EP1814737A1

EP1814737A1 - Density measurement, colorimetric data, and inspection of printed sheet using contact image sensor

Info

Publication number: EP1814737A1
Application number: EP05800110A
Authority: EP
Inventors: Asa Ziv
Original assignee: Vcortex Ltd
Current assignee: Vcortex Ltd
Priority date: 2004-10-28
Filing date: 2005-10-30
Publication date: 2007-08-08
Also published as: WO2006046249A1

Abstract

An image quality measurement apparatus measures quality of an image printed on a substrate. The apparatus comprises illumination elements and image sensors, both mounted in close proximity to the substrate, a controller of the image sensors and illumination elements, and an analyzer of the measurements of the image sensor. The measurements can be used as real time feedback to a printer. The measurements may include density, colorimetric data, and inspection of printed sheet using Contact Image Sensors (CIS).

Description

DENSITY MEASUREMENT, COLORIMETRIC DATA₅ AND INSPECTION OF PRINTED SHEET USING CONTACT IMAGE SENSOR

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to printing technology and, more particularly, but not exclusively to quality assessment of printed matter, and, even more particularly, but still not exclusively to the assessment of color quality of the printed image.

In the sheet fed printing process of today, whether it is digital printing or legacy offset printing, gravure printing or flexographic printing, there is a need to assess the quality of the printed image within the printing machine. The prime measure of quality is the quality of the colors of the printed image.

As with any other manufacturing process, the sooner a problem is located the simpler it is and the cheaper it is to correct. Industrial printing is a very fast process and any stoppage is inherently expensive. The best solution is to have a continuous quality assessment with a feedback mechanism that can correct quality drifts on-the- fly, before any stoppage is required.

In any common printing technology, the quality of the printed image is bound to change with time, during the printed job run, for various reasons:

• The amount of ink over the substrate varies constantly.

• Wear of the printing plate • Change of pressure between the printing plate and the surface

• Change of ink pressure

• Temperature change

• Changes of ink viscosity

• Change of ink density • Change of balance between the ink and the water

• Changes of the quality of the substrate

Obviously, a continuous colorimetric measurement and a resulting modification of the printing calibration such as ink pressure, ink temperature, ink density, water to ink balance, press pressure and other parameters can enhance the color stability and the quality of the printed image.

Furthermore, based on accurate colorimetric measurement it is possible to profile the printing parameters to achieve the best printed image for a certain combination of plate type, substrate type, ink type, etc. Using the adequate profile may considerably decrease the set-up time of a print job run.

The common methods of acquiring colorimetric data for the sheet-fed printing comprise human inspection, Automatic Optical Inspection (AOI), and offline color measurement tables.

Human inspection

Human inspection is performed by the operator of the printing machine. The operator randomly samples a printed sheet from the press delivery area, and checks the sampled sheet against a proofing sheet. If the operator detects a color change the operator manually manipulates a set of ink keys to correct the color in the designated area.

This method is disadvantageous for the following reasons:

• Subjectivity - the result is as good as the operator's capabilities at the time of printing and is therefore variable from person to person, from day to night, and prawn to errors • Delay - the sampling is erratic, it takes the operator 2-3 minutes to pick up a sheet, analyze it and correct the printer settings while the printer is running. It therefore takes hundreds of sheets of low quality image until a problem is rectified.

• Instability - as the sheets may be sampled before the colors have stabled it is possible the coloring is fluctuating faster than the sampling and the correction is performed based on inadequate data, which may lead to over correction and deterioration of the quality.

Automatic Optical Inspection systems (AOI) Currently available AOI systems are, for example, EagleEye manufactured by ISRA Vision Systems AG, Industriestr. 14, D-64297 Darmstadtand, Germany and installed on a MAN Roland presses (MAN Roland Druckmaschinen AG, Stadtbachstraβe 1, 86153 Augsburg, Germany), and Qualitronic II manufactured by Koenig & Bauer (Koenig & Bauer AG, Wϋrzburg Facility, Friedrich-Koenig-Str. 4, D-97080 Wurzburg, Germany). AOI systems employ a commercial CCD or CMOS line scan camera. The camera is typically mounted at an optical distance from the printed surface to capture the entire width of the printed image, while the illuminating fixture is positioned closely to the surface.

This method is disadvantageous for the following reasons:

• Difficult to calibrate - the distance between the camera and the imaged sheet complicates the calibration of the optical system. Calibration must be performed each time any optical component is replaced.

• Vibration — the large optical distance between the camera and the printed image translates the small vibrations of the press frame into a significant movement of the camera, thus limiting the useful resolution of the camera.

• Human interference - the optical path crosses the area where the operator stands to control the ink-feeding tower. The operator body and limbs affects the illumination and interfere with the optical path.

Offline color measurement tables

Various offline color measurement solutions are available, such as ROLAND CCF (Computer-controlled inking) from MAN Roland Druckmaschinen AG, Stadbachstrasse 1, 86153 Augsburg, Germany, and Prinect Image Control from Heidelberger Druckmaschinen AG, Kurfϋrsten-Anlage 52-60, 69115 Heidelberg, Germany. These solutions provide an offline scanning of the printed sheet. Offline scanning is similar to human inspection except that it is not subjective and enables and can automatically provide ink keys correction commands.

This method is disadvantageous for the following reasons: • Manual - operator's attention is still required in taking the sheet out of the collector area to the offline scanner.

• Delay - the sampling is erratic, it takes the operator 2-3 minutes to pick up a sheet, analyze it and correct the printer settings while the printer is running. It therefore takes hundreds of sheets of low quality image until a problem is rectified.

• Instability — as the sheets may be sampled before the colors have stabled it is possible the coloring is fluctuating faster than the sampling and the correction is performed based on inadequate data, which may lead to over correction and deterioration of the quality.

• Slow - the available scanners scan the sampled sheet much lower than the speed of the press and even slower than the human eye. It is therefore impossible to use the scanner within the printing machine.

• Erroneous - as the method employs densitometers or spectrophotometers with narrow field of view, and does not involve image processing to ignore print imperfections that should not affect the color settings, the method produces correction errors.

There is thus a widely recognized need for, and it would be highly advantageous to have a system for assessing the quality of a printed image devoid of the above limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided scanning apparatus for print control comprising: a plurality of contact image sensors, each having respective output connections, to obtain image quality data from a substrate, wherein said contact image sensors are arranged with their respective outputs connected in parallel for fast data readout; and a feedback unit for feeding back said image quality data for setting of parameters of a printer. According to a second aspect of the present invention there is provided an image quality measurement apparatus for measuring quality of an image printed on a surface, said apparatus comprising: at least one illumination element mounted in close proximity to said surface and operative to illuminate at least a portion of said surface; an array of image sensors mounted in close proximity to said surface and operative to produce measurements of light reflected from said illuminated portion of said surface; a controller operative to control said image sensors and said illumination elements and to receive said measurements of light; and an analyzer operative to analyze said measurements of light and produce a measure of quality of said image printed on said surface. wherein said image quality measurement apparatus is capable of being integrated with a printing apparatus; and wherein said image quality measurement apparatus is operative to perform said measurements of light and to produce said measure of quality at printing speed. According to a third aspect of the present invention there is provided a method of measuring quality of a printed image, said method comprising: illuminating said image at close proximity; measuring light reflected from said image at close proximity; and analyzing said measurement to form a quality analysis; wherein said steps of illuminating; measuring and analyzing are performed at printing speed.

According to a second aspect of the present invention there is provided a printing apparatus for printing on a surface, said apparatus comprising: an surface moving apparatus for moving a surface to be printed under at least one printing head at printing speed; said at least one printing head for printing an image on said surface; at least one illumination element mounted in close proximity to said surface and operative to illuminate at least a portion of said image; an array of image sensors mounted in close proximity to said surface and operative to produce measurements of light reflected from said illuminated portion of said image; an analyzer operative to analyze said measurements of light and produce a measure of quality of said printed image; and a controller operative to control said image sensors, said illumination elements and said printing heads in accordance with said analysis of said measurement of light; wherein said array of image sensors, said analyzer and said controller are operative to perform said measurements of light and to produce said measure of quality at said printing speed.

According to a fourth aspect of the present invention there is provided a printing method for printing on a surface, the method comprising: moving a surface to be printed under at least one printing head at printing speed; printing an image on said surface; illuminating at least a portion of said surface at close proximity; measuring light reflected from said illuminated portion of said surface, said measuring performed at close proximity to said surface; analyzing said measurements of light and producing a measure of quality of said printed image; and controlling said image sensors, said illumination elements and said printing heads in accordance with said analysis of said measurement of light; wherein said steps of illuminating, measuring, analyzing and controlling are performed at said printing speed.

According to a further aspect of the present invention there is provided scanning apparatus for print control comprising: a plurality of contact image sensors, each contact image sensor having respective output connections, and located in non-contact relation with a substrate to obtain image quality data from said substrate, wherein said contact image sensors are arranged with their respective outputs connected in parallel for fast data readout; and a feedback unit for feeding back said image quality data for setting of parameters of a printer. That is to say, the contact image sensors are not actually placed in contact with the substrate since they are optical devices and are able to scan without being in contact. The sensors are able to scan from distances in the order of magnitude of tens of centimeters and from further away if suitable optics are provided. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples provided herein are illustrative only and not intended to be limiting.

Implementation of the method and system of the present invention involves performing or completing certain selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of preferred embodiments of the method and system of the present invention, several selected steps could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, selected steps of the invention could be implemented as a chip or a circuit. As software, selected steps of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In any case, selected steps of the method and system of the invention could be described as being performed by a data processor, such as a computing platform for executing a plurality of instructions.

BRIEF DESCRIPTION QF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in order to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention, hi this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is a simplified illustration of an image quality measurement apparatus 10 mounted within a sheet-fed printer system 11 according to a preferred embodiment of the present invention.

Fig. 2 is a simplified drawing of the quality measurement apparatus of Fig. 1 according to a preferred embodiment of the present invention.

Fig. 3 is a simplified drawing of another view of the quality measurement apparatus of Fig. 1 according to a preferred embodiment of the present invention.

Fig. 4 and Fig. 5, are simplified illustrations of two configurations of the array of image sensors, according to a preferred embodiment of the present invention.

Fig. 6 is a simplified block diagram of an image processing part of the quality measurement apparatus according to a preferred embodiment of the present invention.

Fig. 7 is a simplified block diagram of a process executed by the quality measurement apparatus, according to a preferred embodiment of the present invention.

Fig. 8 is a simplified block diagram of a program executed by the data processing module, according to a preferred embodiment of the present invention.

DESCRIPTION QF THE PREFERRED EMBODIMENTS

The present embodiments enable high-speed, high-resolution imaging of a sheet within a sheet-fed offset press, and to use this imaging to assess the quality of the printed image. Particularly, the present embodiments provide information about optical densities, color composition, color registration, etc. Furthermore, the present embodiments provide information regarding the image quality in real-time, to analyze this information and to produce feedback to the printing machine so as to correct drifts in image quality without stopping the printer or decreasing the printing speed.

The present embodiments implement illumination elements and light sensors in close proximity to the printed surface and use for this purpose an array of Contact Image

Sensors (CIS). Preferably the sensors are connected in parallel so as to output information at high speed and allow real time processing, and the information is fed back to the printing machine to change settings as necessary.

Consequently, the preferred embodiments confer the following features:

• Automation - a printed image quality control system that measures, analyses and controls the relevant settings of the printing machine without human intervention.

• Continuity - a printed image quality control system that inspects every single printed sheet and determines color drifts and color fluctuations.

• Integration with the printing machine - a printed image quality control system that is integrated into the cavities of a printing machine.

• Minimal optical distance - a printed image quality control system that has a minimal distance between the sensor and the printed sheet is able to minimize the effect of vibrations in reducing measurement accuracy.

• The preferred embodiments use Contact Image Sensor (CIS) chips to provide colorimetric data.

• The preferred embodiments connect the CIS chips to the control circuitry in parallel, rather then serially. The parallel connection allows high speed data output and consequently enables sampling of the sheet at full printing speed in the highest possible resolution.

The printed image quality control system as described hereinafter may be used for other than sheet-fed offset presses.

The principles and operation of a printed image quality control system according to the present invention may be better understood with reference to the drawings and accompanying description.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Reference is no made to Fig. 1, which is a simplified illustration of an image quality measurement apparatus 10 mounted within a sheet-fed printer system 11.

As shown in Fig. 1, the printer 11 consists of a printing drum 12 and an impression drum 13. Typically the printing drum 14 carries the ink bearing the image to be printed and transfers the ink onto a sheet 44, which is mounted on the impression drum 13. The printing drum 12 and the impression drum 13 are termed herein a drum system 15. Typically, the drum system 15 described and shown in Fig. 1 prints only one color. A multi-color sheet-fed printing system thus incorporates several drum systems 15 as shown in Fig. 1, each printing a single color, and the sheet is transferred sequentially between the drum systems 15. In this case of a printing system comprising several drum systems 15 the drum system of Fig. 1 is typically the last drum system. It is appreciated that though the quality measurement apparatus 10 is preferably mounted within the last drum system 15, the quality measurement apparatus 10 can be mounted also within other drum systems 15 of the printer system 11.

In some printing systems there may be a third drum, not shown in Fig. 1, typically named an image drum, on which the image is first created by ink applying devices. The ink is then transferred onto a blanket covering the printing drum, and hence onto the sheet mounted on the impression drum. Some printing systems may print several colors within a single drum system.

It is noted that drum-based printing is merely given as an example, and the scanner of the present embodiments is¹ relevant for any kind of printing in which output quality measurements can be fed back to alter printer settings. This includes any kind of color printer.

Reference is now made to Fig. 2, which is a simplified drawing of the quality measurement apparatus 10 of Fig. 2 according to a preferred embodiment of the present invention. The quality measurement apparatus 10 preferably consists of illumination elements 16, image sensors 17, a data compression board 18 and a data processing unit 19. The illumination element 16 typically consists of a lamp 20 and an optical system such as a lens 21. The lamp 20 can be any type of lamp such as incandescent lamp, fluorescent lamp, light emitting diode (LED), Cold Cathode Fluorescent Lighting (CCFL), Metal Halide, etc. The optical system can be, for example, a lens or a light guide such as a fiber optic.

The illumination element 16, the image sensors 17 and the data compression board 18 are preferably mounted within a frame 22 of the printing system 11. The illumination element 16 and the image sensors 17 are preferably mounted in close proximity to the printed sheet which is located on surface 45. Preferably the image sensors 17 are mounted 5 centimeters from the printed surface or closer.

The data compression board 18 is preferably mounted in close proximity to the image sensors 17.

Typically and preferably the illumination element 16 and the data compression board 18 are connected to the data processing unit 19. The data processing unit 19 preferably comprises various communication facilities, such as control link 23 connected to the printer controller and display link 24 connected to various display units. It is appreciated that the quality measurement apparatus 10 is capable of providing a displayable scan of the printed image, to be displayed on a display unit, preferably via the display link 25. It is also appreciated that such display units can be co-located with the printing system 11 or located remotely from the printing system 11.

Reference is now made to Fig. 3, which is a simplified drawing of another view of the quality measurement apparatus 10 of Fig. 1 according to a preferred embodiment of the present invention.

As shown in Fig. 3Error! Reference source not found., typically and preferably, the illumination elements 16 of Fig. 2 are two arrays 25 of illumination elements 16 and the image sensor 17 of Fig. 2 is an array 26 of image sensors 17. Reference is now made to Fig. 4 and Fig. 5, which are simplified illustrations of two configurations of the array 26 of image sensors 17, according to a preferred embodiment of the present invention.

As shown in fig. 4, the array 26 consists of a single line of image sensors 17. The image sensors 17 of Fig. 3 are preferably arranged in a single row sequence of elements:

RGBRGBRGBRGB

where the R image sensor element 27 senses red, the G image sensor element 28 senses green, and the B image sensor element 29 senses blue. Typically and preferably the image sensors 17 are equipped with R, G and B filters.

Alternatively, as shown in Fig. 5, the array 26 consists of a single line of image sensors 17. The image sensors 17 of Fig. 3 are preferably arrange in a dual row format (Bayer filters):

R,G,R,G,R,G,R,G,R G,B,G,B,G,B,G,B,G

Typically and preferably, each image sensor 17 senses a single picture element (pixel)

Reference is now made to Fig. 6, which is a simplified block diagram of an image processing part of the quality measurement apparatus 10 according to a preferred embodiment of the present invention.

As shown in Fig. 6, the image processing part of the quality measurement apparatus 10 consists of the array 26 of image sensors 17, an array of analog-to-digital converters (ADC) 33, an array of the data reduction modules 18, and the data processing module 19. As shown in Fig. 6, each image sensor 17 is preferably equipped with a color filter 34. Preferably the color filters are red, green and blue filters (RGB) as described above with reference to Figs. 4 and 5.

As shown in Fig. 6, each image sensor 17 is preferably connected to an ADC 33, which converts the analog measurement of the image sensors 17 into a digital signal. The ADC 33 is preferably connected to a data reduction module 18, which receives the digital signals, compresses and multiplexes them, and sends the signals in a continuous bit stream to the data processing module 19. Preferably the data reduction modules 18 are daisy-chained as shown in Fig. 7. The data processing module 19 typically and preferably comprises various communication facilities, such as control link 24 connected to the printer controller and display link 25 connected to various display units.

It is appreciated that the output from the CIS chip can be either analog or digital. Fig. 7 illustrates CIS chips with analog outputs.

It is appreciated that the front CIS chips can be single chips or a device comprising several CIS sensors.

It is appreciated that the digital connection between the CIS chips and the data compression board are preferably implemented using standard communication technologies such as camera link, USB2, RS485, Ethernet or similar high speed communication technologies.

It is appreciated that the data compression boards preferably implement binning and ROI (region of interest) functions to reduce the amount of data to be transferred to the processing unit. These functions are preferably implemented simultaneously for each frame, or separately, different from frame to frame.

It is appreciated that the digital connection between the data reduction board and the processing unit is preferably implemented in standard communication technologies such as camera link, USB2, RS485, Ethernet or any other high speed communication technologies.

It is appreciated that scanning images in high resolution and in high printing speed creates a large quantity of data to be processed. For example, a 40" x 30" sheet scanned at 400DPI at 3 colors (R₃G₅B) and at 12bits per pixel at the printing speed of 18,000 sheets per hour generate a bit stream of 36G bits/Second. Therefore the bit stream is preferably compressed before it is transferred to the processing unit. The reduction unit performs the following steps:

• Compensate for Fixed Pattern and other noises; • Designate lower scanning resolutions to areas of lower interest;

• Designate lower scanning bit depth to areas having lower color interest;

• Apply selective Gamma corrections, converting pixel sampling from 12 bits to 8 bits through Gamma function conversion

Reference is now made to Fig. 7, which is a simplified block diagram of a process executed by the quality measurement apparatus 10, according to a preferred embodiment of the present invention.

As shown in Fig. 7, the process starts with step 37, by setting the parameters of the illumination elements 16 and proceeds to step 38 to set the parameters of the image sensors 17, according to the characteristics of the printed image. The process than proceeds to step 39 to receive image data from the image sensors 17 and then to step 40 to process the image data, as will be explained below in further details. The results of the processing of step 40 are provided in step 41 as feedback, either manually or automatically, to the printing system 11. The results of processing of step

41 are also provided in step 42 as feedback to the illumination elements 16 and preferably also to the image sensors 17.

Reference is now made to Fig.8, which is a simplified block diagram of a program executed by the data processing module 19, according to a preferred embodiment of the present invention.

As shown in Fig. 8, the program executed by the data processing module 19 preferably consists of 12 modules:

Receive Image module 51

Collate module 52 Compensation module 53

Pad and Place module 54

Display Manager module 55

Display Communication module 56

Locate Colors module 57 Registration Control module 58 RGB to Density Conversion module 60,

Density Analysis module 61

Color control module 62

Printer Communication module 63

The Receive Image module 51 receives the pixel bit stream from the reduction units or reduction boards 18, and performs the following functions: Identify a sheet image block; Identify image type;

Calibrate the color bar; Calibrate viewing;

Calibrate lighting reference; Identify the sheet number; Calibrate the physical position; Calibrate the pixels¹ positions; Calculate a binning factor;

Calculate de-gamma compensation, preferably to 12 bits.

and then transmits the processed sheet image and its associated parameters to the Collate module Error! Reference source not found..

The Collate module 52 collects images from the same sheet and the same type and transfers the data, comprising the collection of sheet images, to the compensation module 53.

The compensation module 53 process the sheet's collection of images to provide compensation for the following artifacts:

Dark signal resulting from fixed pattern noise; White non-uniformity resulting from variations between sensors;

White non-uniformity resulting from illumination non-uniformity; Light level from light reference; Chromatic aberrations.

The compensation module then transfers the compensated data to the Pad and Place module 54. The above operation is used to compensate for positioning errors that may have occurred during the physical butting of image sensors 18. More particularly, when a pixel is missing or moved due to sensor positioning errors, padding and placing algorithms are used to compensate for the missing pixel.

The Pad and Place module 54 relocates the pixel information to create an image of equally spaced pixels. The Pad and Place module 54 then transfers the image data to the Display Manager 55, which reformats the image for the target display and transfer the reformatted image to the Display Communication module 56 to be transmitted to a local display, or to a remote display (or both). The Pad and Place module 54 also transfers the image data to the Locate Colors module 57.

The Locate Colors module 57 identifies and locates color patches within the sheet image and transfers this information to the Registration Control module 59 and to the RGB to Density Conversion module 60.

The Registration Control module 58 processes the image information, generates registration correction data, and transfers this data to the Printer Communication module 63.

The RGB to Density Conversion module 60 converts the RGB signals into color density parameters and transfers these parameters to the Density Analysis module 61 and to the color control module 62.

The Density Analysis module 61 analyses the changes in density values, preferably both the temporal and the spatial changes, generates density trend parameters and sends them to the color control module 62.

The color control module 62 processes the density parameters received from the RGB to Density Conversion module 60 and the density trend parameters received from the Density Analysis module 61 and produces ink-key correction parameters, which it sends to the Printer Communication module 63.

The Printer Communication module 63 transmits the registration correction parameters and the ink-key correction parameters to the printer system 11 to compensate for drifts in the image quality. Thus the image quality is corrected in real¬ time and without affecting the printing speed. Alternatively, if the printer system 11 is not equipped to receive feedback signals, the registration correction parameters and the ink-key correction parameters are provided to an operator of the printer system 11 so that he may carry out manual modification of the settings of the printer system 11.

Thus the present embodiments enable high-speed and high resolution imaging of a sheet inside a sheet-fed offset press and as an integral part of the printing process. The present embodiments further use this imaging data to provide: optical density measurements; color measurements; inspection of print quality; color registration analysis; and other optional information.

Preferably, the present invention uses Contact Image Sensor (CIS) chips for its image sensors 17. The use of CIS technology enables a faster scanner and a smaller scanner. Preferably the present embodiments can be mounted within a common sheet- fed offset press.

It is appreciated that the present embodiments can be used for printing systems other than sheet-fed offset printers.

The image sensors 17 preferably consist of Contact Image Sensor (CIS) chips that are preferably physically butted to each other to create a sensor of a preferred length.

Preferably, to enable fast scanning of the image, the image sensors 17 are operated simultaneously. As described with reference to Fig. 7, the outputs of the CIS chips are preferably connected in parallel, rather than in daisy-chain configuration, to enable higher clock rates and to provide faster sampling rate, thus supporting imaging of the printed sheets at full printing speed and in the highest resolution required.

Color filters 34 are preferably mounted on the image sensors 17 (preferably CIS devices), preferably in a single line or a dual line configurations as described with reference to Figs. 5 and 6. As described with reference to Fig. 7, the outputs of the CIS devices are preferably connected in parallel, rather than in daisy-chain configuration, to enable higher clock rates and to provide faster sampling rate, thus supporting imaging of the printed sheets at full printing speed and in the highest resolution required.

The RGB information taken from each group of three CIS devices in single row format (or a group of four CIS devices in dual rows format) is converted to density information as described below. Alternatively, the RGB information is converted into XYZ values and then to LAB values to create Delta-E and LAB information readouts.

Preferably the CIS devices are calibrated in the manufacturer's laboratory and all artifacts and differences between pixels and changes over time are rectified in real¬ time during operation, using the information created during the calibration procedure. The calibration procedure preferably includes the following steps:

• Measure the flat-field correction needed (differences between pixels) • Adjust RGB gains and offsets

• Calibrate the responsiveness and light energy with an optical reference

• Measure the spectral response of the sensor

• Measure the spectral energy of the light source

• Alternatively, use an ICC profile of the scanner to achieve colorimetric data

The extraction of density values from the RGB measurements is possible when the spectrophotometric response of the sensor and the spectrum of the radiated light is known. A preferred method for converting RGB values to density values is as follows: The RGB outputs for a specific substrate are:

700

R= ∑r(λ)*S(λ)*β(λ)

1=400

S is the illumination function β is the substrate color function

R, G and B are the response functions of the image sensor

The R, G and B functions are preferably integrals over the visible light, preferably estimated using 32 or 64 points, preferably every 5 or 10 nanometers.

The RGB functions are preferably written as a matrix equation as follows:

R

G Sv B

Similarly and preferably, for a standard, theoretical CMYK sensor:

C M ..m» Y A

-y_n K

A is the standard A illumination type (CGATS standards) c,m,y,k are the "ANSI status T (or A,E or I)" standard responses

The above matrices are preferably solved to produce the CMYK value from the known response matrices of the light and the sensor, and the RGB values, as follows:

Alternatively, the measured spectrum of the illumination is used, together with the measured spectrophotometric response, providing prediction of the RGB values to be measured by the sensor, for each combination of β{λ) (2=380 to 740 in 5 or 10 nanometers steps).

Further alternatively, the CMYK values are predicted using the standard T (or A,E or I) functions and type A illumination parameters. Having multiple pairs of CMYK and RGB vectors from the same color sample, the following K matrix is solved:

C Ir Ir

M k₂yk_{2 3}

Y Jr Ir

K

For example by using the Generalized minimal error method (GMERR method) from the known RGB and CMYK values.

The quality measurement apparatus 10 preferably enables the imaging of the entire sheet to provide the following functions (in addition to measurement of color density):

• Print defects inspection - comparing the printed image with a scanned proofing image; • Digital proofing - comparing the printed image with a computer generated proofing image;

• Variable data verification - comparing selected areas of the printed image with variable data retrieved from a database, checking for the correctness of the variable data, registration to the regular, fixed print, existence of all variable data elements, etc;

• Extracting Delta-E, delta-LAB, LAB values and any color information from the RGB information of the sensor;

• Barcode inspection - validating the printed barcode information and grading it according to ANSI (grades A₅B₅C₃D or F) or DIN (grades 1,2,3,4 or 0) standards;

• Character reading;

• Remote proofing - transferring the image for verification to a remote site. It is appreciated that any press, sheet-fed or web-fed, using offset, flexo, gravure, letterpress or any other technology, conventional press or digital press, is appropriate for mounting the quality measurement apparatus 10. Integrating the quality measurement apparatus 10 into a printing system provides: • Illuminating and sensing of the printed image at close proximity;

• Mechanical stability ensuring long term calibration of the optical system;

• Reduce energy consumption due to shorter optical distances;

• Minimal modifications of the printing system due to the small size of the quality measurement apparatus 10;

• Mapping the spectral response of the CIS based sensor enables measuring the colorimetric data of the scanned image for CMYK, or LAB values.

• Density reading enables continuous and automatic control of ink-keys, ink-to-water balance and other printer parameters that affect color reproduction. It is appreciated that the use of the quality measurement apparatus 10 enables approving of the print job without physically visiting the print shop to verify the printed image visually. Having a reliable and accurate reading of the colors from the printed material while in the press, and furthermore having a reliable and accurate verification and correction of text and variable images, enables the printing customer to rely on the digital data and approve the print job from a remote location.

It is appreciated that the use of the quality measurement apparatus 10 enables inspection for process control by comparing the printed image with a required standard image (proofing image, master image). The proofing images are preferably obtained from the pre-press design or by scanning an approved sheet. When detecting a faulty image the quality measurement apparatus 10 preferably sends an alarm to an operator of the printing system, preferably indicating the detected defect. When the current image is different from the proofing image the differences are analyzed and compared against a pre-defined or user defined threshold to reduce false alarms. The detected defects are also analyzed for their type and origin, to assist the operator in resolving the problem. It is appreciated that such inspection can be performed by sampling sheets, since print defects tend to build up slowly. It is appreciated that some functions of the quality measurement apparatus 10 can be processed using relatively low resolution, thus requiring lower processing power, and therefore can be performed continuously for each sheet. Hence defective sheets may be prevented from reaching the final customer. It is appreciated that this screening process can be performed on-line or off-line in a post-printing process.

In a further embodiment of the present invention the contact image sensors may be located in non-contact relation with the sheet or other print substrate. That is to say the image sensors are removed a short distance away from the sheet.

It is expected that during the life of this patent many relevant printing systems and image sensing devices will be developed and the scope of the terms herein, particularly of the terms "Offset", "Gravure", "Flexographic" and "CIS", is intended to include all such new technologies a priori.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

WHAT IS CLAIMED IS:

1. Scanning apparatus for print control comprising: a plurality of contact image sensors, each having respective output connections, to obtain image quality data from a substrate, wherein said contact image sensors are arranged with their respective outputs connected in parallel for fast data readout; and a feedback unit for feeding back said image quality data for setting of parameters of a printer.

2. Apparatus according to claim I₃ wherein said image quality data comprises colorimetric data.

3. Apparatus according to claim 2 wherein said contact image sensors are fitted with respective color filters, thereby to provide said colorimetric data.

4. Apparatus according to claim 3, wherein said color filters are for at least three colors.

5. Apparatus according to claim 1, wherein said contact image sensors comprise CMOS Contact Image Sensor Integrated Circuits.

6. Apparatus according to claim 1 , wherein said color filters are for four colors.

7. Apparatus according to claim 1, further comprising a color density unit, associated with said feedback unit, for calculating color density values from said colorimetric data, for use by said feedback unit.

8. Apparatus according to claim 1, wherein said feedback unit is configured to control at least one member of the group consisting of ink keys, ink water balance, and press parameters of said printer.

9. Apparatus according to claim 1, further comprising a white light illumination unit for illuminating said substrate.

10. An image quality measurement apparatus for measuring quality of an image printed on a surface, said apparatus comprising: at least one illumination element mounted in close proximity to said surface and operative to illuminate at least a portion of said surface; an array of image sensors mounted in close proximity to said surface and operative to produce measurements of light reflected from said illuminated portion of said surface; a controller operative to control said image sensors and said illumination elements and to receive said measurements of light; and an analyzer operative to analyze said measurements of light and produce a measure of quality of said image printed on said surface. wherein said image quality measurement apparatus is capable of being integrated with a printing apparatus; and wherein said image quality measurement apparatus is operative to perform said measurements of light and to produce said measure of quality at printing speed.

11. An image quality measurement apparatus according to claim 10 wherein said close proximity is substantially 5 centimeters or less.

12. An image quality measurement apparatus according to claim 10 wherein said printing apparatus is at least one of a sheet fed printing apparatus and an offset printing apparatus.

13. An image quality measurement apparatus according to claim 10 and additionally comprising a feedback unit operative to provide said measure of quality to said printing apparatus.

14. An image quality measurement apparatus according to claim 10 wherein said analyzer comprising a color analyzer.

15. An image quality measurement apparatus according to claim 10 wherein said color analyzer comprises at least one of a color density analyzer and a color registration analyzer.

16. An image quality measurement apparatus according to claim 14 wherein said color analyzer comprises an analyzer of at least one of the group consisting of Delta- E, delta-LAB and LAB color measurements, and dot gain, dot area, trapping and gray balance.

17. An image quality measurement apparatus according to claim 13 and any of claims 14 to 16 wherein said measure of quality comprises a measure of color and said feedback affects composition of printing colors of said printing apparatus.

18. An image quality measurement apparatus according to claim 13 and any of claims 14 to 16 wherein said feedback affects at least one of calibration of ink keys and calibration of color registration.

19. An image quality measurement apparatus according to claim 10 wherein said analyzer is operative to compare said measurements of light with a proofing image.

20. An image quality measurement apparatus according to claim 19 wherein said proofing image is at least one of a computer generated image and a scanned image.

21. An image quality measurement apparatus according to claim 10 wherein said analyzer comprises of an analyzer of at least one of a barcode and a text.

22. An image quality measurement apparatus according to any of the claims 19 - 21 wherein said analyzer comprises an analyzer of at least one of a fixed data and a variable data.

23. An image quality measurement apparatus according to claim 10 wherein said controller is additionally operative to provide said measurement of light as a displayable image of said image printed on said surface.

24. An image quality measurement apparatus according to claim 10 wherein said image quality measurement apparatus is additionally operative to send at least one of said measurement of light and measure of quality to a remote site.

25. An image quality measurement apparatus according to claim 10 wherein said image sensors are contact image sensor (CIS) devices.

26. An image quality measurement apparatus according to claim 25 wherein outputs of said CIS devices are connected to said controller in parallel.

27. An image quality measurement apparatus according to claim 25 wherein said CIS devices are mounted in a single line.

28. An image quality measurement apparatus according to claim 27 wherein said single line comprises a plurality of triplets of red, green and blue sensors.

29. An image quality measurement apparatus according to claim 25 wherein said CIS devices are mounted in two lines.

30. An image quality measurement apparatus according to claim 29 wherein said two lines comprise a first line comprising a plurality of a first pair of color sensors and a second line comprising a plurality of a second pair of color sensor, said second pair being different from said first pair, said first and second pairs being selected from red, green and blue sensors.

31. A method of measuring quality of a printed image, said method comprising: illuminating said image at close proximity; measuring light reflected from said image at close proximity; and analyzing said measurement to form a quality analysis; wherein said steps of illuminating; measuring and analyzing are performed at printing speed.

32. A method of measuring quality of a printed image according to claim 31 wherein said close proximity is 5 centimeters or less.

33. A method of measuring quality of a printed image according to claim 31 additionally comprising a providing said quality analysis as a feedback to a printing apparatus.

34. A method of measuring quality of a printed image according to claim 31 wherein said steps of measuring and analyzing comprises measuring and analyzing color of said image.

35. A method of measuring quality of a printed image according to claim 34 wherein said step of analyzing color comprises analyzing at least one of a color density and a color registration.

36. A method of measuring quality of a printed image according to claim 34 said step of analyzing color comprising analyzing at least one of Delta-E, delta-LAB and LAB color measurements.

37. A method of measuring quality of a printed image according to claim 33 and any of claims 34 to 36 wherein said step of providing feedback affects composition of printing colors of said printing apparatus.

38. A method of measuring quality of a printed image according to claim 37 wherein said feedback affects at least one of calibration of ink keys and calibration of color registration.

39. A method of measuring quality of a printed image according to claim 31 wherein said step of analyzing comprises comparing said measurements of light with a proofing image.

40. A method of measuring quality of a printed image according to claim 39 wherein said step of comparing comprises comparing said measurements of light with at least one of a computer-generated image and a scanned image.

41. A method of measuring quality of a printed image according to claim 31 wherein said step of analyzing comprises analyzing at least one of a barcode and a text.

42. A method of measuring quality of a printed image according to claim 39 - 41 wherein said analyzer comprises an analyzer of at least one of a fixed data and a variable data.

43. A method of measuring quality of a printed image according to claim 31 additionally operative comprising displaying said measurement of light.

44. A method of measuring quality of a printed image according to claim 31 additionally comprising sending at least one of said measurement of light and measure of quality to a remote site.

45. A printing apparatus for printing on a surface, said apparatus comprising: an surface moving apparatus for moving a surface to be printed under at least one printing head at printing speed; said at least one printing head for printing an image on said surface; at least one illumination element mounted in close proximity to said surface and operative to illuminate at least a portion of said image; an array of image sensors mounted in close proximity to said surface and operative to produce measurements of light reflected from said illuminated portion of said image; an analyzer operative to analyze said measurements of light and produce a measure of quality of said printed image; and a controller operative to control said image sensors, said illumination elements and said printing heads in accordance with said analysis of said measurement of light; wherein said array of image sensors, said analyzer and said controller are operative to perform said measurements of light and to produce said measure of quality at said printing speed.

46. A printing apparatus according to claim 45 wherein said close proximity is less than 5 centimeters.

47. A printing apparatus according to claim 45 wherein said image sensors are contact image sensor (CIS) devices.

48. A printing apparatus according to claim 45 additionally comprising a feedback unit operative to provide said measure of quality to said printing head.

49. A printing method for printing on a surface, said method comprising: moving a surface to be printed under at least one printing head at printing speed; printing an image on said surface; illuminating at least a portion of said surface at close proximity; measuring light reflected from said illuminated portion of said surface, said measuring performed at close proximity to said surface; analyzing said measurements of light and producing a measure of quality of said printed image; and controlling said image sensors, said illumination elements and said printing heads in accordance with said analysis of said measurement of light; wherein said steps of illuminating, measuring, analyzing and controlling being performed at said printing speed.

50. A printing method according to claim 49 and additionally controlling said printing according to said measure of quality.

51. Scanning apparatus for print control comprising: a plurality of contact image sensors, each contact image sensor having respective output connections, and located in non-contact relation with a substrate to obtain image quality data from said substrate, wherein said contact image sensors are arranged with their respective outputs connected in parallel for fast data readout; and a feedback unit for feeding back said image quality data for setting of parameters of a printer.