CA2235015A1 - Real time colour calibration and correction (rt3) - Google Patents

Abstract

This invention is directed to a method and system for real time colour calibration and correction during a colour printing run that is by an RTC
computer.
This method involves creating a computer generated output file that is loaded onto the RTC computer, modifying the file as needed to remove any extraneous data, scanning the file in order to preset colour keys and set the RTC scanning controller with initial colour values, preparing a high resolution colour proof, determining suitable areas within the image region of the proof for use as calibration points that are monitored during the printing run, selecting a printed output during the printing run, scanning the pre-selected suitable areas of this printed output, storing the information within the RTC scanning controller and sending the information to the RTC computer for comparison with the initial colour values, and adjusting the required colour keys to correct for any differences noted in the printed output. The use of the RTC
system ensures real time colour correction during a printing run with minimum down time and colour consistency over an entire print run. This is especially useful for large printing jobs where colour consistency is an issue and difficult to maintain.

Description

.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.

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 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.

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.

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.

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)).

_7_ 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 _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 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 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 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.

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 _13-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 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.

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.

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 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.

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 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;

-2.0-5) 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.

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 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 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.

Claims (12)

1. 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.

2. The method of claim 1 wherein the computer generated file is selected from one of the following: a scanned image file, a direct input file, a processed file, or a combination thereof.

3. The method of claim 2 wherein the processed file is selected from the following group: bitmap, or TIFIT.

4. The method of claim 1 wherein said at least one suitable area is of continuous tone.

5. The method of claim 1 wherein the suitable area is a plurality of suitable areas.

6. The method of claim 5 wherein said plurality of suitable areas includes at least one area of continuous tone.

7. The method of claim 6 wherein said plurality of suitable areas includes a colour control bar.

8. The method of claim 1 wherein the step of obtaining a first and second set of suitable data involves scanning said suitable areas.

9. The method of claim 8 wherein the scanning is done using a device selected from the following group: video camera, CCD camera, or densitometer.

10. The method of claim 1 wherein steps g) to j) are repeated at least once during the print run.

11. The method of claim 1 wherein in step e) a predetermined tolerance range is established, and wherein step i) 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.

12. 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 are 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.