
It is important for customers of digital press equipment
to be able to take into account the quality of the equip-
ment, typically to be able to make a trade-off between
price and quality. The total quality of a device is, how-
ever, a very complex entity, involving technical aspects
such as expected lifespan, printing speed, accepted media,
as well as customer relation aspects such as service agree-
ments. Also customers who do not intend to own dig-
ital press equipment, but rather are looking for instance
to have a publication printed by a print bureau, need toconsider the quality of service they can get with differ-
ent providers. While different customers have different
requirements, we believe that knowledge of the color im-
age quality that different equipment can provide is of great
importance to customers.
In our study we have evaluated the color image qual-
ity of four different commercially available digital color
presses, namely Oce CPS700, Xerox DC2060, Nexpress2100, and Canon CLC5000. Note that because of the ob-
vious interest this will have to customers and others, we
have decided to publish the names of the devices under
test, similarly to what has been done in other benchmark-
ing studies (den Engelsman, 2002, Lindberg et al., 2001,
Bolanca et al., 2001). A popularized version of the re-
sults has been published in a Swedish graphic arts maga-
zine (Skarsb and Hardeberg, 2002), and presented at two
Nordic conferences. However, we also believe that not
only the results, but also the methodology is of interest to
the engineers and scientists working with printing technol-
ogy.
Electrophotography is the most widespread non-
impact-printing technology that exists. Three of the
presses in the test (Xerox, Canon, and Nexpress) use vari-
ants of the electrophotographic principle with electrostatic
powder toner, based on an invention of Chester Carlson
from 1939 (Kipphan, 2001).
The fourth press, Oce CPS700, uses a different princi-
ple that may be classified under the category magnetogra-
phy even if Oce does not use that designation and calls
the method Direct Imaging Printing Technology. The
imaging carriers are cylinders fitted with individually con-
trollable ring electrodes, protected by a dielectric coating.
A magnetic, single component toner is fed to the imagingcylinders by a magnetic roller. The imaging is achieved by
charging the ring electrodes with image-dependent voltage
pulses. An imaging magnetic roller achieves the imaging
by removing magnetic toner from the non-printing areas
of the imaging cylinder.
For multicolor printing with Oce CPS700, instead of
only using the usual four colors, it uses seven colors,
CMYK plus the three complementary colors red, green
and blue. Seven imaging cylinders are therefore posi-
tioned in a satellite configuration around one common in-
termediate cylinder, which transfers the toner to the paper.
The multicolor printing is not achieved by overprintingof four colors. Instead, pixels of the seven colors are po-
sitioned alongside each other, and the press is thus relying
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on additive color mixture. This may be the advantage as
well as the disadvantage of the Oce direct imaging print-
ing technology. Just one thin layer of toner can be fixed
with less heat supply than four layers. The mono-layer
also prevents the brutal relief building typical for photo-electric color prints. On the other hand, pixels of different
colors placed alongside each other tend to make smooth
tones look grainy.
In the remainder of this paper we first give an overview
of the field of color image quality, in Section 2. We then
proceed in Section 3 to a description of the experimental
setup and results of our study in which the color image
quality of four digital presses were evaluated. The results
are further discussed in Section 4, along with a higher level
discussion of factors surrounding this study. Finally, we
round off by a conclusion and discussion of further work
in the area of color image quality.
2. Color Image Quality
In recent years, the concept of image quality has received
quite much attention within the imaging science and tech-
nology community. The subject has for instance been
extensively discussed at the PICS conferences (see e.g.
Stokes, 1998, Rasmussen et al., 1998, Yendrikhovskij,
1999, Topfer and Cookingham, 2000, Kane et al., 2000,
Jung et al., 2001, Engeldrum, 2001, Hardeberg, 2002), and
at more graphic arts oriented conferences (see e.g. Lind-
berg et al., 2001, Bolanca et al., 2001, Norberg et al.,
2002, Edinger, 2002). But still, image quality often re-
ceives a rather stepmotherly treatment in the industry
probably because of its somewhat awkward position be-
tween subjectivity and objectivity (Yendrikhovskij, 1999).
The concept of quality, typically defined in dictionaries as
degree of excellence is inherently a subjective entity. An
engineer and scientist, however, generally prefers to deal
with objective quantities, backed by scientific evidence.
On the web site of a major French consumer electronics
retailer, a formula for the image quality of a printer was
given approximately as follows: Image quality = Resolu-
tion x Color Depth. This is an example of another com-
mon misconception regarding image quality its over-simplification.
There are indeed many factors that contribute to the
quality of an image, such as spatial resolution, color depth,
the nesses (sharpness, naturalness, colorfulness, etc.),
and visual artifacts (banding, streaking, grain, blocking,
mottle, moire, etc.). There exist an ongoing effort to stan-
dardize the definitions of these and other image quality
factors, as well as their assessment methodology, see for
example a recent paper by Grice and Allebach (1999). It
is out of scoupe of this paper to give an extensive descrip-
tion of these quality factors. We will, however, present and
discuss the factors included in our analysis, in Section 3.Potential uses of quantifiable data on color image qual-
ity for manufacturers include the following:
Tradeoff analysis of speed and implementation cost
versus color image quality in image processing algo-
rithm development.
Benchmarking of imaging systems and algorithms toother vendors products.
Documentation of color image quality improvements
resulting from efforts spent on optimization of tech-
nology parameters.
For customers, it would obviously be advantageous to
have access to reliable and objective information about the
image quality that devices can provide when considering
several alternatives for purchasing.
3. Experimental Setup and Results
To carry out our color image quality evaluation, we first
designed two test targets. The paper size used was A3,
the resolution 600dpi, and the file format PDF. The tar-
get shown in Figure 1 was specified using CMYK color
space, while the one shown in Figure 2 uses the sRGB
color space (Anderson et al., 1996, IEC 61966-2.1, 1999).
The targets contain several graphical and pictorial ele-
ments which were used for our quality evaluation.
Figure 1: The CMYK test target designed for our study.
The actual test printing was carried out in the manu-
facturers offices in Norway, except for that of Nexpress,
which was carried out at Heidelberg in Great Britain, since
there were yet no such presses installed in Scandinavia.
The presses were operated by the manufacturers own per-
sonnel, in the presence of the authors.
The CMYK target was printed without color manage-
ment, that is in particular, not with the intention of proof-
ing or simulating another press. The sRGB target, how-
ever, was printed using ICC-based color management with
four different rendering intents perceptual, saturation,media-relative colorimetric, and ICC-absolute colorimet-
ric (ICC.1:2001-12, 2001). For each of these, 20 copies
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Figure 2: The sRGB test target designed for our study.
were printed. The manufacturer then chose the render-
ing intent that they preferred, and 600 more copies were
printed with this intent. After this, the press was turned off
and restarted after at least 5 minutes, whereby a new set of
20 copies was printed. The manufacturers were allowed to
choose printing parameters such as raster frequency, RIP
software, and paper, with the goal of achieving the best
color image quality.
The quality analyzes were done at the Color Lab at
Gjvik University College, and included quantitative an-
alyzes based on measurements, psychophysical experi-
ments, as well as expert evaluations. The quantitative an-
alyzes included colorimetric measurements to determine
color gamut (Section 3.1), colorimetric reproduction (Sec-
tion 3.2) and stability (Section 3.3). The psychophysical
evaluations (Section 3.4) were done to determine color
pleasantness, total image quality, smoothness, and de-
tail rendition. In the expert evaluation we examined the
halftoning, text readability, and alignment.
3.1. Color Gamut
The color gamut of a digital press (or any other imaging
device) is the sum of all colors it can reproduce. Colors
that are outside of this gamut cannot be reproduced. Thecolor gamut, and in particular its size, is thus a quality fac-
tor. A press with a larger gamut than another is typically
able to reproduce more saturated colors, which can be ap-
preciated for many applications.
The color gamut of a digital press depends on many fac-
tors such as toner/colorant, substrate, and halftoning algo-
rithm.
The color gamuts were quantified based on spectropho-
tometric measurements of the TC3.5 CMYK target (Fig-
ure 1. For the case of the Oce press, however, we used the
union of this data and data from the RGB target, since
some colors were found to be attainable only in RGBmode.
From these measurements, we created solid 3D objects
representing the gamuts in CIELAB color space, using the
convex hull method, as implemented in the ICC3D tool
(Farup and Hardeberg, 2002, Hardeberg and Farup, 2002).
Projections of the gamuts onto the ab-plane is shown in
Figure 3.
(a) Oce CPS700 (b) Xerox DC2060
(c) Canon CLC5000 (d) NexPress 2100
Figure 3: The color gamut of the four devices, projected on the
ab-plane.
As a supplement to the visual appreciation of the gamut
shapes and sizes, we also calculated its volume (CIE TC8-
05, 2001), see Table 1. We see that the Xerox press has
a larger gamut than the other electrophotographic presses,
and that Oce CPS700 has a significantly smaller gamut.This is probably due to unwanted mis-coloring of the toner
from the iron oxide particles, and it is expected that Oce is
working to improve this.
Table 1: Size of the color gamuts, quantified as the volume of the
convex hull of the gamut in CIELAB color space.
Device CIELAB volume Relative volume
Canon CLC5000 437000 91%
Xerox DC2060 480000 100%
Nexpress 2100 345000 72%
Oce CPS700 269000 56%
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As references for gamut size calculations, it can be
mentioned that the gamut volume of the sRGB color space
is found to be approximately 821000, and all the devices
under test thus have significantly smaller gamut volume
than sRGB. Even so, there are parts of color space wherethe gamuts of all the presses exceed that of sRGB. Klaman
(2001) evaluated the gamut volume of different presses by
using a dodecahedron with the vertices defined by the pri-
mary and secondary colors, black and white. For different
types of presses, she found gamut volumes ranging from
approximately 100000 to 350000, with electrophotogra-
phy situated in the range of 160000 to 210000. Because
of the methodological difference in how to quantify the
gamut volume, these numbers are not really relevant to
our study, however.
3.2. Colorimetric reproduction
In some cases accurate reproduction of colors is important
for the customer, for example when printing color sam-
ples or when digital prints shall be used as proof prints
and the aim is to match offset prints. To investigate how
accurate the presses can reproduce defined colors, 125
color patches defined in sRGB were printed on all the four
presses using ICC-based color management and relative
colorimetric intent.
One means of evaluating color quality is thus through
the measurement of color differences between the actual
reproduction and a preferred color reproduction. The pre-
ferred color reproduction typically relates to an original
document when evaluating color copy, the colors as they
appear on the monitor for a typical WYSIWYG evalua-
tion, or specific colors of which the color reproduction is
particularly important for a given reproduction.
The color differences are typically measured in terms
ofEab the Euclidean distance between two colors
in the CIELAB color space (CIE 15.2, 1986, Wyszecki
and Stiles, 1982). Since natural images rarely contain
sufficiently uniform areas to allow consistent color mea-
surements, a color target with several uniformly colored
patches should be used. Simple statistical measures suchas maximum and average color differences are then typ-
ically used as an indication of the color quality. Note
that newer formulae for color difference are slowly replac-
ing the Eab, for instance E
94(McDonald and Smith,
1995).
A very helpful resource for such color quality evalu-
ation is the Microsoft Windows Color Quality Test Kit
(Microsoft Corporation, 2001). This freely available kit
contains descriptive documents, test targets and images,
as well as tools for the calculation of color differences, for
several different color imaging devices. Typically the goal
is for the devices to communicate images using the sRGBcolor space (Anderson et al., 1996, IEC 61966-2.1, 1999).
If certain criteria, in particular in terms of average color
difference, are not met, the device does not receive Mi-
crosofts certification the designed for Windows logo.
The color patches of the 5x5x5 target (see Figure 4)
on the prints were then measured with a GretagMacbeth
SpectroScan/Spectrolino spectrophotometer, and the val-ues were analyzed using a Microsoft Excel spreadsheet
developed on the basis of Microsofts recommendations
(Microsoft Corporation, 2001). The ideal reproduction is
defined according using colorimetry that is relative both
to the paper white and to the media black. The deviation
between the printed colors and ideal colors were specified
asE94.
Figure 4: The 5x5x5 RGB target used for evaluation of colori-
metric reproduction accuracy. The colors are divided into two
groups, the in-gamut colors, which are expected to be within
the gamut of the digital press, and the gamut surface colors
which have a large probability of being out of gamut.
To make an attempt to separate between the unavoid-
able colorimetric errors due to out-of gamut colors, and
the errors on colors that could have been correct, we di-
vided the target into two regions, as shown in Figure 4,
the safe in-gamut colors, and those that lie on the sur-face of the sRGB space, and that are prone to being out of
gamut of the press.
The results are presented in Table 2. For in-gamut col-
ors Oce obtained the best result, while Xerox got the high-
est score for all colors seen together.
Table 2: Colorimetric accuracy, measured as average E94color difference between printed color and an ideal sRGB re-
production defined according to Microsofts color quality speci-
fications (Microsoft Corporation, 2001). The media-relative ren-
dering intent was used.
Device In-gamut Gamut surface Total
Canon CLC5000 10.3 14.5 13.6
Xerox DC2060 7.5 10.9 10.2
Nexpress 2100 9.2 12.7 11.9
Oce CPS700 6.5 13.1 11.7
3.3. Stability
To get an indication of the short-term stability and repeata-
bility of the devices, we measured two color patches be-fore and after the device had been restarted. We report in
Table 3 theEab color difference between the averages of
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10 measurements of the colors before and after restart. We
see that the Xerox DC2060 and the Oce CPS700 have the
highest stability.
Table 3: Stability of the devices quantified as E
ab color dif- ference between averaged measurements of two patches printed
before and after restart.
Device Patch 1 Patch 2
Canon CLC5000 1.24 0.61
Xerox DC2060 0.56 0.39
Nexpress 2100 1.63 1.47
Oce CPS700 0.44 0.58
3.4. Psychophysical evaluation
To evaluate the visual quality that the presses can achieve,
we carried out a psychophysical experiment, or in more
common terms, a panel test. Seven observers were asked
to evaluate three different images (Figure 2) according to
four different quality criteria:
color pleasantness,
total image quality,
smoothness, and
detail rendition.
For each quality criterion, the observers gave ratings on
a scale from excellent (0) to very poor (6). The images
were of course anonymized, presented in a pseudorandom
sequence, and viewed under standard D50 illumination.
The test showed us that typically, the images printed
with saturation and perceptual intents were judged to
be best, and that there was a correlation between total im-
age quality and color pleasantness.
The results of the experiment are summarized in Fig-
ure 5. The averaged result over the three images, and
the best rendering intent, is used to represent a device.
We see that for the three electrophotographical presses,
the differences are small, perhaps with the exeption of
smoothness, for which the Nexpress device came out sig-
nificantly better than the others. That the magnetographic
press came out last can probably be attributed to two fac-
tors smaller color gamut and graininess because of the
additive color mixture.
3.5. Halftoning
To investigate the halftoning methods, we captured micro-
scopic images on different locations on the target, see Fig-
ure 6. For comparison, we also captured a conventionaloffset print, with a raster frequency of 69 lpcm (150 lpi),
as shown in Figure 7.
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
IQS
core
Col or P le as an tn es s T ot al Im ag e Q ua li ty S mo ot hn es s Det ai l r en di ti on
Oce CPS700 Canon CLC5000 Xerox DC2060 NexPress 2100
Neutral
Excellent
Fair
Good
Very good
Poor
Very poor
Figure 5: Results of the psychophysical evaluation
Figure 6: Microscopic enlargements of the raster structure of the
four presses.
Not surprisingly, we see that Nexpress uses a raster
structure that is very similar to that of offset, both with re-
gards to dot shape and raster angles (15, 75, 0, and 45 de-
grees). Xerox also resembles offset, with Postscript angles
(0, 18.4, 45, and 71.6 degrees). Canon is using essentially
a line raster with angles of 105, 75, 50, and 90 degrees.
Oce uses raster structures which appear to be AM/FM hy-
brids not surprisingly given its additive color mixture.
3.6. Summary of the results
All the four presses have support for color management,
and allows the user to choose between the four rendering
intents defined by the ICC, although they are sometimes
given different names in the user interface.
Canon CLC5000 was judged to produce the most pleas-
ing colors, closely followed by the Nexpress 2100. Con-
cerning smoothness, the Nexpress device was a relativelyclear winner, and it was also best on detail rendition, that
is, what we might call visual resolution.
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Figure 7: Microscopic enlargements of the raster structure of an
offset press.
Xerox DC2060 has the largest color gamut, while Oce
CPS700 has the smallest, despite its seven colorants.
Oce was best with regards to colorimetric accuracy of
in-gamut colors, while Xerox came out best when all
the colors of the sRGB target were taken into considera-
tion, probably because of its large color gamut. Xerox and
Oce were the most stable devices, as observed by their low
color difference between the same patches printed before
and after a restart of the device.
The examination of the raster structures showed that
Nexpress uses a raster which is nearly identical to tradi-
tional offset raster. Xeroxs raster is also similar to offset,
while Canon, and to an even greater extent, Oce, have cho-
sen different approaches to halftoning.
All the presses had good alignment between the differ-
ent process colors, the readability of small positive and
negative text was good, and there were no significant er-
rors in how the PDF file was printed.All in all, if we had to elect a best in test it would be
the Nexpress device, but the distance down to the Canon
and Xerox devices is small and hardly relevant. They are
all capable of printing beautiful color pictures.
The main problems with the Oce CPS700 is that it has
a much smaller color gamut than the electrostatic presses,
and that the special additive color mixture, with pixels of
different color side by side, gives a more grainy appear-
ance than its competitors. This is most striking when the
prints are compared visually to the other technologies, and
much less problematic when looking at a print in isolation.
The strength of the Oce device is that it only lays down athin layer of toner on the paper, and it is fixated at low
temperature with little strain on the substrate. It might be
argued that our test was designed in such a way that it to a
great extent revealed the weaknesses of this press, but that
it did not appreciate its strong sides.
4. Discussion
Although the results of our evaluation presented in the pre-
vious section are indeed interesting, and the methods are
valid in themselves, it is appropriate to proceed to a critical
discussion.Concerning the choice of paper, it is obvious that for the
sake of comparison, it would have been better if the dif-
ferent presses had used the same paper. The reason why
we did not want to dictate the use of a certain paper, was
that we would avoid forcing the manufacturer to use a pa-
per that might not be well suited for their printing process,
and thus that would have given sub-optimal results. Wesuggest that a better procedure would be to print on two
paper stocks, one preferred by the manufacturer, and one
common to all.
With regards to the examination of the raster struc-
ture, we argue that even if it is interesting to study close-
up views, comparing to offset raster, this should not be
viewed as a quality criterion. It is much more relevant to
look at how the images look at normal viewing distances.
Are they smooth, with good color, good detail rendition,
and without visible moire?
It has previously been concluded by Klaman (1995) that
the total gamut volume does not correlate well with visualappearance. We do, however, believe that the small gamut
of the magnetographic device contributes to its results.
Concerning the color quality evaluation through color
difference measurements, we would like to remark that
the numbers should be used with care. Although proba-
bly a good indicator for color quality, minimizing the av-
erage and maximum color differences does not guarantee
optimal results in terms of perceived color image quality.
For example, colors that are not in the evaluation target
might be important. Another factor that limits this ap-
proach is gamut mapping (Morovic, 1998). Because of
the differences in color gamut between different devices
and technologies, a colorimetrically exact reproduction is
rarely optimal.
As we have mentioned earlier, the notion of color im-
age quality is ultimately defined by what the customer
wants. Since there does not yet to our knowledge exist
measurement-based image quality models that adequately
quantifies quality in this sense, psychophysical experi-
ments or panel tests is the method of choice. However,
there are many critical factors that may limit the signifi-
cance of such an experiment, such as the number of ob-
servers, the relevance of the questions, and the choice of
representative images.
It should be mentioned that we have received indica-tions that some of the presses were operated using sub-
optimal printing parameters, and that the printed samples
thus do not give a good representation of the achievable
quality. It would probably be much better to run the test
prints, for each device model, independently at several lo-
cations, with different operators, but obviously this would
add to the complexity of the study.
It is also worth discussing the impact that this study
has had, both within the participating companies and to-
wards customers. It was not really our goal to influence
customers to buy one device instead of another. In our
presentation at the Scandinavian conferences, we empha-sized the uncertainties and unknowns in the process, and
that there are much more than color image quality to con-
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sider when buying a digital press, but that message was
not really heard what came through was that one device
is better than the other... In hindsight, we were probably
nave not to realize the impact our study would have, and
the participating companies probably did not realize thiseither.
5. Conclusion and Perspectives
Color image quality is of very high importance in a digital
imaging device such as a digital press. For manufactur-
ers and customers of such devices it is important to be
able to quantify color image quality. However, to do so
is not a trivial task, since ultimately, quality is defined as
what the customer wants. Unfortunately, as of today there
are no analytical techniques that can quantify color image
quality in this context. It is therefore necessary to rely onexperiments involving real observers.
We claim that the notion of color image quality is ulti-
mately tied to the preferences of customers and end users.
Because of this, a very useful tool to quantify color image
quality is psychophysical experiments involving a panel of
human observers.
However, it is clear that such experiments are relatively
time consuming. Definitively, Yendrikhovskij (1999) hits
the nail on the head when he states that most studies on
image quality employ subjective assessment with only one
goal to avoid it in the future. Therefore results from
ongoing research toward analytical models for color im-
age quality is eagerly anticipated. An example of such re-
search is the development of metrics for color differences
between complex images (CIE TC8-02, 2000, Imai et al.,
2001). However, a device or algorithm that takes any im-
age as input, and provides a number that perfectly quan-
tifies its color image quality as output, is still probably
many, many years away.
Acknowledgments
Our thanks go to Peter Ollen and Aktuell Grafisk Infor-
mation AB for their support, and to our contacts at the
four manufacturers for agreeing to spend time and con-sumables on our project.
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