Development of Color Control
System by Measurement ofWhole Print Image for Offset
Printing Press
HIDEAKI NAGAI* 1SHOUJI YAMAMOTO* 2
SHUICHI TAKEMOTO* 3
SAKIO NAKAMURA* 3

In the operation of a printing press, color control depends very much on the operator's skill and experience.
However, with the development of the color control system by color scale, the conventional operator's skill is being
replaced by numerical evaluation of color quality, and productivity is greatly enhanced. Recently, it has been attempted to measure the whole print image and develop a color control system for the next generation based on a newconcept of controlling the print color tone. This system guarantees the quality of the image of the merchandise, andpresents a new solution for digital system of print work flow that is rapidly advancing in the directions of print
standardization and color management.

1. Introduction

2. Concept of color control system

Printing companies are being required to improve theoperation rate of the printing press and to stabilize quality to cope with the needs for smaller lots and shorter
delivery terms of printing production. Accordingly, printing press operators are required to have a very
sophisticated technical level to print products of superior quality. Such requirements include the ability tochange the machine setting in consideration of printing
conditions that change moment by moment dependingon the ambient temperature, humidity, roller temperature in printing press, state of dampening water, type ofink, type of paper, etc. In particular, color matching ofprints depends very much on the skill of an experienced
operator, and lack of experience on the part of operators
can often cause uneven color quality or increase the printing preparation time and the number of sheets withdefective printing.
With the object of enhancing productivity and matching colors of printing without depending on operators'skills, MCCS II (Mitsubishi Color Control System II) wasdeveloped in 1995. This is a system intended to adjust
ink supply amounts automatically depending on the desired color, by measuring the color scale printed on theblank margin of paper at high speed by a spectrophotometer. It is now installed at many printing companies
as a system capable of evaluating the color quality ofprinting objectively and controlling numerically, independently of operators' skills.
This concept is further advanced in a new color control system introduced in this paper. At present, it isbeing verified in the field. The paper reports mainly on
the outlook of the color control system in sheet-fed printing machines, and discusses the technical merits
introduced in this system.

The color control system using a color scale has dramatically promoted automation of color matching jobs,
but has also left some problems unsolved. Usually, theoperator of a printing press is required to match printing colors to color samples. In the case of a color control
system by color scale, when it is controlled on the basisof colors on the color scale and if the printing colors andcolor samples are different, the operator is expected tomake fine adjustments on his own decision.
In addition, color scale is not printed in all jobs - forexample, if there is no blank margin in the paper for theprinting color scale, or plate making data is not compiled in own printing companies. In such cases, the color
control system cannot be used and the quality of operation depends on the operator's skill. These are the mainfactors hindering completion of automation of color
matching.
Quality control of printing will be required at ahigher level henceforth by ISO 9000 and others. In addition, digitization of printing work flow will bepromoted, and efforts toward printing standardization
are indispensable. To cope with these trends, numerical control of color quality of the image of themerchandise is required. Against this background, thenew color control system has been developed on thebasis of the following concepts.
(1) A system capable of checking the quality of the entire printing job by measurement of the whole printimage.
(2) A system capable of controlling the color tones of theimage itself by using measurement data of the wholeprint image.
(3) A system applicable to printing standardization byuse of a full digital system.

*1 Hiroshima Research & Development Center, Technical Headquarters*2 Advanced Technology Research Center, Technical Headquarters*3 Paper & Printing Machinery Division

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

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Therefore, when measuring a printing paper of, forexample, 939 x 636 mm, measurement data of about
240 0000 points can be obtained. In control of the printing press, colors are separated into four process colors(cyan, magenta, yellow, black) from measurement dataof mixed color dots, so the light spectrum of the nearinfrared region can also be detected in addition to thevisible light region.
To realize these functions within a practical measuringtime, an original sensor head that operates by switchinglight source and photo diode array has been developed.
FigFig.. 2 shows the difference in measuring method between
color scale type and this developed system.
The sensor head includes a total of more than 2 000
photo diodes disposed in an array, and the whole printarea can be measured by one scanning. By means of theoriginal parallel processing circuit using various digital
signal processors (DSP), a very large volume of measurement data of the whole print image can be processed athigh speed.
(Note 2) Print production format. A standard format
for obtaining numerical data from information corresponding to job instructions.

3. Configuration of the system
An outline of the color control system is shown in
Fig
Fig.. 1 and its main specifications are given in Table 1
1.
It mainly comprises a suction board for spreading paper, a sensor head, a scanner, a processing board, a touch
monitor as user interface, and a desk unit including acomputer and other devices for controlling and supervising the entire system.
The operator starts operation by using the touch monitor, and necessary information is set automatically by
printing job setting by IPC II (Note 1). By measuring theprinting sample, the optimum control value of the printing press to a preset target color is displayed, fed backand controlled in the printing press as required.
(Note 1) Intelligent press control II. A computer system for supervising manipulation and operation of
the printing press.

4. Technical features of the system
4.1 Sensing technology
The main feature of the system is measurement of
the whole print image. The sensor resolution is 0.5 x 0.4mm, which is nearly equal to the image data stored in
PPF data(Note 2).

Table 1 Main specifications
Specifications

Item

ScannerLine scanning typeMeasuring method

Sensor head
Sensor head

Touch monitor

Max. measuring area (mm)1 020 (W) x 720 (H)Resolution

0.5 (W) x 0.4 (H)(mm)

Paper thickness 0.04 to 1.0mm or lessSuction boardMeasuring time

Central computer
Processing substrate

Fig. 1 Appearance of color control system

Halogen lamp

About 25 seconds(measuring condition: 939 x 636 mm paper,measuring start-result screen display)

Colors to becontrolled

CMYK process 4 colors/
special color (monochromatic)

Number ofcorrespondingcolorsPrint on one side: max. 12 colors
Print on both sides: max. 6 colors on face/
6 colors on backNumber of
IPC unitsconnected

One side: max. 4 units (recommended 2 units)
Two sides: max. 2 units (recommended 1 unit)

Photo diode array
Photo-receiver
selfoc lensSpectral sensor

Photo-receiver lensIlluminating lens
Switching light source
Illuminating cylindrical lens(b) Image color control device(a) Color scale type color control device

Fig. 2 Comparison of sensor measuring methods

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

2Here, D ( λ ) is a density spectral value of measurement data extracted by integrating and averaging
process, and Dc ( λ ), Dm ( λ ), Dy ( λ ), and Dk ( λ )are reference contrast spectra value of cyan, magenta,yellow, and black. Subscripts tc , tm , ty , and tk are ink
component amounts expressing the transfer amountof each ink onto the paper.
In multiple regression calculation, by calculating
the ink component amounts of color samples and printsamples, excessive and insufficient amounts of eachink from the target color can be ascertained. By converting these values into control amounts of ink
supplies in the printing press, color control is achieved.

By the paper suction mechanism based on the airsuction system, paper setting error is prevented during measurement. In addition, by measurement of paper
thickness by a laser displacement meter and lift mechanism of the sensor head, the sensor optical focal positionis adjusted automatically. As a result, the paper thickness can be measured stably in a range from 0.04 to
1.00 mm.
4.2 Color control technology
In color printing using four process color inks, forautomatic control of amounts of inks supplied for correction of color deviation from color samples in theimage area, the required measurement data is extracted
from the measurement data of the whole print image,and excessive and insufficient amounts of each ink must
be calculated in the each ink key unit.
Accordingly, it is necessary to position the color
samples and measurement data of the print sample correctly, and relate to the ink key positions of the printingpress. In measurement, however, various disturbances
can occur, including expansion or contraction of paperwith temperature changes, and deviation of paper position when the paper is set by an operator.
In order to create a system not influenced by disturbances, it is intended here to enhance the working
efficiency by an operator and to stabilize the precisionof measurement by realizing an automatic position
tracking function of measurement data on the basis of
the position of image data.
From position coordinates data and pattern image
data, the points suited to control are selectively integrated and averaged in ink key units. Color deviation
can then be detected even in an ink printed in a small
area without lowering the precision.
At this time, in the steeply changing portions (edges)of image mode, the color difference is largely due to aslight difference in the measuring position. In measurement, edges are likely to be disturbances, and sincehuman visual evaluation tends to feel more acutely color
deviation in the smooth region than at the edges, theevaluation is weighted depending on the smoothness of
the image.
Using the measurement data integrating and averaging the optimum region in each ink key unit and ink
type, excessive and insufficient amounts of ink from thetarget color are calculated. Measurement data after integrating and averaging process may be expressed asfollows.

5. Verification results

Color deviation in image ( E*)

Using the color control system, color control of theprinting press was tested in a state where the color
was intentionally made to deviate from the color
sample of the target color. The results are shown inFig. 33.
In process colors and special colors, the color deviation from the color sample was color difference⊿ E*<2 (Note3) in average, settling within a controllable range, and a satisfactory color control
performance was obtained.
In solid density control using a conventional color
scale, due to temperature changes in the printing pressor fluctuations of the ink and water balance, if theprinting characteristics (dot gain (Note4) , trapping
efficiency (Note5) ) are changed, color deviation occurs
mainly in the intermediate tone area of image if thesolid contrast is controlled to the reference value.
In the case of color control system, since it is intended to control an appropriate ink supply amountfor color matching of pattern, as compared with color
control using color scale, resistance to such situationis enhanced.

8: Process 4 colors: Special colors

64

20
Initial state(0)#1 (70)#2 (140)#3 (210)#4 (280)

Number of times of control (print copies)Fig. 3 Verification results of color matching control
Deviation of color from color sample of target color is expressed byE*.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)3

Color deviation in image ( E*)

3.00: Image control: Color scale control

In this developed color control system, in the light ofprinting standardization, printing a color chart of original 625 colors serves the function of generating a color
table using special software. In this table, the relation
between dot percentage and reflection spectrum is described, and a target color can be preset from the PPF
data by using this color developing table. By making thecolor table in the standard printing condition, proof printing is not required at the printing shop, and print color
control with full digital operation using the present standardization data can be realized.
By this function, digital numerical control of color reproduction hitherto required up to the pre-press process
can be extended to the press process, enabling realization of a new printing standardization system. This color
developed control system also includes the function ofmaking ICC(Note 8) profile in the printing press.
(Note 6) Computer-to-plate. A direct plate making.
(Note 7) Direct digital color proof. A digital output
machine capable of issuing a color sample.
(Note 8) A device profile describing the characteristics of devices defined by the International Color
Consortium.

2.40

2.00
1.30
1.00

0.00
Image control

Color scale control

Fig. 4 Comparison of color control performance by differencein control system
Evaluation of color deviation from color sample in intermediate tone
portion in image (dot percentage 75% or less).

Fig. 4 shows the results of a color control test in theprinting condition by changing the dot gain by 3% of thereference printing characteristic. In this test, with useof this developed system as compared with the conventional color scale type device, the color deviation in theintermediate color tone portion of the pattern has been
improved by about 45%.
In this algorithm, by averaging in each color in theink key unit, in the case of a pattern mainly composed ofsolid area, colors are matched mainly in the solid area,
or if mainly composed of intermediate colors, colors arematched mainly in the intermediate color area. If thesolid area and intermediate color area are similarly contained, color matching is neutral and a generally
satisfactory color matching result is obtained as compared with color samples.
(Note 3) Distance between two points in CIELAB colorspace. Generally, at color difference ⊿E*= 2, color
difference cannot be recognized by comparison between layers.
(Note 4) Phenomenon of growth of printing dots ascompared with printing plate. Dot gain is evaluated at 50% of plate area rate.
(Note 5) Ink transfer efficiency in multi color printing.

7. Conclusions
A color control system has been created from a newconcept of directly controlling the colors of the image ofthe merchandise by measuring the whole print image.
As a result, the quality of the merchandise can be guaranteed, and at the same time this system presents a newsolution for printing standardization and digitization ofprinting work flow. Rapid advances are being made in
the direction of color management and others.
Future digitization in the printing industry is inevitable, whether within a process, between factories, orbetween enterprises. Without being limited to the color
matching process, the entire job flow relating to production of printing must be further analyzed with the common
purpose of developing a new system that can enhance
productivity without depending on operators' skill.
References

6. Toward printing standardization(1) Hattori et al., Development of Color Control System for Offset Printing Press, Mitsubishi Heavy Industries TechnicalReview Vol. 34 No. 2 (1997)(2) Kaji et al., Development of Sheet-Fed Offset Press Control
System, Mitsubishi Juko Giho Vol. 37 No. 4 (2000)(3) Akatsuka et al., Advanced Technologies That Support Printing Machinery, Mitsubishi Heavy Industries Technical Review Vol. 40 No. 6 (2003)

In the trend to digitization of printing work flow,
CTP (Note 6) of printing plate and DDCP (Note 7) of color
sample are promoted, and printing standardization isinevitable at present. In the printing industry, therefore, efforts have been concentrated on creating
standards of reproduction of printing colors, such as Japan Color. For realizing color management, many
printing companies are also trying to set their own corporate standard colors.

Hideaki Nagai

Shouji Yamamoto

Shuichi Takemoto

Sakio Nakamura

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

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