GB2222112A

GB2222112A - Monitoring print quality in multi-colour offset printing machines.

Info

Publication number: GB2222112A
Application number: GB8917898A
Authority: GB
Inventors: Wolfgang Uebel
Original assignee: Kombinat Polygraph Werner Lamberz VEB; Polygraph Leipzig Kombinat Veb
Current assignee: Kombinat Polygraph Werner Lamberz VEB; Polygraph Leipzig Kombinat Veb
Priority date: 1988-08-11
Filing date: 1989-08-04
Publication date: 1990-02-28
Anticipated expiration: 2009-08-04
Also published as: DD274786A1; JPH02175155A; GB8917898D0; DE3925011A1; CN1041563A; GB2222112B; DE3925011B4

Description

19 2 2 112 METHOD AND APPARATUS FOR MONITORING OF THE PRINTING QUALITY OF

MULTI- COLOURED PRINTING ORIGINALS IN AN OFFSET PRINTING MACHINE The present invention relates to a method and apparatus for monitoring the printing quality in multi-coloured raster printing originals in an offset printing machine, especially in conjunction with determination of the printed proportions of the primary colours in selected segments of small area so that an inking change can be carried out on departure from target values.

The judgement of printing quality and regulation of inking usually takes place by densitometric evaluation of an ink check strip. In that case, this evaluation can take place off-line at a statically measuring densitometer or on-line with use of a machine densitometer. It is disadvantageous in this procedure that it is necessary to print an ink check strip,which for reasons of format is frequently not possible and means an increased consumption of material. In patent specifications EP-0 142 469, EP-0 142 470 and EP-0 143 744 there are described a method and apparatus for the judgement of the printing quality and for regulation of ink feed in an offset printing machine without use of an ink check strip. In that case, the printed sheet is divided into image elements of the size 0.5 x 0.5 mm2 to 20 x 20 mm2 and the reflect- ance in four spectral ranges (infrared for black, red for cyan, green for magenta and blue for yellow) is measured photo-electronically in each image element. The reflectance values are then converted by c alculation with the aid of the known Neugebauer equations into areal coverages of the primary colours. Signals for the control of inking are derived by comparison of actual areal coverages with the areal coverages of an OK-sheet or the areal coverages measured at printing plates of the machine. For this purpose, a weighting factor for each printing ink is associated with each image element and indicates the certainty with which the areal coverage of this colour in this image element can be determined. It is disadvantageous in this method that merely the areal coverage is measured for the production of a control signal and it is implicitly presupposed that all colour raster points possess a constant ink layer thickness. As is known, however, both the areal coverage and the ink layer thickness are influenced on an inking change so that sole measurement of the areal coverage is not sufficiently accurate.

There is accordingly a need for a method in which the accuracy of of the judgement of the printing quality in multi-colour printing in an offset printing machine may be able to be increased at the moved sheet directly in the printing image without use of an ink check strip.

In particular it would be desirable to provide a method for monitoring the printing quality of multi-coloured print originals in an offset printing machine whereby measurement can be made not only of the areal coverage of the colour raster points but also of further printing image parameters so -as to allow increase in the accuracy of an inking control.

According to a first aspect of thepresent invention there is provided a method of monitoring the printingquality of multi-colour raster printing originals in an offset printing machine, comprising the steps of determining areal coverage in a measuring segment of an original and determining the mean colour raster point density of primary colours in the segment by producing digitalised colour extract images therefrom in three spectral ranges, the geometric resolution of the image raster 1 i field being higher than that of the printing raster and said mean density in the segment being determined from the extract images by determination of the mean grey value of the purely primary-coloured image points.

According to a second aspect of the invention there is provided apparatus for carrying out the method of the first aspect of the invention, the apparatus comprising an objective, a colour-measuring head for reflectance measurement for determination of the degree of areal coverage of a measuring segment, a first beam splitter for dividing the light beam from the objective and transmitting part of the divided beam to the colour-measuring head, a second beam splitter for receiving the remaining part of the divided beam and having three light exit surfaces respectively provided with red, green and blue filters, respective colour-selective, rastered-field, sensors associated with the filters and arranged to produce colour extract images, and image analysis means connected to output means of the sensors and operable to determine the mean grey value of the pure primary-coloured image points.

In a preferred example of the method, the ink layer thickness of the pure primary colour is determined by estimation of the grey value of the image points, which represents a measure for the magnitude of the absorption, by the scanning of digitalised colour extract iamges in the spectral ranges of red, green and blue by means of image analysis. For this purpose, the measuring segment, in which the determination of the degree of areal coverage takes place, is covered by an image raster and multi-spectral recordings are produced in the given raster field. The image recording takes place by colour-selective sensor fields. For this purpose, a first beam splitter is arranged behind the objective and resolves the light reflected from the measuring segment by way of the objective into two parts, wherein the first part is passed to a.colour-measuring head for reflectance measurement for the determination of the degree of area] coverage, whilst the second part is passed to a second beam splitter having the three light exit surfaces provided with corresponding red, green and blue filters, at which exit surfaces the colour-selective sensor fields are arranged. The outputs of the fields are connected to an image analysis system.

The geometric resolution of the image raster field is higher than that of the printing raster so that its raster values amount to a multiple of the image point spacing. The grey value resolution depends on the required measurement accuracy of the ink layer thickness. The ink layer thickness is estimated from the obtained colour extract images of the multiple print with the aid of image analysis. For this purpose, the colour extract images are initially made binary in a respective first threshold, which represents the boundary between the black colour raster points and the printed material. The three arising binary images are so logically interlinked that an image point in the resultant image receives the value 11111 exactly when the same image points in the three images to be interlinked also have the value 111". The thus obtained image points contAin the colour black or colour coverings equivalent to black. In addition, the same colour extract images a re each made binary with a second higher threshold in such a manner that, apart from the image points of the colour black, they also contain image points of a primary colour and their coverings with the value "V'. Finally, all those image points in each of these binary %C 1 images receive the value M" by the second threshold, which have the value "V' either in the resultant image of the first interlinking of the binary images with the first threshold or in one of the two remaining binary images with the second threshold. After this operation, only those image points still have the value 111" in the three binary images with the second threshold, which contain a pure primary colour without colour coverings. The mean grey tone of these image points represents the desired value of-the ink layer thickness.

Apart from inking change signals from the determined area] cover- ages, further inking change signals can be produced from comparison of the actual ink layer thicknesses with the layer thicknesses of an OK- sheet.

For preference, the monitoring of the printing quality takes place online at the moved sheet immediately after the last colour printing in that the duration of the flash illumination is matched to the speed of movement.

An example of the method and an embodiment of the apparatus will now be more particularly described with reference to the accompanying drawing, the single figure of which is a schematic block diagram of apparatus for carrying out a method exemplifying the invention, in particular for image analysis determination of the ink layer thickness of the colours grey, magenta and yellow.

Referring now to the drawing there is shown in Fig. 1 apparatus tor monitoring the printing quality of multi-coloured print originals. A photo-electronic reflectance measurement for determination of the degree of areal coverage in a measuring segment 1 on a printed material web 2 takes place in known manner by way of an objective 4 with a colourmeasuring head 3. The measuring segment 1 has a size of 2.5 x 2.5 MM2. For determination of the ink layer thickness of the colour raster points, three colour extract images are set up from the same measuring segment 1 by way of beam splitters 5 and 6, wherein the light exit surfaces of the beam splitter 6 are provided with corresponding red, green and blue filters, with the aid of a colourselective sensor 7. The sensor 7 is so constructed that a sharp image is produced for each colour extract on the sensor field.

The scanning takes place with a geometric resolution of 20 micrometres per image point, i.e. it- is about 10 times as high as the print raster resolution and a grey value resolution of 64 grey steps.

A flash light with standard light mode C or similar characteristic with a flash duration of 1 to 5 microseconds serves for illumination according to the speed of the printing sheet of 2 to 10 metres per second. The filtering must occur in the spectral absorption ranges typical for the three colours. In the ideal case, the transmission curve of the filter corresponds to the absorption curve of the respective colour. The determination of the ink layer thickness takes place by image analysis of the three colour extract images in an image analysis system 8. The image analysis system 8 is controlled by a 16-bit personal computer 9. For demonstration, the individual processing steps can be depicted on a colour image monitor 10. In order to be able to analyse the ink layer thickness from the image, those image points must be determined initially, which are printed on only by the colours grey, magenta or yellow and have no superposed printing. They are called monochromatic image points. The -mean grey value of these image points represents the desired estimated value for the thickness. Some definitions will now be given for explanation of the algorithm, wherein gb represents the quantity of a] l image points of the blue extract. The grey values are associated with the quantity elements P9b.

gb [Pgb (i,j): i = 1..128.i = i..128], wherein P9b610,1,...63].

The corresponding applies to the quantity gg of the image points of the green extract and for the quantity gr Of the red extract with correspond ing values pg, and Pgr. In the-case of four-colour printing, the quantity 91(1,b,gj) consists of 16 subquantities of the image points of the four primary colours, the six secondary colours, the four tertiary colours, the quartiary colour as well as the colour w of the printed material, i.e.

g, = [Ecl, [y], [m], Is], ECYL 1CM1, [CS], EYMI, EYSI, lms] ECMY], [CMS], EYMS], ECYS], [CMYS], [Wj For example, c is the quantity of all those image points which correspond only to the colour cyan. In that case, digitalising effects are left out of consideration.

In making binary a grey value image gl with a threshold thk, there arises the binary quantity bl = [pbl (i,j): i = 1-128, j = 1-128], wherein pbl Co,1]1GEb,g,r].

Furthermore, all elements with bl = 0 are combined into the subquantity Cwl so that the remaining 15 subquantities only - - contain elements pbl = 1. According to choice of the threshold, the subquantities will have different powers or -- --,!-be empty.

1.

Finally, the following logical interlinkings are defined:

0 the binary quantity bk and bl results The interlinking bk + bl o., in a quantity bv, the elements pbv(i,j) of which are exactly equal to 1 when pbk(i,j) = 1 and pb (i,j)l = 1.

The interlinking bk-bl of the binary quantities bk and bl results in a quantity bvs the elements pbv(i,j) of which are exactly equal to 1 when pbk(i,j) = 1 and pbl(i,j) = 0.

At first, those image points are determined, which belong to one of the subquantities of Is'] = -rcsl, [CS], Ems], EYSI, 1CMS1, ICYSI, EMYSI, 1CMYS3 or possess their grey values. For this purpose, the colour extract images gb, 99 and gr are to be made binary with the threshold tb, tg or tr, which represent the boundary between the black colour raster points and the printed material.

The obtained images are bb, b 9 and br. The interlinking b = bb+b 9 +br supplies the quantity bs = LESl, [CMY1, [cm"], Ecyll ' wherein [cm] = L[cml, [cml 11 and Ecy] = [Icy,], Icy, 11 That means that the coverings of the colours cyan and magenta as well 20. as cyan and yellow partially supply grey values in the range of the colour black. The cause lies in the printing inks not being optimal.

In a second step, the grey value images gl(iE[b,g,r]) are made binary anew with the thresholds tbmon, tgmon or trmon in such a manner that t, Icy], CS'], 1cmy], Ecy], Emy], 1CM1 [Em], is.], 1CMY1, [my], [CM1, Ecyl [CC], Es."I [CMY1, 1CM1, ECYJ That means that in every colour extract image that has been made binary, apart from the subquantities of bs only not empty subquantites of a colour and their coverings are still contained. Thresholds tbrnon, tgmon and trmon of that kind exist, since the filters are adapted to the absorption ranges of the respective colour. For them, there always applies tbmon tb, tgmon tg and trmon tr. Since the object consists of the determination of monochromatic image points and not ---the determination of all monochromatic image points, the thresholds are always to be so chosen that the bi, (i b,g,r) contain only the above sub quantities. The sought quantities of the monochromatic image points thus C = (br-brn)((bg-bs+(bb-bm)) = [c] M = (b g- bm)-((bb-bs+(br-bm)) = [m] Y = (bb-bm)-((br-bs+(b g- bm)) = Cy] Proof:

br-bm = [Cc], [cm"], [cyl"j b - b CM1, [my], EcM,' 11 9 m = [C bb-bm = [Ey], [my], Ecy '1] It follows from this:

Ecl, [cm], ECY1 [EY], [m], Ecy,'], 1cmX%13 [c] c rmj, [MY], [cm,13 ECY], [MY], Ecm'l, Ecy,2 [M] m [1c], Em], [my], [cm ' 1, ICY1 EY1 Y Ely], [my], Ecy 1 - The computation of the mean grey value of these image point.quantities [c], [m] and Cy] in the associated colour extract images gr, 99 and gb supplies the desired estimated value of the ink layer thickness. By a comparison of the actual ink layer thicknesses of an OKsheet, inking change signals 12 are produced in block 11 with use of the results for area] coverage.

Claims

- 10 CLAIMS 1. A method of monitoring the printing quality of multi-

colour raster print originals in an offset printing machine, comprising the steps of determining areal coverage in a measuring segment of an original and determining the mearl colour raster point density of primary colours in the segment by producing digitalised colour extract images therefrom in three spectral ranges, the geometric resolution of the image raster field being higher than that of the printing raster and said mean density in the segment being determined from the extract images by determination of the mean grey value of the purely primary-coloured image points.
2. A method as claimed in claim 1, comprising the step of producing inking change signals from comparison of the areal coverages and further inking change signals from comparison of actual ink layer thicknesses with layer thicknesses of an OK-sheet.
3. A method as claimed in either claim 1 or claim 2, wherein the three colour extract images are made binary in such a manner with a respective first threshold representing the boundary between the black colour raster points and the printed material that all image points containing the colour black or colour coverages equivalent to black, are determined by logical interlinking of the binary images in which an image in the resultant image receives the value IT' exactly when the same image points in the three images to be interlinked also have the value "V' and wherein the same colour extract images are made binary in such a manner with respective second higher thresholds that apart from the image points of the colour black they also contain image points of a primary colour and its coverages with the value "V' and wherein finally all those image points in each of these binary images, which have the value I'll' either in the resultant image of the first interlinking of the binary images of the first threshold or in one of the two remaining binary images with the second threshold, receive the value 11011 in the second threshold so that now the image points with the value "ll' are only those which contain a pure primary colour without colour coverages. the mean grey tone of which supplies the desired value of the ink layer thickness.
4. A method as claimed in any one of the preceding claims, wherein monitoring of the printing quality takes place on-line at a moved sheet immediately after the last colour printing by matching the duration of the flash illumination to the speed of movement of the sheet.
5. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.
6. Apparatus for carrying out the method claimed in claim 1, comprising an objective, a colourmeasuring head for reflectance measurement for determination of the degree of area] coverage of a measuring segment, a first beam splitter for dividing the light beam from the objective and transmitting part of the divided beam to the colourmeasuring head, a second beam splitter for receiving the remaining part of the divided beam and having three light exit surfaces respectively provided with red, green and blue filters, respective colour- X selective, rastered-field sensors associated with the filters and arranged to produce colour extract images, and image analysis means connected to output means of the sensors and operable to determine the mean grey value of the pure primary-coloured image points.
7. Apparatus substantially as hereinbefore described with reference to the accompanying drawing.

Published 1990 atThe Patent OMce, State House. 86.7 1 High Holburn, London WClR 47?. Further copies zn&Y be obtamedtro Th PatantoffloOSales Branch, St Msi7 Cray, Orpington. Kant 16IM5 3RD. Printed by Multiplex techniques ltd. St MLTY CrAY. Ken'- Con, 1/87