EP0658428B1

EP0658428B1 - Control system for a printing press

Info

Publication number: EP0658428B1
Application number: EP93310110A
Authority: EP
Inventors: Xinxin Wang
Original assignee: Goss Graphic Systems Inc
Current assignee: Goss International LLC
Priority date: 1993-12-15
Filing date: 1993-12-15
Publication date: 1999-08-11
Anticipated expiration: 2013-12-15
Also published as: DE69326010D1; EP0658428A1; DE69326010T2; ATE183137T1

Abstract

A control system (10) for a four-color printing press (11) having a device (21) for detecting the energy reflected from a paper surface in both the visible region and the infrared region of the electromagnetic spectrum, a device (23) for converting the output of the detecting device (21) to a set of variables which represent the amount of ink presented on the paper for any of the cyan, magenta, yellow, and black inks, and a device (25) responsive to the converting device (23) for adjusting the four-color printing press (11) to maintain the color consistency with or without an additional color control target. <IMAGE>

Description

The present invention relates to control systems for a printing press.
In the past, four process inks (cyan, magenta, yellow and black) have been used on a printing press to produce copies with a gamut of colors. To improve trapping and reduce ink cost, various undercolor removal techniques (UCR) and grey component replacement (GCR) techniques have been used in the color separation processing. The UCR and GCR techniques remove a certain amount of the cyan, magenta and yellow ink from some printing area and replace them with a certain amount of the black ink Thus, the black ink has been used to generate not only the text but also the color image. Different color separation equipment manufacturers offer different UCR and GCR techniques to determine when this black ink substitution will take place and what amount of inks will be substituted.
In the past, the press room color reproduction quality control system cart be divided into the following two categories: one is a "control by target" system, and the other is a "control by image" system.
In the "control by target" system, a set of color control targets is printed in a margin. Instruments, such as densitometers, are used to monitor the color attributes, such as the optical density, of these targets. The printing press is then adjusted based on the deviation of these control targets from a predefined attribute value. The application of this "control by target" system is restricted in that an additional process is required to cut off this target from the final product. This system also requires a tight material control for paper, ink, and other printing parameters.
In the "control by image" system, the print image on a live copy is compared with the printed image on a reference copy, called a proof. The press is then adjusted based on the difference between the live image and the reference image. This system is more versatile because it does not require an additional target. This system is also more accurate than the "control by target" system, because in some situations although the measured attributes of control targets on the live and reference images are the same, those two images still look different. Conventionally, both the image comparing task and the press adjusting task are done by a press operator. To improve the productivity and the color consistency, several automatic printing quality inspection systems have been reported recently. These systems use opto-electronic sensor devices, such as a spectrophotometer, or CCD color cameras, to measure the color reproduction quality. Currently, the bandwidth of these sensor devices is limited to the visible region of 400 nm through 700 nm in wavelength of the electro-magnetic spectrum. However, within the visible region, it is not possible for these devices to reliably distinguish the black ink from the process black made by the combination of cyan, magenta, and yellow inks, or to determine whether the black ink or all cyan, magenta, and yellow inks should be adjusted. Although, these devices such as the spectrophotometer might be able to measure the printed colour accurately, it is difficult to use the measured colour information to achieve the automatic control for a four-colour press without a target due to the involvement of the UCR and GCR techniques.
CH-A-649842 discloses a device for determination of colour components in multicolour print. The device includes a filter wheel having three filters for components in the visible light spectrum and an infrared filter to be used with a photodetector of appropriately wide range photodection from visible to infra red. The results are evaluated with the aid of a colour table. It is not disclosed that any other factors are taken into account in the evaluation.
A principal feature of the present invention is the provision of an improved control system for a four-colour printing press.
According to one aspect of the invention there is provided a control system for a (four-colour) printing press, said printing press using cyan, magenta, yellow and black inks to produce a colour image on a paper surface, the control system comprising:
means for detecting energy reflected from a paper surface in both a visible region and an infrared region of the electromagnetic spectrum, the detecting means having an output;
means for converting the output of the detecting means to a data set representative of an ink volume, an average ink film thickness, or an ink dot size for the cyan, magenta, yellow and black inks present on the paper in a given area; and
means for comparing a first data set and a second data set generated by the converting means and for adjusting the four-colour printing press to maintain colour consistency in response to a difference between the first and second data sets, the first data set being representative of ink volume, average ink film thickness, or ink dot size for each of the cyan, magenta, yellow and black inks present in a given area of a reference copy and the second data set being representative of ink volume, average ink film thickness, or ink dot size for each of the cyan, magenta, yellow and black inks present in an area on a production copy corresponding to the given area on the reference copy.
A feature of the present invention is the provision of a sensor structure or device for detecting the energy reflected from the paper surface, with the sensor structure having a minimum of four separate channels, and with at least one channel operable in the infrared region of the electromagnetic spectrum. Another feature of the invention is that the bandwidth of the infrared channel may be between 800nm and 1100nm, which is a portion of the near infrared region and which is compatible with a regular silicon detector.
Yet another feature of the invention is that the working wavelength of the infrared channel may be longer than 1100nm or within the 700-800nm transition region.
A further feature of the invention is that at least three distinct channels are utilized in the visible region. Three of these channels may correspond to red, green and blue (RGB), or cyan, magenta, and yellow (CMY), or other colors. The bandwidth of each channel may be less than 70 nm, more than 100 nm, or any value in between, with channels having a multiple peak in its passing band, such as magenta, being also included.
The sensor device can be constructed from either a single element detector, a one-dimensional (linear) detector, a two-dimensional (area) detector, or other suitable detector structure.
The sensor can be constructed by adding an additional infrared channel to existing devices, e.g., adding an infrared channel to a RGB color camera or a densitometer, or by extending the working band into the infrared region e.g., adding infrared capability to a spectrophotometer.
The light source used provides enough radiated energy in both the visible region and the infrared region, depending upon the sensor working band and sensitivity.
Still another feature of the invention is that all possible values which are output from the sensor device may be used to form a vector space. For example, all possible values output from a sensor device with Red, Green, Blue, and Infrared channels form a four dimensional vector space R-G-B-IR, being termed a sensor space, with each output from the sensor device being termed a vector in the sensor space.
Another feature of the invention is that the minimum number of dimensions required by this sensor structure is four.
Still another feature of the invention is that a set of variables can be defined to represent the amount of ink presented in a given area. For example, a set of variables C, M, Y, and K (black) can be defined to represent or be a function of the amount of cyan, magenta, yellow and black ink in a given area. This set of variables may correspond to the ink volume, average ink film thickness, dot size, or other quantities related to the amount of ink in a given area on the paper surface, with the vector space by this set of variables being termed an ink space, with the ink space having formed a dimension of four for a four-color printing press.
Another feature of the invention is that there exists at least one transfer function which can map a vector in the four dimensional ink space into a vector in the four dimensional sensor space, with the transfer function being termed a forward transfer function.
The forward transfer function can be used in a soft proof system, which can electronically generate a proof image. This electronically generated proof image can be stored in the system as a reference, or can be displayed on a CRT screen for visual inspection.
A further feature of the invention is that there exists at least one transfer function which can map a vector in the four dimensional sensor space into a vector in the four dimensional ink space, with the transfer function being termed a reverse transfer function.
Another feature of the invention is that the printed image on a live copy can be compared with the printed image on a reference copy in the sensor space. If the difference between the live copy and the reference copy is within a predefined tolerance level, at least for all channels in the visible region of the sensor space, the live copy is said to be acceptable by definition.
Yet another feature of the invention is that both the live image and the reference image in the sensor space can be mapped into the ink space by applying the reverse transfer function point by point. The difference between the live image and the reference image in the ink space thus represents the difference of the ink distribution for each of the cyan, magenta, yellow, and black inks.
Another feature of the invention is that the difference between the live and the reference images in the ink space indicates which printing unit should be adjusted, which direction (up or down) it should be adjusted, and the amount of ink which should be adjusted.
A press control Formula can be developed to adjust press parameters, such as ink input rate in lithographic or letterpresses, ink consistency in flexographic or gravure presses, water input rate in lithographic presses, or temperature in any of the above, based on the differences between the live and the reference image in the ink space.
The press adjustment can be achieved by an automatic control system by the press operator alone, or by the interaction between the automatic control system and the press operator.
The sensor device may be used to monitor the printing web of the press directly, i.e., on press sensing, or to monitor the prints collected from the folder of the press, i.e., off press sensing.
If the digital images from the color separation processing, or the film/plate images are available, the image of the reference copy in the sensor space can be generated electronically by the forward transfer function.
The electronically generated reference may be used to set up the press in order to reduce the makeready time.
Yet another feature of the invention is that the colour reproduction quality can be maintained through the entire press run, through different press runs on different presses, or at different times.
A closed loop automatic colour reproduction control system may be formed with or without an additional colour control target.
A further feature of the invention is that the variation of ink, paper, and other press parameters can be compensated, such that the printed copies have the highest possible overall results in matching the reference copy.
According to another aspect of the invention there is provided a method for controlling the operation of a printing press, the printing press using cyan, magenta, yellow and black inks to produce a colour image on a paper surface, the method comprising the steps of:
providing a reference copy of an image to be printed;
measuring reference red, green, blue and infrared (RGBI) reflection values from the reference copy;
converting the reference RGBI reflection values into reference cyan, magenta, yellow and black (CMYK) ink values representative of the amount of ink present on the reference copy;
providing a production copy of the image as printed by the printing press;
measuring production RGBI reflection values from the production copy;
comparing the production RGBI reflection values to the reference RGBI reflection values;
if the comparison reveals a difference between the production RGBI reflection values and the reference RGBI reflection values which is greater than a predetermined threshold amount, converting the production RGBI reflection values into production CMYK ink values representative of the amount of ink present on the production copy, developing a difference between the production CMYK ink values and the reference CMYK ink values at corresponding locations, and adjusting the printing press to correct the difference between the production and reference RGBI reflection values based on the difference between the production and reference CMYK ink values; and
if the difference between the production RGBI reflection values and the reference RGBI reflection values is less than the predetermined threshold amount, accepting the production copy without converting the production RGBI reflection values into production CMYK ink values and without adjusting the printing press.
In the drawings:
FIG. 1 is a block diagram of a control system for a printing press of the present invention;
FIG. 2 is a diagrammatic view of the system of FIG. 1;
FIG. 3 is a block diagram of the control system of FIG. 1;
FIG. 4 is a diagrammatic view of a camera or sensor for the control system of the present invention;
FIG. 5 is a diagrammatic view of another embodiment of the camera or sensor for the control system of the present invention;
FIG 6 is a diagrammatic view of a further embodiment of a camera or sensor for the control system of the present invention;
FIG. 7 is a chart plotting the normalized percentage of IR Reflection against the percentage Dot Area in a printed sheet;
FIG. 8 is a diagrammatic view of a spectrum of electromagnetic waves including the visible spectrum and the infrared spectrum;
FIG. 9 is a diagrammatic view of set of elements for a sensor space and ink space;
FIG. 10 is a block diagram of the sensor space and ink space in conjunction with the control system of the present invention; and
FIG. 11 is a block diagram of the control system for adjusting the printing press.
There now follows a description of the preferred embodiments;
Referring now to FIG. 1, there is shown a control system generally designated 10 for a printing press 11 of the present invention.
The control system 10 has a 4 channel sensor 21, a data converter 23 for processing information from the sensor 21, and a device 25 for controlling ink for the press 11. As will be seen below, the 4 channel sensor 21 detects the energy reflected from a paper surface, such as the paper web for the press 11, in both the visible region and the infrared region of the electromagnetic spectrum. As shown in FIG. 8, electromagnetic waves in the infrared region have a longer wave length than the visible spectrum, with the wave lengths of the electromagnetic waves in the region of visible light being approximately 400 to 700 nanometers (nm), and the wave lengths of the electromagnetic waves in the infrared region, including near infrared, being equal to or greater than 800 nm.
As shown in FIG. 2, the control system 10 has a support 12 for placement of a sheet of paper 14 with image or indicia 16 on the sheet 14 in a configuration beneath a pair of opposed lights 18 and 20 for illuminating the sheet 14. The system 10 has a first color video camera or sensor 22 having three channels for detecting attributes of the inks from the sheet 14 in the visible region of the electromagnetic spectrum, such as red, green, and blue, or cyan, magenta, and yellow, and for sending the sensed information over separate lines or leads 24, 26, and 28 to a suitable digital computer 30 or Central Processing Unit having a randomly addressable memory (RAM) and a read only memory (ROM), with the computer or CPU 30 having a suitable display 32. Thus, the three distinct color attributes of the inks are sensed by the camera 22 from the sheet 14, and are received in the memory of the computer 30 for storage and processing in the computer 30.
The system 10 also has a black/white second video camera or sensor 34 having a filter 50 such that it senses the attributes of the inks in the infrared region of the electromagnetic spectrum, having a wave length greater than the wave length of the electromagnetic waves in the visible region of light. The camera or sensor 34 thus senses infrared information from the sheet 14, and transmits the sensed information over a lead 36 to the computer 30, such that the information concerning the infrared rays is stored in and processed by the computer 30.
The normalized percentage of infrared (IR) reflection vs. the percentage of dot area is shown in the chart of FIG. 7. It will be seen that the infrared reflectance of cyan, magenta, and yellow inks show no significant change as a function of percentage of dot area. However, the normalized infrared reflectance of the black ink displays a significant change as a function of percentage of dot area, and changes from a normalized value of 100% IR reflection for 0% dot area to approximately 18% IR reflection corresponding to 100% dot area. Hence, the black ink may be easily sensed and distinguished from other color inks in the infrared region of the electromagnetic waves.
As shown in FIG. 2, the sheet 14 may contain printed image or indicia 16 which is obtained from a current press run of the press 11, termed a live or current copy. In addition, a sheet 38 containing printed image or indicia 40, termed a reference copy, from a previous reference press run may be placed on the support 12 beneath the cameras 22 and 34 in order to sense the energy reflected from the sheet 38, and send the sensed information to the memory of the computer 30 for storage and processing in the computer 30, as will be described below.
Thus, the cameras or sensors 22 and 34 may be used to sense both the current copy or sheet 14 and the reference copy or sheet 38. The information supplied by the cameras 22 and 34 is formed into digital information by a suitable analog to digital converter in a frame grabber board on the computer 30. Thus, the computer 30 operates on the digital information which is stored in its memory corresponding to the information sensed from the sheets 14 and 38 by the cameras or sensors 22 and 34.
Referring now to FIG. 3, there is shown a block diagram of the control system 10 for the printing press 11 of the present invention. As shown, the four inks (cyan, magenta, yellow and black) of the four-color printing press 11 are first preset, after which a print is made by the press 11 with a current ink setting, thus producing a live or current printed copy, as shown. The color and black/white video cameras or sensors 22 and 34 of FIG. 2 serve as a four channel sensor 21 to capture an image of the current printed copy, and then place this information into the memory of the computer 30 after it has been formed into digital information.
Next, an "Ink Separation Process" 23 is used to convert the red, green, blue and IR images captured by the four channel sensor 21 into four separated cyan, magenta, yellow and black ink images, which represent the amount of corresponding ink presented on the live copy. The "Ink Separation Process" 23 may utilize mathematic formulas, data look up tables or other suitable means to perform the data conversion task.
The similar processes are also applied to the reference copy. First, the four channel sensor 21 is used to capture the red, green, blue and IR images from the reference copy. Then, the "Ink Separation Process" 23 is utilized to obtain the cyan, magenta, yellow and black ink images, which represent the amount of corresponding ink presented on the reference copy.
As shown, the ink images of the live copy are compared with the ink images of the reference copy by the computer 30 to detect the variation of ink distribution for each of the cyan, magenta, yellow and black inks.
The determined differences in ink distribution are then processed by the computer 30 in order to obtain an indication for controlling the keys or other devices of the press 11 in an ink control process, and thus provide an indication of an ink adjustment to the press to obtain further copies which will have a closer match to the reference copy. The indication of ink changes may be automatically supplied to the press 11, or the operator may utilize the indications of ink color attributes to set the press 11, such as adjustments to ink input rate by using the the keys.
In the past, four process inks (cyan, magenta, yellow, and black) have been used on a printing press to produce copies with a gamut of colors. In these systems, the black ink has been used to generate not only the text but also the color image. In a control by image, system, the print image of a live copy is compared with the printed image on a reference copy, termed a proof, and the press is adjusted based on the difference between the live image and the reference image. However, within the visible region, it is not possible to reliably distinguish the black ink from the process black made by the combination of cyan, magenta, and yellow inks, or whether the black ink or all cyan, magenta, and yellow inks should be adjusted.
In accordance with the present invention, the four channel sensor 21 is utilized to sense not only attributes in three channels of the visible region, the fourth channel of the sensor 21 senses an attribute in the infrared region in order to determine the correct amount of inks, including black ink, to correctly reproduce the proof. The printing press control system 10 uses the four channel detector or sensor 21 to detect the energy reflected from a paper surface, such as the sheets 14 and 38, or the paper web of the press 11, with three channels being in the visible region and one channel being in the infrared region of the electromagnetic spectrum. The control system 10 has a device 23 for converting the output of the sensing device 21 to a set of variables which represent the amount of ink presented on the paper for any of the cyan, magenta, yellow, and black inks, and a device 25 responsive to the converting device 23 for adjusting the four-color printing press 11 to maintain the color consistency.
In a preferred form, the bandwidth of the infrared channel may be between 800 nm and 1100 nm, which is a portion of the near infrared region, and which is compatible with a regular silicon detector, although the working wavelength of the 15 infrared channel may be longer than 1100 nm. At least three distinct channels are utilized in the visible region which may correspond to red, green, and blue (RGB), or cyan, magenta, and yellow (CMY), or other colors. The bandwidth of each channel in the visible region may be less than 70 nm, more than 100 nm, or any value in between, with channels having a multiple peak in its passing band, such as magenta, being also included.
The sensor device 21 may be constructed from either a single element detector, a one-dimensional (linear) detector, a two-dimensional (area) detector, or other suitable detector structure, as will be seen below. The sensor device may be constructed by adding an additional infrared channel to existing devices, adding an infrared channel to a RGB color camera or a densitometer, or by extending the working band into the infrared region, e.g., adding infrared capability to a spectrophotometer. The light source 18 and 20 used provides sufficient radiated energy in both the visible region and the infrared region, depending upon the sensor working band and sensitivity.
All possible values which are output from the sensor device 21 may be used to form a vector space. For example, all possible values output from the sensor device 21 with red, green, blue and infrared channels form a four dimensional vector space R-G-B-IR, with the vector space being termed a sensor space S₁, with each output from the sensor device 21 being termed a vector in the sensor space S₁, with the minimum number of dimensions required by the sensor structure being 4. Thus, as shown in FIG. 9, a set S₁ of elements e_i1 and e_i2 being given, with the elements e_i1 of the set S₁ being the vectors v_i1 corresponding to the output from the sensor device 21 of sensing a live or current printed copy, and with the elements e_i2 of the set S₁ being the vectors v_i2 corresponding to the output from the sensor device 21 sensing a reference printed copy. In accordance with the present invention, the printed image on a live or current copy may be compared with the printed image on a reference copy in the sensor space, and if the difference between the live copy L.C._s and the reference copy R.C._s is within a predefined tolerance level delta, at least for all the channels in the visible region of the sensor space, such that, |L.C.s - R.C.s| ≤ delta , the live or current copy is said to be acceptable by definition.
A set of variables may be defined to represent the amount of ink presented in a given area. For example, a set of variables, C, M, Y, and K can be defined to represent or be a function of the amount of cyan, magenta, yellow, and black ink in a given area. This set of variables may correspond to the ink volume, average ink film thickness, dot size, or other quantities related to the amount of ink in a given area on the paper surface. The vector space formed by this set of variables is termed an ink space S₂, with the ink space S₂ having a dimension of 4 for a four color printing press 11. Thus, with reference to FIG. 9, a set S₂ of elements d_i1 and d_i2 are given, with the elements d_i1 of the set S₂ being the vectors v_j1 corresponding to the variables associated with the live or current copy in the ink space S₂, and with the elements d_i2 of the set S₂ being the vectors v_j2 corresponding to the variables associated with the reference copy in the ink space S₂.
With reference to FIG. 9, there exists at least one transfer function or transformation phi which can map the elements d_i1 and d_i2 of the set S₂, or the four dimensional ink space, into the elements e_i1 and e_i2 of the set S₁ or the four dimensional sensor space, with the transformation phi being termed a forward transfer function, as shown in FIGS. 9 and 10. It is noted that the subsets in each set S₁ and S₂ may overlap or may be the same.
The forward transfer function may be used in a soft proof system which can generate a proof image which can be stored in the system as a reference or can be displayed on a CRT screen.
With further reference to FIG. 9, there exists at least one transfer function or reverse transformation phi^-1 which can map the elements e_i1 and e_i2 of the set S₁ of the four dimensional sensor space into the elements of d_i1 and d_i2 of the set S₂ of the four dimensional ink space, with the transfer function being termed a reverse transfer function. Thus, both the live image and the reference image in the sensor space or set S₁ can be mapped into the ink space or set S₂ by applying the reverse transfer function phi^-1 point by point as shown in FIGS. 9 and 10.
The difference between the live image and the reference image in the ink space S₂ thus represents the difference of the ink distribution for each of the cyan, magenta, yellow, and black inks, as shown in FIG. 11. The difference between the live and reference images in the ink space S₂ indicates which printing unit should be adjusted, which direction, up or down, it should be adjusted, and the amount of ink which should be adjusted. A suitable press control formula may be developed to adjust press parameters, such as ink input rate in lithographic or letterpresses, ink consistency in flexographic or gravure presses, water input rate in lithographic presses, or temperature in any of the above, based on the differences between the live and the reference image in the ink space S₂.
The press adjustments can be achieved by the automatic control system 10 or by the interaction between the automatic control system 10 and the press operator. Also, the sensor device 21 may be used to monitor the printing web of the press 11 directly, i.e., on press sensing, or to monitor the prints collected from the folder of the press, i.e., off press sensing. If the digital images from the color separation processing, or the film/plate images are available, the image of the reference copy in the sensor device 21 can be generated electronically by the forward transfer function phi. The electronically generated reference may be used to set up the press 11 in order to reduce the makeready time.
In accordance with the present invention, the color reproduction quality can be maintained through the entire press run, through different press runs on different presses, or at different times. Thus, a closed loop automatic color reproduction control system may be formed without an additional color control target. The variation of ink, paper, and other press parameters can be compensated such that the printed copies have the highest possible overall results in matching the reference copy.
As shown in FIG. 4, the camera or sensor 22 may be associated with a rotating filter member 52 having filters which only transmit the desired colors F₁, F₂, and F₃, such as red, green, and blue during rotation, such that the camera or sensor 22 senses and records the colors F₁, F₂, and F₃ sequentially or separately from the printed material which may be taken either from the current press run or from the reference press run. In addition, the filter member 52 may have an infrared (IR) filter F₄ in order to sense and record the energy reflected from the printed material in the infrared region. The information received by the camera or sensor 22 from the filters may be recorded in the computer or CPU for use in forming the desired data to control the inks, as previously discussed.
In another form, as shown in FIG. 5, the camera or sensor 22 may comprise a charge coupled device (CCD) with built in filters which converts light energy reflected from the printed material into electric energy in a video camera, i.e. F₁, F₂, F₃, and F₄ (IR), such as the distinct colors red, green, and blue in the visible region, and the near infrared energy in the infrared region, in order to supply the information to the computer 30 for storage and processing, as previously discussed.
Another embodiment of the camera or sensor 22 of the present invention is illustrated in FIG. 6, in which like reference numerals designate like parts. In this embodiment, the camera or sensor 22 has a beam splitter in order to separate the incoming light reflected from the printed material into an infrared beam for a first CCD 1, F₁ such as red for a second CCD 2, F₂ such as green for a third CCD 3, and F₃ such as blue for a fourth CCD. In this embodiment, suitable prisms, lenses, or mirrors may be utilized to accomplish the beam splitting of light in order to obtain the desired color attributes in the various charge coupled devices to supply the information to the computer 30 for storage and processing in the computer 30, in a manner as previously described. Of course, any other suitable camera or sensing device may be utilized to obtain the desired colors.
Thus, in accordance with the present invention, a control system 10 for a printing press 11 is provided which ascertains three distinct attributes, such as colors, in the visible region of electromagnetic waves and an attribute in the infrared region of the electromagnetic spectrum for the printed inks. The control system 10 utilizes these four attributes in a four channel device to indicate and control the ink colors for use in the press 11.
Thus, the colors may be sensed from a sheet taken during a current press run, and from a sheet taken during a reference press run, after which the sensed information is utilized in order to modify ink settings of a press 11 in order to obtain repeatability of the same colors from the reference run to the current press run. In this manner, a consistent quality of colors may be maintained by the printing press 11 irrespective of the number of runs after the reference run has been made, and may be continuously used during a press run if desired.

Claims

A control system for a printing press, said printing press using cyan, magenta, yellow and black inks to produce a colour image on a paper surface, the control system comprising:

means for detecting energy reflected from a paper surface in both a visible region and an infrared region of the electromagnetic spectrum, the detecting means having an output;

means for converting the output of the detecting means to a data set representative of an ink volume, an average ink film thickness, or an ink dot size for the cyan, magenta, yellow and black inks present on the paper in a given area; and

means for comparing a first data set and a second data set generated by the converting means and for adjusting the four-colour printing press to maintain colour consistency in response to a difference between the first and second data sets, the first data set being representative of ink volume, average ink film thickness, or ink dot size for each of the cyan, magenta, yellow and black inks present in a given area of a reference copy and the second data set being representative of ink volume, average ink film thickness, or ink dot size for each of the cyan, magenta, yellow and black inks present in an area on a production copy corresponding to the given area on the reference copy.
The system of claim 1 wherein the detecting means senses at least four separate channels, at least three of said channels being operable in the visible region of the electromagnetic spectrum, and at least one of said channels being operable in the infrared region of the electromagnetic spectrum.
The system of claim 2 wherein the bandwidth of the infrared channel is between 800nm and 1100nm.
The system of claim 2 wherein the working wavelength of the infrared channel is longer than 1100nm or within the 700-800nm transition region.
The system of claim 1 wherein the energy reflected in the visible region comprises the attributes of the colours red, green, and blue.
The system of claim 1 wherein the energy reflected in the visible region comprises the attributes of the colours cyan, magenta, and yellow.
The system of claim 2 wherein the output from the detecting means comprises a plurality of elements comprising vectors in a sensor space.
The system of claim 7 wherein the elements in the sensor space include vectors designating an image from the reference copy and the production copy.
The system of claim 1 wherein the comparing means compares a third data set corresponding to an image from the reference copy with a fourth data set corresponding to an image from the production copy, the third data set and the fourth data set comprising the output of the detecting means.
The system of claim 9 wherein the comparing means accepts the production copy if a difference between the third and fourth data sets is within a predetermined limit.
The system of claim 7 wherein the vectors in the sensor space are at least four dimensional.
The system of claim 1 wherein the first and second data sets comprise a plurality of four dimensional vectors.
The system of claim 9 wherein the converting means converts the third data set into the first data set and the fourth data set into the second data set.
The system of claim 1 in which the paper comprises a web of the press.
A method of controlling the operation of a printing press, the printing press using cyan, magenta, yellow and black inks to produce a colour image on a paper surface, the method comprising the steps of:

providing a reference copy of an image to be printed;

measuring reference red, green, blue and infrared (RGBI) reflection values from the reference copy;

converting the reference RGBI reflection values into reference cyan, magenta, yellow and black (CMYK) ink values representative of the amount of ink present on the reference copy;

providing a production copy of the image as printed by the printing press;

measuring production RGBI reflection values from the production copy;

comparing the production RGBI reflection values to the reference RGBI reflection values;

if the comparison reveals a difference between the production RGBI reflection values and the reference RGBI reflection values which is greater than a predetermined threshold amount, converting the production RGBI reflection values into production CMYK ink values representative of the amount of ink present on the production copy, developing a difference between the production CMYK ink values and the reference CMYK ink values at corresponding locations, and adjusting the printing press to correct the difference between the production and reference RGBI reflection values based on the difference between the production and reference CMYK ink values; and

if the difference between the production RGBI reflection values and the reference RGBI reflection values is less than the predetermined threshold amount, accepting the production copy without converting the production RGBI reflection values into production CMYK ink values and without adjusting the printing press.
A method as defined in claim 15 wherein the reference and production CMYK ink values are representative of ink volume, average ink thickness, or ink dot size.