Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41F—PRINTING MACHINES OR PRESSES
  • B41F33/00—Indicating, counting, warning, control or safety devices
  • B41F33/0027—Devices for scanning originals, printing formes or the like for determining or presetting the ink supply
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41F—PRINTING MACHINES OR PRESSES
  • B41F33/00—Indicating, counting, warning, control or safety devices
  • B41F33/0036—Devices for scanning or checking the printed matter for quality control
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F3/00—Colour separation; Correction of tonal value
  • G03F3/10—Checking the colour or tonal value of separation negatives or positives
  • G03F3/101—Colour or tonal value checking by non-photographic means or by means other than using non-impact printing methods or duplicating or marking methods covered by B41M5/00
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/40—Picture signal circuits
  • H04N1/40075—Descreening, i.e. converting a halftone signal into a corresponding continuous-tone signal; Rescreening, i.e. combined descreening and halftoning

Definitions

• the invention relates to a method for process control in printing according to the preamble of claim 1 and to a device designed for carrying out this method for image capture on a printing substrate according to the preamble of claim 10.
• the object of the invention is to provide a method which, for process control during printing, measures the measurement of halftone values on various printing substrates, such as printed paper, all types of printing plates, films and in particular also on smooth printing forms such as those used in digital printing in the press, with a high degree of accuracy.
• Another object of the invention is to provide an image capturing device with which suitable images for carrying out such a method of various printing substrates, in particular also smooth printing forms with reflecting surfaces, can be recorded.
• the method according to the invention makes it possible to automatically determine the halftone value of a section of a printing substrate with high accuracy on the basis of a recorded digital image.
• the method can be used with all types of substrates, in particular also with those with difficult contrast ratios, which lead to a gray value distribution with a blurred transition from covered to free surface in a digital image. It enables meaningful comparisons between different substrates.
• the method also provides, in the form of the halftone value variance, an integral quantitative statement about the homogeneity of the halftone value in a section of a substrate under consideration and in the form of a gray scale image a spatially resolved graphic representation of the fluctuations in the halftone value, which enables conclusions to be drawn about the causes of the fluctuations.
• the invention further provides an image capture device, the lighting device of which comprises a separately activatable and adjustable bright and dark field illumination.
• light-emitting diodes are used as light sources, the light of which is directed to the substrate surface via plastic rods functioning as light guides and via deflection and scattering elements.
• plastic rods functioning as light guides and via deflection and scattering elements.
• FIG. 1 shows a frequency distribution of the gray value measured on a section of a printing substrate.
• Fig. 2 is a - enlarged detail • - the central region of the frequency distribution of Figure 2.
• Fig. 4 is a diagram showing a halftone value variance as a gray value image
• FIG. 5 shows a schematic illustration of an image capturing device according to the invention
• Fig. 6 embodiments of light sources for use in the device of Fig. 5, and
• Fig. 7 digital images of sections of various printing r substrates.
• the starting point of the method according to the invention is a digital image of a section of a printing substrate, the quality of which provides information about the quality of the printing process to be monitored.
• These can be substrates such as film or paper that can already be measured satisfactorily with conventional densitometers, but also substrates that cannot be measured with sufficient accuracy using conventional measuring devices, such as printing plates and, in particular, smooth printing forms.
• An example of the second aspect of the invention, namely a device for capturing such a digital image on any printing substrates, will be explained later.
• a digital image of the type of interest here consists of a rectangular matrix of pixels, also called pixels, each of which is at the Image acquisition in a camera module has been assigned a gray value in the form of a digital number. For example, this exists.
• a known parameter for characterizing a printed image is the so-called halftone value R, also called area coverage. It specifies the proportion of the area of a print image that is covered when printing ink.
• the halftone tone value R of a print image present on a printing substrate cannot easily be determined from a digital image captured by the substrate with a camera module, since there are always numerous pixels that cover the edges of halftone dots and therefore have a gray value that differs from the particular one Extent of coverage depends. Furthermore, due to imperfections in the lighting and the substrate surface, even pixels that are completely in halftone dots do not appear to be completely covered and pixels that are completely outside of halftone dots do not appear to be completely free, i.e. the contrast between covered and free space is always limited.
• the raster tone value R is determined by evaluating the frequency distribution of the gray values of the captured digital image.
• An example of such a frequency distribution is shown in FIG. 1.
• This is actually a discrete frequency distribution, i.e. a histogram of the gray value, which in this case is between 0 and 255 according to a resolution of 8 bits, with the value 0 corresponding to a purely black and the value 255 to a purely white pixel.
• the height of the curve shows the number of pixels with this gray value above each gray value. The curve shown results from connecting the individual points with straight lines.
• the distribution has two distinct maxima, one in the lower half of the gray value range, the other in whose upper half is. These two maxima correspond to the covered or free area of the picture. They can always be expected in the gray-scale histogram of a digitally recorded print image and the more pronounced the higher the contrast between the covered and free area.
• the middle area between the two aforementioned maxima is evaluated by means of a compensation curve, which is shown in FIG. 2 which shows the central region of FIG. 1 enlarged.
• FIG. 2 shows the frequency distribution of the gray value is actually discrete and in the area in question is not monotonous with a considerable scatter of the individual points around the compensation curve entered.
• the use of the gray value with the absolutely lowest frequency as a limit value for distinguishing between pixels that are to be considered covered and those that are to be viewed freely would therefore not be a sensible procedure because it is too imprecise.
• a compensation curve is to determine a low-order polynomial, in particular a parabola, using the method of least squares, whereby it must always be ensured that the curve in the area of interest should rise monotonically on both sides of its minimum.
• Methods for determining compensation curves with constraints are well known as such in mathematics.
• a parabola is preferably used as the compensation curve.
• the curve shown in Fig. 2 is a parabola.
• the invention preferably provides for the two previously mentioned absolute frequency maxima to be searched first and then the gray value lying between them with the absolute minimum of Determine the frequency as the middle of the range in which the distribution is to be approximated by the compensation curve.
• a first criterion that seems reasonable for determining the width of the range of validity of the compensation curve is half the distance between the two maxima. This criterion was used in the example shown in FIGS. 1 and 2. It provides a compensation curve with a satisfactory course, however, only if the minimum is approximately in the middle between the two maxima.
• the width of the validity range is greatest when the minimum is exactly in the center between the two maxima and is then half the distance between the two maxima. However, it gets smaller the closer the minimum moves to one of the two maxima, i.e. the more asymmetrical the distribution is.
• the minimum G G of the compensation parabola is at a gray value of 91. All pixels below this value G G are now counted as covered, while all pixels above it are counted as free. The frequencies of all gray values below the minimum GG ZU are therefore added up to a number Ni.
• a further evaluation can begin with the integral area coverage R obtained in this way, ie related to the entire substrate section, namely the determination of the screen tone variance ⁇ R 2 as a measure of the homogeneity of the area coverage.
• a square unit cell is used, the side length of which is preferably as large as the smallest line spacing of the halftone dots on the substrate in the coordinate directions of the digital image, or can be an integral multiple. If the directions of the halftone dot lines on the substrate match the coordinate directions of the pixels of the digital image, the side length of such a unit cell thus corresponds exactly to the line halftone spacing of the halftone dots on the substrate.
• the unit cell is determined from the periodicity of the gray value curve of the pixels in the coordinate system of the digital image.
• Each pixel at least half a side length of a unit cell from the edge of the image is now circumscribed with a unit cell of the type mentioned, as is illustrated for a pixel in the top left of FIG. 3.
• the unit cell is identified by the drawn-in square spanned by the arrow cross and the circumscribed pixel by the small cross in the middle of the square.
• Fig. 3 the previously explained relationship between the side length of the unit cell and the line spacing of the halftone dots at a position of the line grid at 45 ° to the coordinate system of the digital image can be seen immediately.
• ⁇ (ni 1 N ⁇ ) -100 for the gray values of the pixels located within the respective unit cell.
• nn is the number of pixels within the respective unit cell with a gray value below G G and NE the total number of pixels of a 'unit cell.
• a local halftone value ⁇ and its deviation ⁇ -R from the integral halftone value R is determined for each such pixel, the calculation expediently being carried out incrementally when the entire image is processed, ie each time the unit cell under consideration is shifted from one pixel to the next, those below G G subtracted gray values of the falling out pixels and added those of the added pixels.
• a grid tone variance ⁇ R 2 is now assigned to the entire substrate section as an integral homogeneity parameter, which is calculated according to the rules of mathematical statistics as follows:
• n is the number of pixels for which a local screen value n has been calculated and i is a running index.
• ⁇ 2 the greater the fluctuations in the halftone tone value within the considered substrate section.
• the local deviation of the halftone tone value from its value R averaged over the substrate section under consideration can be converted into a gray-scale image for each pixel that can be rewritten with a unit cell, which gives location-resolved information about inhomogeneities, such as trends in the exposure width of the imagesetter or local exposure fluctuations.
• G M is a medium gray value and ⁇ a scaling factor.
• FIG. 4 An example of such a conversion of the local fluctuations of the halftone value into a gray value image is shown in FIG. 4.
• This shows on the left the digital image of a substrate section and on the right the associated gray value image of the deviation of the local halftone value n from its mean value R.
• Three different unit cells are entered in the left image and the assignment of these unit cells to the corresponding points in the right image is made clear by arrows.
• the dimensions of the right image in both coordinate directions are each smaller by the side length of a unit cell than those of the left one, i.e. for an edge region of the left image the width of half a unit cell, there is no implementation for the reason explained above.
• the unit cell could also be chosen to be larger than was previously proposed, but this would mean averaging over a larger area when calculating the local screen tone values ⁇ and thus reducing the local resolution.
• the unit cell could be transformed by a suitable transformation, i.e. by using an appropriately rotated unit cell, its dimensions are reduced and the local resolution is thereby increased somewhat.
• a universally applicable image capture device is provided according to a second aspect of the invention.
• This initially includes a camera module 1 with a CCD image sensor and a frame grabber or a USB or Firewire interface.
• the camera module 1 is intended for connection to a computer which is programmed for evaluating the recorded image and on whose screen the image can be viewed.
• a low-noise measuring camera with at least 300,000 pixels (eg as a 640X480 matrix) and 8-bit gray value resolution is preferably used.
• pixel-wise averaging is carried out over a large number, ie preferably over 20 to 100, of digitized images and only the noiseless image generated in this way is used for evaluation.
• a microscope objective 2 with a large working distance is selected for the imaging.
• the magnification is selected such that an area of a few square millimeters, for example 2.5 mm ⁇ 2 mm, is imaged. This minimizes the measurement error in the later evaluation, since a sufficient image resolution is combined with a sufficiently large image section (depending on the raster, an average of 100 raster points). Too small a resolution would mean that halftone dots would only be partially captured and the measurement accuracy would be reduced.
• the illumination device 3 combines a plane glass illuminator 4, 5 with a directly illuminating dark field light source 6.
• the plane glass illuminator consists of a light source 4 which emits transversely to the beam path running from the substrate 7 to the objective 2, and a partially transparent mirror 5 arranged in this beam path at an angle of 45 °, which deflects the light from the light source 4 onto the substrate 7 , where it is partially reflected and falls through the mirror 5 into the lens 2.
• the dark field light source 6 illuminates the substrate 7 at an angle of 45 ° to said beam path, so that light originating from this light source 6 can only enter the objective 2 by scattering on the substrate 7.
• the light sources can be switched on individually or together and their intensity adjusted to optimize the contrast of the captured image.
• the light sources 4 and 6 schematically shows a bright field light source 4 emitting straight ahead at the top and a dark field light source 6 emitting obliquely at approximately 45 ° below.
• the light sources 4 and 6 have a modular structure, in that light-emitting diodes 8 and 9 are respectively fitted and / or glued into a rod or bar 10 or 11 made of transparent plastic, as is available, for example, under the name "Plexiglas" 10 and 11 also acts as a holder for the light-emitting diodes 8 and 9 and as an optical waveguide, in that a large part of the coupled-in light is held in bars 10 and 11 by total reflection and only leaves it at the desired location, as is shown in FIG is indicated.
• a plurality of light-emitting diodes 8 and 9 can be arranged next to one another in one or also in both cross-sectional directions of the bars 10 and 11 in order to achieve a sufficiently high and evenly distributed light intensity.
• Another reason for the arrangement of several light emitting diodes 8 and 9 is. the combination of different emission colors to vary the spectral composition of the incident light, which is of particular interest in the case of at least partially colored substrates in order to optimize the contrast.
• the differently colored light-emitting diodes In order to be able to vary the spectral composition, the differently colored light-emitting diodes must be switched on separately and their intensity must be separately adjustable.
• the use of blue-emitting light-emitting diodes has proven to be advantageous for measuring black halftone dots on a reflective metal surface, as it has a smooth printing form.
• a diffuser '12 and 13 in the form of a film is attached to each of the exit surfaces of the plastic bars 10 and 11 to the most uniform possible illumination of the captured by the camera module 1 cut to reach the substrate. 7 Alternatively, the light can be scattered at the exit surface by roughening the surface in this area.
• the bar 11 of the dark field light source 6 has one taking into account the Refractive index, ie the additional deflection when exiting due to the refraction, beveled surface section 14.
• the width of the ingot 10 . and 11 are only limited by the available space. ,
• annular light source By means of a transparent plastic tube, an annular light source can also be realized, which can be arranged in a space-saving manner coaxial with the objective 2.
• the one end face of the tube is provided with a plurality of bores into which light-emitting diodes are fitted.
• the other end surface is chamfered axially symmetrically in such a way that the light which propagates in the longitudinal direction inside the tube is deflected towards the central axis upon exit and is focused on the region of the substrate 7 to be illuminated.
• Such a tubular light source is particularly suitable as a dark field light source.
• the lighting device 3 is mounted together with the camera module 1 and the lens 2 and an intensity control device (not shown) for the light sources 4, 6 in a housing 15 which is in the passage area 16 of the
• FIG. 1 Examples of digital images of various substrates, which were recorded with an image capturing device according to the invention, are shown in FIG.
• the substrates at the top left are a printing form digitally created in the printing press, known as "DicoForm", at the top right an aluminum printing plate, at the bottom left a film, and at the bottom right paper bright field illumination was used for the two left photographs, and dark field illumination was used for the two right photographs, as can be seen in FIG Device according to the invention from a wide variety of substrates obtain images of sufficient quality for automatic evaluation.
• DicoForm a printing form digitally created in the printing press

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Quality & Reliability (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Inking, Control Or Cleaning Of Printing Machines (AREA)
• Image Processing (AREA)
• Accessory Devices And Overall Control Thereof (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Manufacture Or Reproduction Of Printing Formes (AREA)
• Investigating Materials By The Use Of Optical Means Adapted For Particular Applications (AREA)

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• 2003
  • 2003-08-28 DE DE10339651A patent/DE10339651B3/de not_active Expired - Fee Related
• 2004
  • 2004-08-17 EP EP04764181A patent/EP1663653A2/de not_active Withdrawn
  • 2004-08-17 JP JP2006524287A patent/JP2007504964A/ja active Pending
  • 2004-08-17 WO PCT/EP2004/009189 patent/WO2005028197A2/de active Application Filing
  • 2004-08-17 US US10/569,748 patent/US20060219120A1/en not_active Abandoned
  • 2004-08-17 CN CNA2004800248092A patent/CN1842417A/zh active Pending

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