Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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4
23,440
PRODUCT FOR ENSURING CONSISTENT ACCURACY IN COLOR
PRINTING AND METHOD OF ITS USE
S The present invention is directed to a method of eruuring consistent and
accurate color reproduction with printing devices using different priniizig
media or with
different devices using a given medium. The invention is further directed to
the com-
bination of printing media bundled with look-up tables that provide profiles
specific to
a given printing device using a given medium
Background of the Invention
In recent years printing devices using a digital input have become of
great importance in reproduction of documents. These devices include, but are
not
limited to, xerographic, inkjet, dye sublimation, thermal transfer, and wax
transfer
l5 types. Included as well are other conventional printing devices such as
o~'set presses
that racy also obtain their inputs in digital form; e.g., the digital files
used in the prepa-
ration of printing plates. Accurate color reproduction is subject to many
factors which
introduce color variation between the object imaged and its appearance on a
printed
page. There is significant variation in the color image recorded between
imaging dc-
ZO vices of different types and even variation between presumably identical
models of the
same type madc by a given manufacturer. Image recording devices are as yet
unable to
capture colors in the same way that it is perceived by the human eye.
Scanners or digital cameras are common color image rapture devices
although computer generated images; e.g., eornputer art, are also very
important. The
25 digital image capture device will create an image by encoding intensity
levels of red,
green and blue (RGB) for each individual pixel recorded. These color channels
are
typically represented by 8 bit encoding each having 2S6 intensity levels. If
printed
without modification the resulting file would form a color image noticeably
different
from that of the object imaged. In other words, it would reflect the inherent
and un-
30 avoidable bias of the image capture device. Manufacturers have long been
aware of the
inherent bias in their devices and normally include irnernaI software (or
firmware) to
compensate for it. This software is in the form of so-called "look-ap tables".
These are
initially developed empirically or mathematically using standard colortmetry
and appli-
cation of color reference standards. The look-up tables enable a Color
Management
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2
Module (software application) to accurately map the red, green and blue (RGB)
values
ofeach pi.~tel in the source image so that when it is then rendered on a
monitor or other
destination device, the image mute closely resembles the object originally
imaged_
Commercially available Color Management Profiling software or proprietary
software
can used. for creating or modifying look-up tables_ This software, developed
for a nar-
row specialty market, is complex and expetuive and requires considerable time,
e$'ort,
and skill for its e$ective use.
The problem of acourate color rendition dots not stop with the digital
image source. Biases also exist in the image reproduction device; e.g., a
printer or
1 U monitor, so that without additional correction the color values of an
input image file
would be output somewhat differently on the display or printed medium. This
bias ex-
ists between different printer models and types and even between apparently
identical
printers. Normal variations between different ink or pigment batches can
create further
coloc error. In similar fashion to that used with the imaging devices, look-up
tables are
used with image reproduction devices to compensate for internally introduced
color
error as much as possible. Here also, Color Managetneni Profiling software can
be em-
ployed to effect color modification by altering existing look-up tables or
creating new
ones.
Even as modified by a look-up table, the output of an image capture de-
vice cannot normally be used directly as the input to an image reproduction
device. It
is as if they speak mutually unintelligible languages and require a
translator. This is
because the image encoding, e.g., in RGB values, is generally local to the
device.
Stated otherwise, the encoded color information is device dependent. This
necessary
translation is accomplished by mapping the RGB values of the image capture
device, as
given by its look-up table, into some form of absolute rnlor description. This
absolute
color description is often referred to as the Profile Connection Space. Most
typically
this Profile Connection Space is the L*a*b* color system. L~a'b* coordinates
(also
called CIELAB) are a device independent color difference space established by
the
Commission lnterrrationale d' Eclairage that quantifies object color under
standard
3U viewing and illuminant conditions. The h* axis represents white/black
values, the a'
axis is the redlgreen value, and the b* axis is the yellow/blue value. An
L*a*b* triple
represents a fixed point in three-dimensional color space.
The source image look»up table (or Source Characterization Profile)
consists of its R,G,B, triple and associated Lia*b* coordinates. In a color
managed
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International Color Consortium (ICC) compatible workflow the L*a*b*
coordinates are
attached or embedded into the RGB image tile to form a tagged source image
file that
can be output as a TIFF, JPEG, or similar data format. Once a source image
file is
"tagged" with its Source Characterization Profile, embedding allows the file
to be ex-
changed or rendered on a different device since the source image file now
carries de-
vice independent color information.
Downstream, the tagged image file (along with its Source Characterize-
tion Profile) carrying L*a*b' values for each pixel, is loaded into a
computer. In prac-
tice, software called a Color Management Module relies on the Profile
Connection
Space as a common link to connect the Source Characterization Profile of the
scanner,
monitor, or camera to the Destination Characterization Profile of the monitor
or printer.
'This complex chaining of source to destination profiles is the mechanism that
enables
accurate, device specific, color signals to be sent to the printer. As was
noted earlier,
the reproduction device may use xerography, ink jet technology, or other
conventional
marking technology to produce an image on a given medium. More specifically,
the
Color Management Module software initially maps the RiG,B, source values of
the im-
age into the Profile Connection Space L*a*b*. It then,, for each pixel, maps
the L*a*b*
values into either RZG2Ba or CMYK (cyan, magenta, yellow, black) values using
the
printer's Destination Characterization Ptoflle, depending on the nature of the
printer
driver. To keep the size of profiles small. look-up tables are sparsely
sampled and re
sponsibility falls to the Color Management Module to use interpolation to
overcome the
coarse quantization. This remapping from R,GiBi to L~a*b* back to RzGZ$= (or
CMYK) accurately directs the deposition of appropriate amounts of cyan,
magenta,
yellow, and black colorants onto the medium for each pixel in the input or
source image
file.
It should be apparent that to ensure accurate color reproduction across
devices each individual make or model of image capture device and reproduction
de-
vice will require custom derived source and destination look-up tables
(proRles) re-
spectively.
There is one more very significant source of error in rendition of the ul-
timate printed image. That is the nature of the media itself Considering paper
as the
most typical printing media, the many types and brands available each have
different
chemical, physical, and optical properties that affect a printers Color
reproduction. Im-
age degradation or demodulation can occur in many ways. Some typical
interactions
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that affect color rendition are diffusion of light by the substrate, adverse
chemical reac-
dons of colorant or pigment with the usual additives of the paper, show
through from
excessive wetting, loss of sharpness from colorant spreading, and loss of
density from
colorant absorption or irLSUt~cient colorant transfer. A given color of red
may be used
S as an example. Using a given printer with the same input data file, the red
would ap-
pear different to a viewer if printed on plain xerographic, laser, coated
inkjet, or photo
glossy grade papers, to name but a few types. Similar differences would also
be noted
using the same grades of paper supplied by different vendors on the same
printer. The
color would further appear different using a given paper and the same input
data file
with different printers using the same marking technology or with printers
using differ-
ent marking technologies.
This effect of the reproduction.medium on color rendition is well recog-
nized by printer nnanufacturers. Manufacturers of more sophisticated printers
provide
several individual look-up tables, each developed to represent different
generic grades
1 S of paper. However, these tables are usually created from a very limited
sampling of the
particular medium type. In practice such profiles do not adequately represent
the vari-
ability found with commercial papers belonging to a given grade class. The
generic
grade profile does not capture media variability such as fiber type arid
pigment content,
sheet topography, optical differences, type and concentration of internal and
surface
sizing, surface treatment, and the unique chemical and electrical properties
of that
grade. Generic profiles have not been able to take into account the
manufacturing
variation within a given media type or grade from an individual rnanufacturez
or be-
tween the same grades of rnedia made by different manufacturers. The
assumption that
like fades from different manufacturers are homogeneous rarely holds true. To
etzsure
2S accurate color reproduction the Destination Characterization Profile should
be modified
every time there is a change of medium. Stated otherwise, for a given output
device
and marking method, its optimum Destination Characterization Profile is media
de-
pendent. Sometimes the profile must even be modified between different orders
from a
given paper supplier due to manufacturing variability. At best this represents
a consid-
erable nuisance factor. At its worst, if not done with care and precision, it
can result in
costly rejection of a job order by the customer because accurate color
reproduction has
not been precisely achieved.
The present invention is directly suitable for any type of printing device
receiving a digital signal and indirectly suitable for conventional presses
relying on
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digital image plate setting technology. The invention deals with media
specific output
characterization products and methods. It assumes that input and output
imaging de-
vices used in a given workflow can be calibrated; i.e., set to operate in a
known state of
physical condition. The further assumption is nnade that the color
reproduction device
has been linesrized or that its behavior is essentially linear.
Summary of the Invention
The present invention offers a solution to the problem of media varia-
tion in color hard copy reproduction. It is a vendor supplied product and
method en-
sating accuracy of color. reproduction for each speciFe type of printing
medium. The
media specific profiles of the invention effectively reduce or eliminate the
need far
look-up table development and/or modifications that must be performed by the
user of
the media. While the medium most frequently used will be paper, the terms
'=medium"
or "media" should be read more broadly to include paper-like substrates
comprising
synthetic polymers and/or fillers or other materials that can be handled in a
similar
fashion to paper.
Throughout most of the paper industry, manufacturing tolerances for
paper grades are closely held within commercially practical limits. However,
them is
still some unavoidable variation between different production lots. The method
of the
present invention first requires determining the range of variability in color
reproduc-
tion or printing properties for each specific print medium. By "specific
print, medium"
is meant a given product grade from a given manufacturer. It may be necessary
to treat
nominally identical products made by a particular manui-acturer at different
production
times or in dif~'erent mills as different specific print media. Average values
of color
rendition of the media will be determined on printed images by multiple
measurements
using statistically valid sampling techniques. The term "average values"
should be read
sut~iciently broad to include either arithmetic mean, geometric mean, or
median values.
This measurement will determine the maximum variation in color reproduction
which
might occur between different production lots of the medium or within any
given lot_
Variation is defined here as deviation from expected color. Manufacturers
strive to
keep this variation quite small. However, manufacturing realities are such
that some
degree of variability is always present. Color measurements are usually made
using a
colorimeter or spectrophotometer. These are compared with the expected values
de-
fined by the target L'a'b* coordinates using standard Color reference samples-
At least
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one look-up table will then be constructed for each specific print medium
using the av-
erage values of color rendering parameters averaged over each type of medium
and im-
age reproduction or printing device. The preferred statistic indicating
central tendency
is the median. The term "look-up table" refers to device characterization
profiles for
both input and output devices. Given types of image reproduction devices are
those
which are nominally identical to each other and which use pigment or ink
supplies
which are nominally identical from lot to lot.
In essence, media specific profiles are constructed for optimizing the
hard copy imaging system The resulting look-up table or tables will most
preferably
be supplied or made available with the medium by the medium vendor. They can
be
supplied in a number of known ways of digital data transmission; e.g., by
floppy disks,
CD-ROM compact disks, digital phone transmission, spanner or visually read
labels,
embedded in computer operating systems, embedded in printer drivers, embedded
in
raster image processors, or by downloading from the Internet. A UPC bar code
label
1 S supplied with the shipment can provide a code or key used to identify an
appropriate
look-up table. The supplier can have an TP/LJRL address on the World Wide Web
where the customer can readily download look-up table data relative to the
supplier's
specific media products. The latter method has a particular advantage to the
imaging
community that uses a number of different image reproduction devices and
different
types of paper. They can enter a medium shipment number or other media
identifica-
cion code and specify the particular device or devices to be used with the
medium Any
of these methods enables the look-up table data to be readily assigned as a
medium
specific Destination Characterization Profile for the specific printer to be
used.
It is thus an object of the invention to provide a product comprising in
combination a custom derived look-up table bundled with a specific lot of
print me-
dium.
It is also an object to provide printing media so that consistent and accu-
rate color reproduction can be readily achieved on a given printing device
using the
media without the need for the user to derive look-up tables specific to the
media
3U It is a fiuther object to provide a product with multiple custom derived
look-up tables for a particular grade of print trsedium to readily accommodate
use of the
medium with printers having different color rendering characteristics.
It is another object to provide a unique method of ensuring corLSistent
and accurate color reproduction that compensates for medium and device
variability.
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It is yet a further object to eliminate the need for the imaging community
to create or modify look-up tables for every new shipment of print medium.
It is still another object to provide a unique method for overcoming me-
dia limitations in color reproduction due to possible adverse media-colorant
interac-
S tions.
These and many other objects will become readily apparent upon read-
ing the following detailed description taken in conjunction with the drawings.
Brief Description of the Drawings
FIG. I is a flow chart showing the usual progression of data through an
image capture device creating an outflow of input data to a printer.
FIGS. 2 and 3 are illustrative of look-up rabies describing respectively
Source ChatacteriTation Protiles and Destination Characterization Profiles to
make the
input of an image capture device compatible with the output of an image
reproduction
device.
FIG. 4 is a flow chart showing the progression of input data to a printer
or other image reproduction device_
FIG. S is illustrative showing how customized look-up tables can be
used to optimize print quality of a particular printer using a given medium.
FIGS. 6A and 6B are illustrative of the method of derivation of media
specific custom Destination Characterization Profiles.
Detailed Description of the Preferred Embodiments
The present invention will be more readily understood by reference now
to the drawings. FIGS. 1-4 represent the present state-of the-art color
imaging system
processing input data into a form suitable for a data supply to a printer. As
seen is FIG.
1, a digital representation of the object, drawing, painting, etc_ to be
reproduced is cap-
tured by a device such as a digital camera or scanner. The digital
representation tray
also be created as original art or illustrations on a computer using various
forms of
graphics,software. The image capture device output is a raw digital file in
which each
pixel is assigned a numerical value for the red, green, and blue composite
channels of
the object represented by that pixel. The particular image capture device or
method
used is known to have inherent color representation limitations that introduce
biases or
inaccuracies into the raw RGB image data file. When it is desired to render
this image
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8
on another device such as a monitor or printer it is necessary to compensate
or correct
for these inherent biases. The raw RGB pixel data are used to enter a look-up
table
where appropriate corrections, unique to the particular image capture device,
are made
in the initial RGB data, FIG. 2 is illustrative of a hypothetical look-up
table for an
eight bit data capturing system. The R,G,B, values are associated with their
CIE
L'~a*b* coordinates to form a Source Characterization Profile. Commercially
available
Color Mana,g~ment Profiling software may be used to assign appropriate L*a*b*
values
to the R,G,B, values based on measurement ofcolor reference standards.
The look-up table may be one supplied by the manufacturer of the image
capture device as internal firmware or software. Alternatively, it may be
~rea~ed or
modified by the use of any of the various Color Management Profiling software
pro
grams. To ensure greatest accuracy of color reproduction, the imaging device
captures
or scans a set of physical color reference standards. These physical standards
are also
read by a spectrophotometer or colorimeter and the reference data are compared
with
the device data using the Color Management Profiling software to construct
appropriate
look-up tables.
The Source Characterization Profile data body from the imaging opera-
tion of FLG. 1 is sent to a computer. FIG. 2 illustrates how the R, G, B i
data are linked
to L*a*b; data residing in the image's Source Characterization Profile. FIG. 3
illus-
trates a second look-up table specific to the image reproduction device. This
image re-
production device look-up table consists of L*a*bi coordinates associated with
appro-
priate RaGzBx or CMYK values to form a Destination Characterization Profile.
Here
the L*a'b* data are linked to RZGiB2 or CMYK data. FLG. 4 shows Color Manage-
ment Module software chaining the Source Characterization Profile to the
Destination
2S Characterization Profile via the Profile Connection Space. The L*a*b*
coordinates of
the Source Characterization and Destination Characterization Profiles serYe as
a Profile
Connection Space used by the Color Management Module software in the computer.
Mapping of the image's color information by the Color Management Module is
done
by first using the Source Characterization Profile to convert the R,G~B~ data
to L*a*b*.
This then enters the Destination Characterization Profile with L*a*b# and
extracts ei-
then an RZGZBZ or CMYK file to arrive at color codes specific to that device.
In es-
sence, this enables the image input and image reproduction devices to
communicate
with each other in a common language. The image reproduction device will
typically
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9
be a printer of some kind such as a laser or inkjet type or could even be a
commercial
printing press.
Color Management Profiling Software titay again be used for creating or
modifying the printer look-up table in similar manner to that explained
earlier. Quality
control of the ultimate printed image is again achieved by using color
reference stan-
dards as an input tile. A colorimeter or spectrophotometer scans the printed
output and
the Color Management Profiling software compares it to the reference data,
using the
results to either construct or modify the look-up table. This is the method
used prior to
the present invention by the imaging community to correct for color
inaccuracies intro-
duced by the specifc device and medium employed.
By characterizing each specific medium for each specific image repro-
duction device the need for adjustment by the medium customer is eliminated.
This is
illustrated in FIG. 5 where two different printers are using two different
media. It must
be emphasized again that Destination Characterization Profiles are media
dependent.
Different media will give different color renditions using the same printer.
I~owever,
accurate color rendition may be achieved by cresting a Destination
Characterization
Profile (DCP) that is specific to both the printer and to the print medium
employed.
"Color accuracy" includes, but is not limited to, colorimetric and perceptual
rendering
intents. Colorimetric accuracy ensures that the L*a*b" values for the
reproduced ins
age match the original image within the color gamut limitations of the device.
Percep-
tual accuracy is aimed at obtaining an acceptable visual or appearance match
even if
colorimetric accuracy is relaxed somewhat.
In FIG. 5, DCP-I is used with printer type A using medium x and DCP-
2 is used with medium y. Similarly, DCP-3 is used with printer B when print
medium
x is used. DCP-I and DCP-2 will compensate for the differences in media. DCP-1
and
DCP-3 will compensate for the differences in printers using the same print
medium.
Similar profiles are prepared for each medium and for each printer type.
It is unique in the graphic arts for a media vendor to supply custom Des-
tination Characterization Profiles with their traditional media products.
Without such a
product there is no method for ensuring consistent and accurate color
reproduction
since the media by itself can be thought of as its own imaging device. These
Profiles
wilt have been developed with full knowledge of the characteristics and
variability
within the particular grade of paper and knowledge of the characteristics and
variability
of the particular printer type that ~i11 use the product. Custom look-up
tables specifi-
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cally tailored to the devices and media product represent a very significant
advantage to
the imaging community as a means of saving time and reducing rejected printed
prod-
uct or unprintcd media.
The present invention overcomes the limitation of the current methods
S used for destination ch$racterization. Because the aew method explicitly
incorporates
media characteristics into the image reproduction system, the system is fully
closed
since all system components have been identified. Implementation of the method
en-
sures consistent and $ccurate color reproduction in hardcopy output regardless
of the
manufacturing variation in the media and output device.
10 The invention provides Media Characterization Pmfile software that rep-
resents a significant advance over any generic correction for different media
that the
imaging device supplier might have been able to supply. The custom profiles
may be
supplied in numerous ways, e.g_, as scanner read labels on the medium shipment
or by
downloading from electronic transfer means such as the Internet.
The present invention should not be considered as limited to systems
that convert initial RGB values to a common language; e.g., L*a*b* via the
Profile
Connection Space. The oppominity exists for developments that will enable
image in-
put devices to communicate directly with image reproduction devices without
the need
for embedding a profile. This could be the case if in the future the color
encod-
ing/exchange architecture way modified so that all images were encoded and
exchanged
in Profile Connection Space (PCS) units. An example might be calibration in
[RGB)~$ rather than the present device dependent units. Further, the invention
is not
limited to color reproduction as the term "color" is usually considered. The
customized
Destination Characterization Profiles of FIG. 5 are capable of encoding
various ren-
dering features such as reducing a color image to black and white or duotone.
Con-
versely they may be used to enhance a black and white image by converting it
into
duotone or pseudo color. The invention is also useful for rendering black and
white
input images into black and white output images. All of these variations are
considered
to be encompassed within the teem "color reproduction", It will also be
understood
that CIE XYZ. CIE L*C*H*, or other color descriptions are suitable and the
invention
is independent of the color definition used for the ProFle Connection Space.
Nor is the
invention limited to reproduction devices having only a three or four color
system.
CA 02327749 2000-12-06
Examcle_
The following example illustrates the steps used in preparation of a
customized Destination Characterization ProFle for one selected paper grade
and
printer. No particular products need be specified since the method is
applicable to all
different grades of media and all types and brands of printers receiving
digital inputs.
For convenience in the present example assume an inkjet printer and an
appropriate
mufti-use paper. Characterization will normally be done by the paper supplier
who will
have full access to data regarding the paper source; e.g., mill location or
specific paper
machine within the mill, date of manufacture, changes in chemistry or furnish,
and any
other variables that might af~'ect paper performance. The series of steps will
be re-
peated for each grade of paper to be used with a given printer and again for
all of the
various grades to be used with a number of different printers.
FIGS. 6A and 68 illusuate graphically the process now to be described.
. Step 1. A paper grade is selected. Randomly selected reams of paper are
chosen for testing using statistically valid sampling techniques.
Step 2. Select P printers of a given type. Verify the device to be in a
calibrated state and linearized_
Step 3. Assemble from two to four printers P of the selected type and
provide two to three full sets of ink cartridges R to be used with each
printer. Assume
for this example that three nominally identical printers with two nominally
identical ink
cartridge sets per printer will be tested (printers P = 3 and ink cartridges R
= Z).
Step 4. Choose representative samples M of the paper grade. These
samples will include the major sources of manufacturing variability; e.g.,
source, date,
etc., as was noted above. For the present example, let us assume that the
sample in-
eludes paper drawn from three different production runs at each of mill
locations A and
B. In this case M= 6 classes.
Step 5. Randomly assign and equally distribute the ink jet cartridge sets
to the P printers so that each printer has R = 2 sets of cartridges. Install
the first set of
cartridges into each printer. Randomly assign the M= 6 paper classes to the
printers. In
this case two classes will be assigned to each printer.
Step 6. Print a color reference target_ A suitable reference is the industry
standard IT8 digital file consisting of several hundred color swatches. This
first pass
will yield 6 prints (3 printers each with two paper samples).
CA 02327749 2000-12-06
12
Step 7. Eor each of the above prints use any of the commercially avail-
able Color Management Profiling software packages and a spectrophotometer to
meas-
ure the desired color rendering properties. In this example L*a*b* values were
meas-
ured for each swatch resident in the color reference target. Store the L*a*b*
values
S cottesponding to each RGB triplet or CMYK quadruplet in a digital file.
Step 8. Remove the ftrst set of colorant cartridges and install the second
set for all three printers.
Step 9. Repeat steps 6-8.
Step 10. When all color measurements have been completed a total of
MR (6 X ?) or 12 Profile Connection Space measurement files v~iill have been
created.
Step I 1. Poot the data from all MR files. For each color swatch calculate
and store the average values of the L*a*b* parameters. The median is the
preferred
measure of central tendency.
Step 12. Construct the averaged Profile Connection Space measurement
file by assigning the average value of L*a*b* for each color swatch entry.
Step 13. Again use cocrunercial Color Management Profiling software to
process the data file created in Step 12 to construct a print medium specific
Custom
Destination Characterization Profile.
Step 14. fivaluate the Custom Destination Characterization Profile by
Visually examining color reproduction quality on selected sheets of stock
representative
of the range of variability used in the above medium-device ch~aracterizstion
analysis.
Appropriate evaluation images are those that showcase various hardcopy
reproduction
challenges.
Step I S. If the Custom Destination Characterization performs ade-
quately, the product is complete and ready for dissenvnation. If not, the
Profile Con
nection Space measurement is again analyzed and the Custom Destination
Chatacteri-
zation Profile modified as necessary until visual and instrumental performance
criteria
are met.
It will be evident that variatioru in the process not described herein
3d might occur to those skilled in the art. The inventors regard these
variations to be in-
cluded within the scope of their invention if encompassed within the following
claims.
