Note: Descriptions are shown in the official language in which they were submitted.
CA 02235015 1998-04-16
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-1-
The present invention relates to a method for ensuring colour consistency
during a printing run. More specifically, this invention relates to a computer-
mediated
method for real time colour calibration and correction during printing runs.
BACKGROUND OF THE INVENTION
In the printing industry more reliance is being placed on printed output
obtained
directly from a computer. Matching colour and density from the output computer
file to
the paper output on a 4 colour press or copier has always been an issue. There
are many
factors that can affect the colour of the final output.
Typically, one of the largest and costliest facets of the pre-press arena is
that associated
with stripping. This is where traditional camera work and layout combine
computer
generated output for the final production of a plate to go on a press.
Stripping is very
time consuming and labour intensive. Components of the final output are
combined by
cutting and pasting the various components, in register, on four (for process
CMYK) or
more (Varnish or Pantone) separate layers to produce the desired output. In
recent years
computers have been used more frequently to create larger pieces of film.
Today there
are image setters capable of producing a sheet of film the size of the plate
required,
virtually eliminating the need for stripping. The file is sent to the
stripping department
for the creation of colour proofs or "blues" to check content and layout.
Stripping
departments have shrunk appreciably as these system have become cheaper and
more
proven.
The latest stages of pre-press are capable of entirely eliminating the film
stage from the
production process. Systems are now becoming available that can create a
plate, ready
to go on the press, directly from a computer file. There is also technology
that can etch
an image directly on a cylinder from a computer file. The cost savings in
eliminating
film, developer and stripping is making direct to plate and direct to press
technology
extremely attractive.
CA 02235015 1998-04-16
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Traditionally, the printing industry has relied on film to produce colour and
monochrome
(blues) press approvals and client approvals. However, computer technology has
progressed such that proofing technology is available directly from the output
file.
Current systems rely on scanning a plate to get colour information used to
preset a press.
Colour control and calibration largely relies on the human eye, in some cases
video or
colour scanners are used to check colours on output. These values are recorded
and
compared to desired levels in an off line method.
Large printing j obs require consistency over the entire printing run. This is
currently
done by periodically checking the colour bars by eye, and re-calibrating the
output
device. Outputs on a many presses rely on colour bars along the sides of the
colour
output in waste axeas to check primary colours. These are used to calibrate
the press for
primary colours. Colour copiers use calibration sheets to check colour,
however, this
process is time consuming and interrupts the timely printing of the job.
U.S. 5,149,960 and 5,015,098 disclose the conversion of scanner signals into
colour
signals for the reproduction of colour originals.
In U.S. 5,218,671 there is disclosed an image colour correction system that
involves
assessing the image produced by the computer-aided design system to ensure
that the
desired appearance is obtained before the product is reproduced on an output
device.
There is no disclosure of detecting the colour within the printed product and
using this
information and correcting the colour during a printing run. Lloyd et al
(I1.S. 5,508,826)
describe a self calibrating colour printer based on the comparison of a known
standard
colour test patch which is stored within the printer prior to its use, with a
printed test
patch during a printing run. However, there is no assessment and comparison of
the
actual printed product against itself during the print run, once again
compromising colour
quality and consistency during a print run.
A method for calibrating a colour copying apparatus is described in U.S.
4,464,045. This
method uses a device that detects a calibrated seven-step grey stepped wedge
produced
CA 02235015 1998-04-16
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upon a printed product. The calibration signal obtained from the grey stepped
wedge is
compared with that of the original calibration signal. This method does not
permit
continuous monitoring of the printed product, and requires the use of a grey
stepped
wedge to be printed alongside the printed product. Furthermore, since the
actual printed
product is not being sampled for colour quality, there is a greater chance of
error
associated with improper colour correction. Similar methods, that of
evaluating colour
test strips, is also disclosed in Lecha (U.S. 4,752,892), and Brunner (U.S.
4,852,485).
The process of this invention involves using a computer output file that
creates a high
resolution colour proof, and plates. The information obtained from the output
file is also
used to pre-set the colour keys on the press and monitor and control colour
correction as
required during a printing run to ensure consistency, minimize down-time and
human
intervention during a printing run. As this process is performed continuously
during the
printing run, there is little or no interruption to the run itself and the
accuracy and
consistency of the output product is maintained. As plates or cylinders are
changed on
a press, a process begins that is known as a make-ready. The press is running,
but the
material coming off the press is not of sufficient quality to be sold as a
final product. The
operators of the press make many adjustments to control inputs (colour keys,
register,
plate twist and lead/lag) until the output is acceptable. By using the RTC
system, the
colour keys can be set more accurately the first time to reduce the make ready
time. The
RTC system can access historical press run data and use information in memory
to pre-
set colour keys much as an operator would, although with less error and in
less time.
Operators of presses rely heavily on past experiences to assess and correct
problems with
output. The RTC system could be programmed in a similar matter to recognize
patterns
such as impure inks, dirty blankets or other press run problems.
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SUMMARY OF THE INVENTION
The present invention relates to a method for ensuring colour consistency
during a printing run. More specifically, this invention is directed to a
computer-
mediated method for real time colour calibration and correction during
printing runs.
According to the present invention there is provided a method for real time
colour
correction during a print run comprising:
a) obtaining an initial computer generated file;
b) presetting the colour keys of a printing press by scanning the initial
computer file using press-specific software;
c) preparing a printed output of the initial computer generated file;
d) determining at least one suitable area of the printed output for
monitoring during the print run;
e) obtaining a first set of suitable area data for comparison during the print
run from said at least one suitable area;
f) starting the print run;
g) selecting a printed product during the print run;
h) obtaining a second set of suitable data from said at least one suitable
area of said selected printed product;
i) comparing said first and second sets of suitable area data;
j) adjusting the colour keys of the printing press as needed in order to
maintain the desired colour during the print run.
In an alternative embodiment of this method, the steps h) to k) are repeated
at least
once during the print run.
This invention is also directed to a method as described above wherein the
computer
generated file is selected from a scanned image file, a direct input file, a
processed file,
or a combination thereof, preferably the processed file is a bitmap, or TIFIT.
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This invention also provides a method as described above wherein the suitable
area is
of continuous tone, or is comprised of a plurality of suitable areas including
at least
one area of continuous tone and a colour control bar.
The present invention also provides the above described method wherein the
step of
obtaining a first and second set of suitable data involves scanning said
suitable areas.
Preferably the scanning is done using a video camera, CCD camera, or
densitometer.
This invention is also directed to the above method wherein in step f) a
predetermined
tolerance range is established, and wherein step j) involves comparing the
second set
of suitable data with the first set of suitable data and determining if the
second set of
suitable data is within the predetermined tolerance range.
An object of an aspect of this invention also embraces a device for real time
colour
calibration and correction during a print run comprising:
a) an RTC computer that receives original input files, determines suitable
areas, receives and processes continuous calibration data derived during
a print run from suitable areas, and presets and resets the colour of a
printing press during a print run;
b) a scanner that obtains suitable area data to be processed by the RTC
computer from the printed output and printed product;
c) an RTC scanning controller that activates an output bypass to select one
printed product for suitable area scanning, obtains CMYK colour values
for each point of a suitable area to produce suitable area data, and down
loads the suitable area data to the RTC process controller;
d) an RTC process controller that compares a first set of suitable area data
obtained by the RTC scanning controller with a second set of suitable
area data, and adjusts colour keys of the printing press if needed.
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It is noted that within the prior art there are employed colour correction
systems that
monitor the colour of test strips printed alongside the printed image,
however, this
approach does not allow for correction arising from differences detected in
the printed
image itself. This invention is directed to colour correction systems that
monitor and
correct print runs using data obtained from the printed image itself.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent from the
following description in which reference is made to the appended drawings
wherein:
FIGURE 1 is a diagrammatic representation of the RTC system indicating the RTC
computer (10), RTC scanning controller (20), scanner bypass (30), press and
data collection devices (40). Also shown are the proofing printer (50),
imagesetter platesetter (60), a four colour press (70) and raster image
processor
(RIP, 80)
FIGURE 2 is a flow chart depicting the overall RTC process
FIGURE 3 shows a flow of an embodiment of the present invention depicting the
overall RTC process.
FIGURE 4 shows an example of a printed output of an initial computer generated
file
(Figure 4 (A)), and a method for selecting suitable areas for monitoring
during
a print run (Figures 4 (B) and (C)).
CA 02235015 1998-04-16
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DESCRIPTION OF PREFERRED EMBODIMENT
This invention is directed to a process for the continuous monitoring and
control
of colour correction and calibration during a printing run.
Files with little colour or large amounts of text may need little colour
control during a
printing run, however, these printing runs may still benefit from the
application of the
RTC system in that the process is continuously monitored and regulated to
ensure
consistency during the print run. On the other hand, more complex files with
varied
patterns by their very nature require constant monitoring of the colour
consistency
during a print run and are problematic in this regard. Therefore, printing of
such files is
especially suited to the method of real time colour calibration and correction
of this
invention.
By "RTC system" it is meant the combination of devices that calibrates,
controls and
corrects the colour printed onto a printed product (see Figure 1 ). This
system comprises
several components including an RTC computer and control console ( 10), RTC
scanning
controller (20), a scanner and output area(30), and RTC process controller
(40). The
RTC computer (10), scanning controller (20) , and process controller (40) are
defined in
more detail below. The system also comprises a proofing printer (50) and
imagesetter
plate setter (60) both operatively connected to a raster image processor (RIP;
80) capable
of converting a computer file to a format that the output can utilize, and, in
the case of
a printing press arrangement, a four colour press (70). The initial computer
file may be
created using a layout/creation computers (90) associated with RIP
By RTC computer (10), it is meant the computer that receives all original
input files,
initial and continuous calibration data derived from the output during a print
run, presets
and re-sets the colour keys of the press during the print run. The RTC system
also has
direct access to controllable parameters on the press so it can make
adjustments as
required. There would also be several terminals hooked up to display
information to
users. The terminals would be used to present information to press and pre-
press
CA 02235015 1998-04-16
_g_
operators such as tolerance levels for output, flags to warn of possible
problems if the
system is unable to compensate for values out of range.
By the RTC scanning controller it is meant a device that can change the
position of a
colour level reader and take, record and download readings. Video technology
can also
be used to take readings from fast moving material if a shutter mechanism is
used to keep
in sync with the press speed and controls a scanner.
By RTC process controller it is meant a device that can accept position
information from
the RTC system and move a colour level reading device to the appropriate
position to
take a reading. The information is stored until all the readings are taken and
then sent
back to the RTC system for analysis.
In the context of this invention "printed output" is defined as the initial
product
produced by the RTC computer (i.e. a proof) that is used for the initial
setting of the
colour keys of the press, and for determining the first set of suitable area
data (see
below). "Printed product" is defined as the product obtained from the printing
press or
the like during a print run. Second, third and subsequent sets of suitable
area data are
typically obtained from the printed product.
The process of this invention (see Figures 2 and 3 ) utilizes a computer
generated output
file , i.e. the "initial file", that is created (90) and sent to film or
plate, and entered into
a computer that monitors overall colour correction and calibration during a
printing run,
i.e. the Real Time Colour (RTC; 10) computer. The initial computer generated
file may
be prepared by an assortment of methods including scanning of an image via
standard
known techniques or direct input of the image data as provided by a customer.
A key
feature of the RTC system is the tie to the original computer file to the
output. The
computer file type is defined by the equipment being used to create the film,
plates, direct
to press or guavre cylinders. Common types are Postscript, TIFIT, SCITEX and
others
CA 02235015 1998-04-16
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are being developed frequently. These files contain all the information that
is needed to
create the output, text, graphics, and most importantly, colour information.
This file is down loaded onto the RTC computer via network connection, or via
tape
(DLT (Digital Linear Tape), Exebyte, DAT (Digital Audio Tape)) or removable
media
such as hard drives or drive shuttles. This will also provide a backup of the
original data.
A large number of output devices that are currently in use, use file formats
that may
contain an excess of data not required for the production of the final printed
product. For
example, files created using Adobe~ Postscript, may need to be converted into
an image
file have extra information that is hidden behind other artwork (two scans
slightly
overlapped or text over a large bitmap), This makes Postscript files easier to
edit but can
make them harder to work with. It is to be understood however, that other file
types are
continuously emerging, and the use of a Postscript file is an example of the
many files
types that may be used. However, the entire bitmap can be stored, even the
areas that are
covered by other objects such as text. The RTC computer converts (RIP; 80) the
postscript to a large image file, and removes extra information such as text
or obj ects that
are hidden behind other obj ects. This reduces the size of the file being
worked on and
increase the speed at which the file can be interpreted. An example of a
commercially
available product that converts postscript to a bitmap is Freedom of Press or
Raster Plus,
which makes a postscript to Bitmap RIP (Raster Image Process) that could be
used for
this purpose. An example of a file type with the extra information stripped
out would
be Standard Interchange Formats such as TIFF/IT (Tagged Image File
FormatlInformation Transfer) and PDF (Portable Document Format). TIFF/IT is a
raster-
based file format that has all the information necessary, but is at a fixed
output resolution.
PDF is a follow-on to Postscript but has not been pre-processed. These file
formats
contain less extra information than a corresponding Postscript file.
Files that are either converted or that do not require conversion, for the
purposes of this
invention, are termed "processed files". However, it is to be understood that
a processed
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file includes both converted files as described above, or files that do not
require any
conversion, and are useable in their initial format.
The processed file within the RTC computer is then scanned (60) by press-
specific
software so that the colour keys on the press are preset (70). Examples of
press specific
software include, but are not limited to Heidelberg, Kimoto or Omni-Adast who
have the
ability to electronically control press from a digital control panel with an
external
interface. This step reduces the make-ready time for presetting the press
itself and is
analogous to the scanning of plates to pre-set a press as would be known to
one of skill
in the art. Typically a processed file comprising a bitmap of sufficient
resolution is used
for the scan, however, as would be evident to one of skill in the art, the
degree of
resolution may be determined by experimentation. The file does not necessarily
have to
be a bitmap. A postscript, or other file can be scanned and interpreted as
well, but due
to file size and complexity, bitmaps of lower resolution may be used with
varying
degrees of success.
The next step involves making a high resolution colour proof of the processed
file (50).
This colour proof is termed "printed output" and is defined as the product
produced by
the RTC computer that is used for determining the first set of suitable area
data (see
below). This printed output may be approved by the client, if such approval is
required,
and used to compare desired colour levels of the output file as defined by the
processed
computer file, with the actual colours of the printed output. This is done
using
densitometers, colorimeter, reflective spectrophotometer and the like,
interfacing with the
RTC system as described in more detail below (30).
The RTC system can bring up on screen a representation of the output, and show
the user
where to scan. Once the scan is done (30), these readings are stored for
further
interpretation. An automated method using a robotic arm could also be used
where the
2 dimensional robotic arm could move itself to the correct position over the
output and
take readings. This comparison calibrates the RTC computer and provides the
RTC
computer with a baseline for all further comparisons during the printing run.
This can
CA 02235015 1998-04-16
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be used where differences in the computer file from working on a monitor are
significantly different from reflective output from a proofing device. This
information
can be used in adjusting the colour proofing device or to indicate that
monitors being
used are not properly calibrated. However, during the printing run the entire
surface of
the printed output need not be examined for comparative purposes. Rather,
areas of the
printed output may be identified so that the comparative process proceeds in a
timely
manner. These areas are termed "suitable areas" for the purposes of this
invention. It is
to be understood, however, that the entire surface of the printed output may
be considered
a suitable area, and used for comparative purposes during a printing run if
this is desired,
for example when less complex output files (output that has many small highly
detailed
areas where colour is very important (for example, company Logos or pictures
of cars...
etc.) are being printed. Furthermore, suitable areas may be determined
directly from the
computer file.
Suitable areas of the printed output fulfilling the criteria listed below may
be identified
for continuous monitoring during the print run.
1 ) the suitable area is of continuous tone - this is important, as areas of
continuous
tone provide the most accurate readings;
2) the suitable area should be large enough - the size of the area to be
measured is
largely dependent on the scanning device with different devices matched with
the
output product as required;
3) the suitable area should be large enough to necessitate multiple readings -
this
criteria ensures consistency in output as large areas of continuous tone
colour
permit, multiple readings;
4) readings should be obtained from several suitable areas of the output - it
is
desirable to obtain readings from many evenly spaced areas of the output
unless
the output is relatively uniform.
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The printed output is scanned (30) for colour information and suitable areas
that meet the
above criteria are located using pattern recognition software. There are many
different
scanners that could be used scan the printed output and store the information
within the
RTC computer. For example, and this list is not intended to be limiting in any
manner,
video, CCD camera, densitometry, colorimeter, spectrophotometer, or other
optical
methods know to one of skill in the art may also be used for this purpose. The
information obtained from the scanned suitable areas is termed "suitable area
data" for
the purposes of this invention and this data is stored in the RTC computer.
More
specifically, the data obtained from the scanning of the printed output is
termed "a first
set of suitable area data". Data obtained from the printed product during a
print run is
identified as "a second, or third, set of suitable data" etc as the case may
be. Also, it is
to be understood that suitable areas may be obtained directly from the
computer file.
Once the RTC computer has a list of suitable areas to choose from, a process
begins
that will select the correct number of areas (typically user defined) to
provide the
greatest coverage. Since having all the readings taken on one small comer will
not
provide colour consistency over the entire output, selection of suitable areas
typically
is based on the proximity of the selected areas so that they are neither too
close nor too
far. This could be done, for example, with a grid where all areas are
superimposed
on a grid, and the computer randomly picks the suitable areas to be scanned. A
user
interface could also be implemented if specific areas were identified that
required
scanning (e.g. company logos or trouble areas). Random selection of suitable
areas
is easy to implement, however, it may not always afford the best selection of
suitable
areas.
It is common for the printed output to have colour control bars located in the
waste areas
that are trimmed or folded at latter stages. These colour bars can also be
used as a
suitable area for scanning and comparison during a printing run. One of the
goals in the
selection of points for colour reading is consistent coverage of the page. The
number of
points selected on a page for analysis as defined above may be a user defined
parameter
or a system default. Depending on the customer, the j ob type and requirements
for
CA 02235015 1998-04-16
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accuracy, the number of points to take readings from can be adj usted to
provide more or
less accuracy. The operator might want to pick the top 10 points to take
manual readings
from or might want the system to use a robotic scanner to take readings from
100 points.
Automated collection devices will be able to take more readings than a human.
An automated method uses a pattern recognition program to find suitable areas
to scan.
The suitability of the area is determined by the scanner head size,
resolution, and
viewable area. The scanner must be able to read continuous tone (one coor
only) so
the area must be of sufficient size (determined by minimum scannable area for
the
specific scanner) and not so small that neighbouring colours cause erroneous
readings.
The pattern recognition software searches the output file for areas that meet
the
requirements. A grid could be used to separate large areas into multiple
reading areas
(see Figure 4 (B)) . Once all the suitable areas are found (to a user defined
maximum)
a number (user defined) of them are selected for use as colour calibration
points (e.g.
see Figure 4 (C)). The grid size would depend on the output size as well as
the
scanner head size. Once the points are defined, the RTC system would choose
randomly among the points to select areas to scan. The user would have the
option of
overriding or selecting specific points. The grid method with a random
selection
would provide a stable method of choosing points to be scanned and assuring
average
coverage. By choosing 15-50 points on the output to scan out of several
hundred
possible areas, good coverage can be achieved. The grid method also allows the
RTC
system to provide a scanner controller with X and Y coordinates to perform the
scans .
This grid method can be applied to all methods in the RTC system. The file
scanned
to determine the scan points can be the bitmap generated from the image taken
off
output in the bypass as in Method 1 (see below) . The file scanned can be the
original
output file from method 2 or 3 (see below).
Colour photocopiers may also use the RTC process. Colour copiers may already
have
a built in scanner suitable for this purpose, or they may be modified in order
to add-on
a scanner suited for this purpose. A colour copier can be easily calibrated by
placing a
CA 02235015 1998-04-16
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colour target on the glass (as would regularly be done for regular
photocopying), and the
scanner-read colour information can be compare to predicted values, and
adjustments
made accordingly. The advantage to using the RTC process, is that, prior art
methods
require the use of colour bars, however, by placing a piece of output on the
glass
(manually or by an automated bypass) colour values can be read and compared to
colour
values in the original output file. Since the scanner and glass are already
present within
such copiers, they may be used for this purpose, however, it to be understood
that, a
suitable scanner may also be mounted on the output path for this purpose.
Parameters, such as paper type (typically selected from a library) and output
orientation
(which edge of the printed output is the leading edge), temperature, humidity,
varnish
(coating on printed product) are entered on the RTC computer. The plates are
installed
on the press and their register and orientation are checked or displayed to a
user with a
handheld scanner on a screen. For each point of the image there is an X and Y
coordinate as well as cyan, magenta, yellow, and black (CMYK or CMYB) colour
value.
The position and colour information for each point is recorded and down loaded
to the
RTC scanning controller .
Once the press is at speed, the RTC scanning controller activates an output
bypass (30)
to select one sheet of printed product upon which pre-selected suitable areas
are scanned
and a second set of readings is obtained. This is applicable largely for
sheetfed
operations (presses). An inline video system that can look at web press output
at speed
would be a way of taking readings from a web press. For example in a web
press, a piece
of output can be pulled and placed on a vacuum table and scanned. The scanner
reads
colour information from each point, and stores these values in the RTC scanner
controller
and moves to the next point. Once all the readings have been taken, the second
set of
readings have been obtained and the RTC scanning controller sends all the
information
back to the RCT computer for comparison with the first set of suitable area
data already
entered within the RTC computer.
CA 02235015 1998-04-16
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The colour readings are taken and broken down into their respective four
colour levels
CMYK (note "V" in Figure 1 refers to varnish; both K and B refer to black).
The
colour levels of the current and previous readings are compared to one another
and to
the predicted values from the original computer file entered within the RTC
computer.
If the values are of the most recent set of readings are within predefined
tolerances to
the computer-file values, no more adjustments are necessary, however, readings
will
continue to be taken at pre-defined intervals throughout the print run. The
chart
presented below shows how colour keys effect colour levels in output. By
opening or
increasing the colour at one colour key, the colour level for that colour is
increased and
the others lowered. The amount of colour changed per 'turn' of colour key is
press
dependent:
Cyan colourMagenta colourYellow colourBlack colour
key key key key
+1 +1 +1 +1
1$ Cyan Colour+ - - -
Level
Magenta - + - -
Colour
Level
Yellow Colour- - + -
Level
Black Colour- - - +
Level
This chart may also be expanded to include variables such as temperature,
humidity,
paper type and PMS colours and their respective impacts on measured colour
levels.
Colour keys are used to adjust colour levels on Bone Web and Sheetfed presses.
A
colour key refers to an adjustable screw, key or lever that can be adjusted to
push a
small plate against a roller or drum. This resulting pressure allows more or
less ink
to get on the roller. If more or less ink is on the roller, the resulting
output will reflect
this. Colour copiers use a different method to adjust colour levels using
static
electricity to attract toner to the paper.
CA 02235015 1998-04-16
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Different presses and output devices have different methods of adjusting the
colour
levels of output. The RTC system requires a computer control and interface to
the
colour level adjustments on the press. The RTC corrections to colour keys on
the press
would be press specific and would have to be programmed into the RTC system.
Presses of the same make and model might have significant differences in their
output.
When selecting the appropriate colour keys to adjust to make a change to an
output
colour, decisions must be made by the computer based on the entire output, not
just
that one area. Adjusting colour keys is a balancing act where adjustments made
to one
area to bring it within tolerances can have adverse effect to colours in other
areas.
Thus the colour levels must be adjusted to achieve the best possible overall
output.
This is especially true where multiple copies are being printed on one sheet
of output.
Press operators are able too used past experience to recognize and correct
many different
1 S problems that can occur during the course of a run. The RTC system will
use an adaptive
controller to look at a database of historical information to see if it can
recognize a
pattern is the current job or data that it has dealt with in the past. When it
first examines
the output file and gets the j ob information (paper and ink types, press), it
can look to see
if a similar job has been done in the past. Using historical information can
help reduce
the number of steps necessary to set up subsequent press runs.
The RTC system using adaptive process control will have the ability to make
adjustments
as it gains experience during a printing run, or over several printing runs.
The adaptive
multivariable process controller is the computer that controls the RTC
process. It is
adaptive because it is able to use historical data to increase e~ciency and
decrease the
time to make adjustments. It will also be able to predict, again through
historical data,
when or why colours might begin to drift. Temperature or humidity variables
might
not come into play in some areas of the world, but in others they can have a
large
impact. For example, if a controller identified that historically, the last
time it was
28C and 95 % humidity, after 25,000 impressions, the magenta values dropped,
it
would begin to make adjustments to compensate. Also, other variable may need
to be
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considered including the reflective effect of the varnish coating applied to
the printed
output, paper weight etc.
Therefore, it is understood that by RTC process controller, any adaptive,
multivariable
process controller or the like may also be used. The RTC process controller
uses a
process model that calculates values from the output file and compares these
with
readings (suitable area data) obtained by the scanner to make corrections on
the output
device. Temperature and humidity readings may also be evaluated as required
and used
for the correction of the printed output. The same j ob may not require the
same
adjustments as temperature or humidity change. By taking readings of
temperature and
humidity the controller will be able to reduce the number of steps to get the
output within
tolerances.
For the RTC system to operate effectively, an appropriate design and testing
of a working
process model is required, for example the process model is used to determine
whether
a colour has the correct density or CYMK values as compared to the output
computer
file. Canon uses software to calibrate its copiers to check the density of
CYMK sheets
that print out. It does not test spot or mixes of the colours.
All of these adjustments are made based on quantitative measurements and are
performed
in real time, while the press is operating thereby reducing down time of the
printing press
itself. These adjustments are carried out by the process model by comparing
predicted
colour values to actual values determined by the scanner. For example this
algorithm
essentially performs the following process, however, it is to be understood
that any
adjustments, and colour keys, will be press-specific:
1) an area is required to be 10% cyan, 40% magenta, 20% black and 30% yellow.
If there are no differences between the printed output , as determined by the
scanner, and the reference values, then no changes need be made through the
RTC system.
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2) However, if the actual readings are 10%Y, 40%M, 18%B and 32%Y, then black
and yellow deviate from their expected values.
3) Once a deviation is detected, then the reading s for the entire page are
compared
to verify whether or not the colour is off for the entire page.
4) If the colour is off over the entire page, then the required yellow and
black colour
keys are adjusted by an appropriate amount as defined in the individual press
information table.
5) If the colour is off in a localized region, then the required colour keys
on yellow
and black for that region need to be adjusted by the appropriate amount.
As there are hundreds of thousands of possible colour combinations from the
same set
of plates by adjusting all the colour keys, an important feature of the
process model is the
determination of which colour needs to be adjusted in order to make a colour
change. By
simply adjusting the black level, the appearance of a process colour can
change
dramatically. Every output device is different and will require some custom
setup.
After taking a set of readings during the printing run, and making
adjustments, the
calibration process would restart using the same set of previously selected
points or
suitable area. Once a set of corrections has been made via the colour keys, or
toner levels
(in the case of a colour copier), the bypass is activated by the RTC scanning
controller
and a third set of readings is obtained on a selected printed product using
the same set of
previously selected points or suitable regions,. The RCT computer, which can
be one
computer running multiple programs and processing different inputs (readings
from
scanners or postscript file scans, an appropriate program would run and
manipulate the
data as required), compares this third set of readings to the initial
predicted values, or first
set of suitable area data, as well as the second set of readings to see if the
corrections
implemented after the first set of suitable data (readings) have resulted in
an
improvement. The amount of change between two sets of readings (0) is used to
make
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further adjustments. An example of how an adaptive multivariable process
controller
will be able to use past corrections and readings to decrease the number of
steps required
to bring a piece of output within tolerances is given as follows:
1 ) If the first reading, black was off by 10%, then a correction was
performed and
the historical database is written to within +5 black points on a given colour
key.
2) The second reading indicates that black off by 8%, therefor, following the
second
correction, the historical database is written to and an adjustment of +20
black
points is made to the given colour key.
The software will get the output as close to the predicted colour values as
user defined
parameters dictate Depending on the device, paper type and j ob qualities,
operators may
elect to allow a great range of output colour levels. This means some output
may not
require as exacting tolerances as high-end work. Low quality newsprint cannot
yield the
quality and consistency that a heavier weight glossy paper can. If the
historical data
being used to make corrections is unable to correct colour variances, or the
results of the
corrections are not what the RTC systems, the RTC system will revert to the
original
process model.
Therefore, this invention provides for a method for the for real time colour
correction
during a print run. This method (see Figures 1, 2 and 3) involves:
1 ) creating an initial computer file (90), for example as a Postscript,
TIFF/IT, PDF
etc. file in layout creation;
2) Rasterizing output files in RIP (80) and sending RIPped file to image
setter (60),
proofer and RTC computer ( 10). RTC computer may assign a j ob number which
is used to track the job;
3) producing plates or film from imagesetter (60) and plates put on press
(70);
4) producing a full size proof using proofing printer (50), determining
suitable areas
for colour readings;
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5) Examining output file and using data supplied by RTC consol (15; i.e. press
being used, output size, leading edge, process or Spot colour), presets colour
keys
on the press (70), and this data is sent to the Press Control (40);
6) Presetting colour keys on press (70) via Press Control (40) or other
suitable
method;
7) Initiating press run;
8) Monitoring status of print run - RTC scanner controller (20) receives
information
from RTC Computer (10) on position and colour levels for every point where
readings take place, and the interval of the reading;
9) Obtaining reading of the printed output using the scanner (30) and sending
information to RTC Computer (10);
10) Analyzing information and new colour key settings generated and sent to
press
control system (40);
11) Setting colour keys to new levels and repeating steps 8-11 until output is
within
predetermined tolerances;
12) Repeating steps 8-11 at predetermined intervals, following obtaining an
output
that is within predetermined tolerances (step 11 ), and using scanner and
bypass
(30) as required to monitor print run.
There are three distinct methods in which the RTC system can be used.
Method 1
The most basic method involves providing colour control and computer based
correction
to a job throughout the course of a run. This requires the user to correct the
colour
manually on a press control panel, and once the colour is correct according to
the
operator, the RTC system will be activate. The RTC system maintains the colour
at
current levels by scanning a piece of output, storing the readings, and
comparing future
readings to the first set, and making corrections as required.
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In addition to a scanner, this method would require the use of a digital
camera or video
system to locate areas of the output to scan. The camera would be mounted
above the
bypass area, and be able to capture the entire output in one image. The bitmap
image
from the camera is used to fmd points on the output to read. Colour
information of the
image would not be necessary as the user is in control of the press and sets
the colour
levels. The same pattern recognition software is used to find areas to scan
that will yield
the best output consistency. These readings are sent to the RTC scanner
controller, in X
and Y position format. Readings would be taken and down loaded to the RTC
system.
These readings are then used by the RTC software system to compare to readings
during
the course of a run.. The RTC scanner controller activates the bypass at a
user defined
interval and checks the colour of the same suitable areas it checked for the
first reading.
Corrections are made as needed and the user is notified if the output is out
of preset
tolerance levels.
Method 2
The second method involves the use of the computer generated postscript or
other output
file to find areas to scan and colour values, which are processed by the
process model
outlined above. This method requires full sized output files, with no
stripping of parts
in or out of the final film. This allows the RTC system to determine exactly
what is on
the output, and the location of items within the output.
Direct to plate, direct to press, and full size film output from computer
files (postscript)
are all considered to be variations on the method as described herein.
Modifications of
the RTC system for each output device may need to established as needed,
however,
these modifications do not depart from the spirit of the method as disclosed.
Method 3
The RTC system can also be applied to use for colour proofing with slight
modifications
to the workflow. Once a file is ready for the colour proofing stage,
information is sent
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to the RTC system information in a mariner similar to that if the output was
going to
press. A colour proof is created from the output file and then placed in a
scanning area
similar to the one outlined in Method 1. The RTC system may use a digital
camera,
calibrated video, offline or inline colorimeters or densitometers to create an
image of the
proof. The RTC software analyses the bitmap and find suitable areas to scan,
then the
XY information is sent to the scanner controller and colour readings taken.
These
readings are stored along with the XY position information.
After customer approval of the colour proof, the plates are created and the
press set up.
The RTC system can monitor colour correction and quality control during the
print run
from known colour values approved by the customer.
This method combines the colour proofing and outputting stages with one
system. The
RTC system maintains logs of colour readings, corrections, and tolerances and
create
reports should the customer so desire.
Other benefits of RTC
In order to implement RTC, a relatively modern press is required. Modern
presses are
capable of keeping them selves in register and maintaining some consistency.
It is
contemplated that designers and manufacturers of presses will be able to use
RTC as a
diagnostic tool as well.
If RTC is unable to correct a colour or the corrections are not producing the
desired
changes, other factors might be at work. Factors such as old blankets, ink
contamination,
and poor water levels could be responsible. Ink contamination can diagnosed by
the RTC
system due to the accuracy of the colour scanner.
An important benefit of the RTC system will be quality control. The ability to
quickly
make corrections on during a print run will reduce waste and improve customer
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satisfaction. The ability of the RTC system will allow the operator to provide
and audit
trail of colour corrections, readings and calibrations.
The present invention has been described with regard to preferred embodiments.
However, it will be obvious to persons skilled in the art that a number of
variations
and modifications can be made without departing from the scope of the
invention as
described in the following claims.
