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Comparing color image qualityof four digital presses

Jon Y. Hardeberg and Sven E. SkarsbGjvik University College, Gjvik, Norway

jon.hardeberg@hig.no, sven.skarsbo@hig.no

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

mailto:sven.skarsbo@hig.nomailto:jon.hardeberg@hig.no

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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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