EP0143744A1 - Method and device for rating the printing quality and/or controlling the ink supply in an offset printing machine, and offset printing machine with such a device - Google Patents

Abstract

The print sheets are scanned immediately after the last printing unit. The printed sheets are scanned pixel by pixel with one or more measuring heads. A reasonable picture element size is approx. 1 × 1 mm² - 10 × 10 mm². The reflectance on each picture element is measured in four spectral ranges (infrared for black, red for cyan, green for magenta and blue for yellow). The measured reflectance values are converted into surface coverings (unmasked) using the Neugebauer equations and fed to the computer (5) for evaluation. Both the target values (OK sheet) and the actual values (continuous printing) are measured with the same measuring device. The computer compares the measured data, weights it depending on the area covered, the amount of extraneous color and the surroundings of the respective picture element and provides a control signal for each zone and color to control the color control. The color coordinates (X, Y, Z) can be determined from the area coverage values of the four colors in a parallel calculation process for each picture element. Important picture elements for the visual impression of a picture are given a high weight. A quality measure for the change in the visual impression can be determined from the weighted comparison of the target and actual value. With this method it is possible to control the color guidance of a multicolour printing machine online without using a color measuring strip by direct measurement in the printed image.

Description

The invention relates to a method and a device for assessing the print quality and / or regulating the ink flow in an offset printing machine according to the preamble of claim 1 and 15, and to an offset printing machine equipped with a corresponding device according to the preamble of claim 17.

The assessment of the print quality and regulation of the color routing is usually carried out with the help of standardized color control strips. These printed control strips are evaluated densitometrically and the color values of the printing press are then adjusted accordingly. The measurement of the color control strips can be done on the running machine with so-called machine densitometers or off-line using e.g. of an automatic scanning densitometer, whereby the control loop towards the inking units can be open (quality assessment) or closed (machine control) in both cases. A representative example of a computer-controlled printing machine with a closed control loop is in U.S. Patent 4,200,932.

In practice it happens e.g. for reasons of format it is very common that the use of a color control strip is not possible. In these cases, the quality assessment is usually still carried out visually and the color management accordingly based on the visual assessment by hand.

US Pat. No. 3,376,426 describes a multicolor printing machine which is controlled via a machine densitometer and does not require color measuring strips. With this printing machine, the individual printed sheets are scanned point by point, the reflectance values obtained are converted into densities (logarithmic) and the color densities are transformed into analytical color densities in a nonlinear unmasking operation. These analytical color densities are compared directly with the analytical color densities of an OK sheet obtained and stored in the same way, and a signal is obtained from the comparison result that indicates the deviation of the color guide from the target setting and by means of which the color guide can be adjusted.

However, this system described in US Pat. No. 3,376,426 has not proven itself in practice. One of the main reasons should be seen in the fact that the consideration of the secondary absorptions and the superimposed pressure is not sufficiently solved with this system.

Recently, a system has also become known (see, for example, the published UK patent application 2 115 145) which enables machine evaluation of printed products even without color control strips. In this system, the printed products on the running printing press are photoelectrically scanned picture-by-picture over the entire image area by means of a machine densitometer. The sample values from the individual picture elements, possibly after special preparation, are compared with the sample values of a reference printed product (OK sheet), which may have been prepared, and a quality decision "good" or "bad" is made on the basis of the comparison result according to certain criteria. The decision criteria include the number of picture elements that differ by more than a certain tolerance measure from the corresponding picture elements of the reference, the sub-sum of selected picture areas the sample values differ from the corresponding sample values of the reference and the differences of the sample values summed up over certain scanning tracks from the corresponding values of the reference.

While this new system is already making some progress, there is still room for improvement in some respects.

The aim of the invention is to provide a system for mechanical quality assessment of printed products and corresponding regulation of the ink guide members on a printing press, which is improved compared to the prior art, in particular with regard to accuracy and reliability, and does not require color control strips.

The method according to the invention, the device according to the invention and the corresponding offset printing machine according to the invention, which do justice to the object on which the invention is based, are described in claims 1, 15 and 17. Preferred embodiments and further developments result from the dependent claims.

• Fig. 1 eine stark vereinfachte, schematische Darstellung der erfindungsrelevanten Teile eines Ausführungsbeispiels einer erfindungsgemässen Offset-Druckmaschine und
• Fig. 2 ein Blockschema einer Ausführungsvariante.

In the following, the invention is explained purely by way of example with reference to the drawing. Show it:

• Fig. 1 is a highly simplified, schematic representation of the parts of an embodiment of an offset printing machine according to the invention and
• Fig. 2 is a block diagram of an embodiment.

Of the actual, conventionally constructed printing machine, only the last printing unit 1 and the ink guide elements 2 are indicated in the drawing. 3 with a machine densitometer is also of conventional design, which scans the printing units (printing sheets 4) photoelectrically. An electronic system in the form of a process computer 5 is attached to the machine densitometer 3 connected, which controls all functional sequences of the machine densitometer and evaluates the remission data generated by it. The result of this evaluation are control values or signals with which the ink guide elements 2 of the printing press are influenced. Instead of control signals or in addition to this, the process computer can also process the measurement data into quality measures for assessing the print quality.

So far, the arrangement shown largely corresponds to the known state of the art, as it is among others. is given by the publications mentioned at the beginning. The main difference compared to this lies primarily in the acquisition of the measurement data and in their processing.

The photoelectric measurement of the individual printed products takes place picture-wise, ie the printed sheets are divided into picture elements and the reflectance in each picture element is determined in four spectral ranges (infrared for black, red for cyan, green for magenta and blue for yellow). Are x 0.5 at 0.5 for the practice reasonable picture element sizes mm ² to about 2 ₀ x 20 mm ^2, preferably about 1 x 1 mm ² to 10 x 10 mm ^2. Of course, the reflectance values for the individual picture elements do not all have to come from the same printed sheet; the reflectance recording can rather also be distributed over several printed sheets (less effort in terms of apparatus). Examples of suitable machine densitometers with which the printed products can be scanned pixel by pixel in the manner described are described, inter alia, in US Pat. No. Nos. 2 968 988, 3 376 426, 3 835 777, 3 890 048 and 4 003 660.

According to an important aspect of the invention, the measured remissions are not converted into density values but are immediately "unmasked", ie the associated area coverings for the printing inks are calculated from the four remissions of each image element. This calculation is done in a more detailed way Way by solving the Neugebauer equations. The unmasking of the remissions is indicated in the drawing by the box 51 within the process computer 5.

The further processing of the measurement data is only indicated in the drawing for a single printing ink (black). The measurement data relating to the other printing inks is processed analogously.

If the printing process (by hand) is set correctly and the desired print quality is achieved, the printer gives the OK for continued printing. The printed sheets created at this point in time and immediately afterwards can be used as a reference (OK sheets). This reference (in the form of a single sheet or in the form of several successive sheets) is now measured and unmasked according to the picture element above. The area coverings of all image elements of the reference, hereinafter referred to as target area coverings, calculated in this way are stored in four area coverage matrices 52 each associated with a printing ink. Furthermore, based on these surface coverings, four weight matrices each assigned to a printing ink are calculated (block 53) and stored (block 54). Each image element is assigned a weighting factor for each printing ink, which indicates the certainty with which the area coverage of this color can be determined in this image element. More details about these weight factors are explained below.

The color of the press is divided into zones. The weight factors are therefore added up in block 55 according to their belonging to a zone. A total weight is then obtained for each zone and printing ink, which represents a measure of the security with which a change in the color guidance in this zone can be measured.

Of course, the calculation of the weight matrices and the zonal total weights only has to be carried out once.

In order to assess the print quality and / or to regulate the ink flow, production sheets are measured continuously or from time to time in the same way as the reference (OK sheet) and compared with the reference.

The surface coverings of the continuous printing (actual surface coverings) obtained after the unmasking from the remissions are compared picture element by picture element with the corresponding target surface coverings of the reference (subtraction level 56) and the deviations from the target surface coverings with the associated ones stored in the weight matrices 54 Weighted factors weighted (multiplier 57). The weighted deviations are summed up zone by zone for each printing ink (summer 58) and finally the zonal sums thus formed are normalized by division (divider 59) by the assigned zonal total weight. The result of these steps is then a weighted, standardized zonal deviation per printing zone and printing ink, which expresses the relative color deviation in the printing zone during the printing process and can be used as a control signal for the assigned ink guide element 2.

The comparison of target and actual area coverings is preferably carried out online, so that the individual measured values of the production run do not have to be saved.

In addition or in parallel to the evaluation explained above, the deviations of the surface coverings can also be converted into coordinate deviations in the color space. Different weights corresponding to the importance of the image can be assigned to the individual image elements and the deviations can thus be weighted. In this way, changes in the visual impression of the printed image or a quality measure can be determined.

Darin bedeuten:

• F.., F.'. _: Flächenbedeckung des Bildelements i bezüglich 1,J 1,J Druckfarbe j für Referenz bzw. Fortdruck
• G_i,j : Gewichtsfaktor des Bildelements i bezüglich Druckfarbe j
• _: Summation über alle Bildelemente i einer Zone z
• ΔF_relj : genormte zonale Abweichung der Flächenbedeckung für die Druckfarbe j

The formation of the standardized zonal deviations, which are used as control values for the color guide elements, can be represented as follows:

Where:

• F .., F. '. _: Area coverage of the image element i with respect to 1, J 1, J printing ink j for reference or production printing
• G _{i, j} : weight factor of the picture element i with respect to the printing ink j
• _: Summation over all picture elements i of a zone z
• ΔF _relj : standardized zonal deviation of the area coverage for the printing ink j

The unmasking and the formation of the weighting factors are explained in more detail below.

The spectral course of the printing inks is not ideal. Therefore, the mutual influences of the secondary absorption must be suppressed as well as possible in the photoelectric measurement. The influence of the individual color components and the statistics of the overprinting depending on the area coverage of the individual printing inks is described by the so-called Neugebauer equations (see, for example, article "Theoretical Foundations of Multicolour Book Printing" in "Journal for Scientific Photography, Photophysics and ^{Photochemistry} ", Volume 36, Issue 4, April 1937). Expanded to four colors j = infrared, red, green, blue:

• ß. : Remissionen gemessen mit Filter der Farbe j
• W. : Weissremission mit Filter j
• B_j, C_j, M_j, Y_j : Remission von Vollton Black, Cyan, _Mag_enta, _Yellow gemessen mit Filter j
• BC., BM., BY_j, Bl_j, G_j, R. : Remission von Vollton-Uebereinanderdruck B+C, B+M, B+Y, C+M (Blau), C+Y (Grün), M+Y (Rot) gemessen mit Filter j
• BBl_j, BG_j, BR_j, N_j : Remission von Vollton-Uebereinander- druck B+C+M, B+C+Y, B+M+Y,
• C+M+Y (Noir) gemessen mit Filter j BN. : Remission von Vollton-Uebereinander-
• druck B+C+M+Y gemessen mit Filter j b, c, m, y : Flächenbedeckungen der Druckfarben B, C, M, Y

Where:

• ß. : Remission measured with a filter of color j
• W.: White remission with filter j
• B _{j, j j} C, M, Y _j: remission of masstone black, cyan, _Ma g _{enta, Yell} ow measured with Filter j
• BC., BM., BY _j , Bl _j , G _j , R.: Remission of full tone overprint B + C, B + M, B + Y, C + M (blue), C + Y (green), M + Y (red) measured with filter j
• BBl _j , BG _j , BR _j , N _j : remission of full tone superimposed printing B + C + M, B + C + Y, B + M + Y,
• C + M + Y (Noir) measured with filter j BN. : Remission of full tone overlap
• print B + C + M + Y measured with filter jb, c, m, y: area coverage of the printing inks B, C, M, Y

B.-BN. are constants that depend on the print order and the solid density. Their values can be measured empirically from corresponding color tables. For the print order B, C, M, Y, for example, they were determined as follows for a solid density of approximately 1.5:

Für die Volltondichten D im Bereich von 1 bis 2 liegen diese Werte in einem engen Bereich von X^0,6 bis X^1,3, wenn X den Tabellenwert bezeichnet.

For the solid densities D in the range from 1 to 2, these values are in a narrow range from X ^0.6 to X ^1.3 if X denotes the table value.

The Neugebauer equations above, in which the ß. the measured remissions mean are resolved iteratively according to the unknown area coverings b, c, m, y. It is assumed that F = 1-ß is met with sufficient accuracy (F = area coverage (b, c, m, y), ß = remission). Due to the mutual influences, the most suitable sequence for iteration is magenta, yellow, cyan, black.

The security with which deviations in the area coverage of a picture element can be determined depends on several parameters. With approx. 50-70% area coverage, the dot increase has the greatest effect when the solid tone increases in density. This means that medium area coverages have to be taken into account more than large or small area covers. In a quiet environment (homogeneous surface coverage), incorrect positioning plays a smaller role than in a moving environment. If several colors are printed together at the same point, the individual color can be isolated less accurately. To take these factors into account, for each The image element and / or printing ink defines three partial weights, namely a partial weight G ₁ that is dependent on the area coverage, a partial weight G ₂ that is dependent on the environment and a partial weight G ₃ that is dependent on the foreign color. The three partial weights are multiplied together and together result in the weight factor already mentioned for each picture element and each printing color. The individual partial weights can also be weighted differently when combined with the weight factor, which can be expressed as follows:

Here, g _l -g ₃ mean the influence weights of the three partial weights. These influence weights are in the range from 0 to 1. Usually G ₁ receives the strongest and G ₂ the weakest influence weight.

For special printing originals, it is conceivable to introduce a fourth weight G ₄ . With G ₄ , certain areas of the printed sheet can be rated stronger or weaker. The areas and G ₄ can be entered interactively by the printer via a computer terminal. For example, G ₄ can be used to suppress the rating of printed text.

Deviations in the color scheme have the greatest impact with approx. 50-70r surface coverage. With medium area coverings, deviations can thus be determined with greater certainty. Accordingly, the area weight-dependent partial weight G ₁ is chosen so that it is maximum for medium area coverings, but drops towards smaller and larger area coverings. Suitable courses (partial weight G ₁ as a function of the area coverage) are, for example, parabolas, triangles, trapezoids, the maximum value 1 of the partial weight in each case at or around 50% area coverage lies. Of course, asymmetrical gradients, which prefer higher surface coverings, are also possible. Some examples of partial weight curves are as follows:

The indexes i, j for picture elements and printing ink are omitted in these formulas for the sake of simplicity.

The more homogeneous the area coverage in the area of a picture element, the less sensitive the measurement value is to incorrect positioning (large influence of edges). Edges can best be determined using differentiation. Steep edges result in large values, which must correspond to a small weight. A Laplace operator of the general type is particularly suitable as a simple differential operator in a 3 x 3 picture element environment:

Use of this operator means that the area coverage of the respective picture element (for one printing ink each) is weighted with the factor c, the area coverage of the picture elements surrounding it with the factors a and b. The sum of the nine area coverings weighted in this way corresponds to the derivation of the area cover in the relevant picture element.

In practice, the lapalace operator can look like this:

For finer gradations, the environment taken into account can be enlarged as desired. The diagonal coefficients can also be taken into account (fO).

worin |L| das Ergebnis des gemäss Obigem auf das betreffende Bildelement und seine Umgebung angewandten Laplace-Operators und c das Mittelelement des Laplace-Operators ist.The environment-dependent partial weight G ₂ (for each picture element and for each printing ink) is then calculated from the following formula:

where | L | is the result of the Laplace operator applied to the relevant picture element and its surroundings in accordance with the above, and c is the central element of the Laplace operator.

The smaller the area coverage of three colors, the more precisely the area coverage of the fourth color can be determined. Not every color has the same influence on the measurement of the others. For this reason, separate influencing coefficients must be taken into account for each color or filter. The partial weight G ₃ then results as the product of the reflectance values of the foreign color components exposed with the corresponding influencing coefficients:

In it, ß _B , ß _C , ß _M and ß _{Y are} the remissions in the colors B, C, M and Y and a _j1 to a _j4 the mentioned _{coefficients of} influence. The index j denotes the respective printing ink for which the partial weight applies. For j = B, C, M and Y these coefficients can be represented in a matrix:

Practical values are for example:

The coefficients depend on the spectral course of the individual colors. Their spreading range is approximately as follows:

The deviations are distorted due to the non-linear weighting. It is therefore not possible to make a precise statement about the absolute degree of the deviation.

If the full tone deviation is constant, the greatest deviation of the surface coverage is around 50-70%. Partial weight G ₁ also focuses on around 50% area coverage. G ₁ thus causes a dynamic compression of the deviations with larger and smaller area coverings. If, for example, the trapezoidal function of G _{1 is} chosen wide enough, there are only slight distortions of the absolute deviations.

The situation is different for the partial weights G ₂ and G ₃ . They distort the deviations due to environmental and external influences and are difficult to calculate. If you do not want to distort the measured size of the deviations by the weighting too much, the partial weights must be either 0 or 1. If, for example, G or G ₃ exceeds a certain value, they are 1, below that they are 0. With this digital weighting, the calculated relative deviation of the area coverage is somewhat proportional to a change in density of the full tone.

With this weighting, the deviations are less distorted. With extreme printing originals, however, there is a risk that all weights in a zone become 0.

For 5 and 6-color printing, an additional scanner must be attached before and after the printing unit of the fifth and sixth color. With the measurement before and after the printing unit, the contribution of the last printed color can be calculated and the deviation from the target value can be determined.

Special colors are often printed in full tone without overprinting. In this case, the area-dependent partial weight G _l must be ₁ for medium and full tones. The foreign color-dependent partial weight G ₃ becomes 0 for each picture element with a (also small) area coverage of a foreign color. This ensures that only pure colors are measured.

According to the above, the target values of the surface coverings are obtained from a reference in the form of one (or more) OK sheet. However, this is not mandatory, but other references can also be used. One such alternative is to use the printing plates themselves as a reference. For this purpose, the individual printing plates are divided into picture elements in the same way as the printed products to be tested. The picture elements are scanned photoelectrically and the area coverage is determined for each picture element. There are two options for further processing. Either the measured area coverage of each image element of each printing plate is converted into corresponding area coverage in the print using the printing characteristic of the printing press (empirically, tables) and then used directly as target area coverage for comparison with the actual area coverage, or the measured area coverage is based on the printing characteristic curve converted into remission values, which are then unmasked again according to the description above and thereby included in the Desired area coverings are transformed. In the latter possibility, the reference is synthesized from the printing plates to a certain extent.

2 shows a block diagram of a device operating according to this variant. As in FIG. 1, the process computer 5 is connected to the already mentioned machine densitometer 3 and the ink guide elements 2 of the printing press. In addition, a plate scanner 6 is also provided, which is also connected to the process computer 5. The plate scanner 6 is of conventional design (e.g. according to US Pat. Nos. 4,131,879 and 3,958,509 or EP Publ. No. 69572, 96227 and 29561) and scans the individual printing plates point-by-point photoelectrically. The scanning points (spots) can coincide with the picture elements or can preferably be significantly smaller than these. In the latter case, the area coverage of the individual picture elements can be determined with greater resolution and thus more precisely and reliably. Details on the prediction of the remissions or the determination of the surface densities from the printing plates can be found in the co-pending US patent application Serial No.

from (corresponding to CH patent application No. 5965/83 of November 4, 1983).

According to the above, the control of the printing process can also take place on the basis of a reference in the form of the underlying printing plates (or perhaps even on the basis of the raster films or the like on which the latter are based). Mixed operation is also possible. This means that the printing plates are used to ramp up the printing process to a satisfactory quality (OK sheet), but an OK sheet is then used as a reference for continued printing. Ideally, the OK sheet already matches the "synthesized" reference calculated in advance on the basis of the printing plates, so that a special measurement of an OK sheet is not necessary.

Claims (17)

1.Procedure for assessing the print quality and / or regulating the ink flow in an offset printing machine with image-wise photoelectric measurement of printed products and a reference, image-wise comparison of the printed products and the reference and determination of a quality measure or control values for the ink guide elements on the basis of this comparison, characterized in that a reference in the form of a print product found to be good or the printing plates on which the printing is based is divided into a plurality of picture elements and for each picture element target area coverings for the individual printing inks are determined, that each picture element is a measure of each printing color the security factor with which the respective area coverage can be determined is assigned to the weighting factor that the printed product to be assessed is divided into picture elements in the same way as the reference and the remiss for each picture element Ions are measured and from this the actual area coverage for the individual printing inks are determined, that for each image element and the individual printing inks the deviations of the actual area coverage from the target area coverage are determined and weighted with the respectively assigned weight factors, and that the quality measure or the control values for the ink guide elements are determined from the weighted deviations. 2. The method according to claim 1, characterized in that the weighted deviations are preferably combined by summing and normalizing pressure zone-wise to zonal weighted deviations. 3. The method according to claim 2, characterized in that the zonal weighted deviations are used as control values for the ink guide members. 4. The method according to any one of claims 1 to 3, characterized in that the mathematical determination of the area coverings from the remissions is carried out by preferably iterative resolution of the Neugebauer equations. 5. The method according to any one of claims 1 to 4, characterized in that the size of the picture elements is chosen to be approximately 0.25 to 400 mm ² , preferably approximately 1 to 100 mm. 6. The method according to any one of claims 1 to 5, characterized in that the weight factor of each picture element for each printing ink from the area coverage, the foreign color portion and depending on the environment of the relevant picture element is determined. 7. The method according to claim 6, characterized in that the weight factor of each picture element for each printing ink is determined as a combination of three partial weights, a first partial weight due to the area coverage, a second partial weight due to the foreign color component and a third partial weight due to the environment of the respective Image element is determined. 8. The method according to claim 7, characterized in that the partial weight dependent on the area coverage is selected such that it has its greatest value for medium area coverage and smaller and larger area coverage smaller values. 9. The method according to claim 7 or 8, characterized in that the partial weight dependent on the environment is selected so that it is the greater, the more homogeneous the area coverage in the environment of the respective picture element. 10. The method according to any one of claims 7 to 9, characterized in that the part weight dependent on the foreign color portion is selected so that it is all the greater as the area coverage in the foreign colors is smaller. 11. The method according to any one of claims 1 to 10, characterized in that the coordinates in the color space are determined for each picture element from the area coverings, the deviations of the coordinates of the reference are determined from those of the printed product to be checked and the importance is set for each picture element Weights are weighted for the visual impression, and that a quality measure for the change of the visual image impression is determined from the weighted deviations. 12. The method according to any one of claims 1 to 11, characterized in that in the case of a print product which is found to be good as a reference, the remissions in the printing inks are measured for each picture element thereof, and from these remissions the desired values are determined in the same way as for the printed products to be assessed. Area coverings can be determined mathematically. 13. The method according to any one of claims 1 to 11, characterized in that in the case of printing plates as a reference for each image element of the same, the area coverings are measured and converted into area coverings in printing using the printing characteristic of the printing press, and that these area coverings in printing are directly as the target - Area coverings are used. 14. The method according to any one of claims 1 to 11, characterized in that in the case of printing plates as a reference for each picture element of the same, the area coverings are measured and converted into expected remissions to be expected in the print on the basis of the printing characteristic curve, and that from these desired remissions In the same way as for the printed products to be assessed, the target area coverings are determined by calculation. 15. Device for assessing the print quality and / or regulating the ink flow in an offset printing press, with a photoelectric scanning device for the printed products on the running printing press and an electronic system for Control of the scanning device and for evaluating the measurement data generated by the scanning device with regard to a quality measure or manipulated values for the ink guide elements of the printing press, characterized in that the electronic system uses means for converting the remissions recorded by the scanning device into surface coverings, and means for determining weighting factors of the calculated area coverings, means for determining the deviations of the area coverings of the printed products to be tested from the area coverings of a reference printed product, means for weighting these deviations with the weight factors and means for zone-wise summarization of the weighted deviations to zone-specific quality measures or manipulated values for influencing the ink guide elements of the printing press . 16. The apparatus according to claim 15, characterized in that it has a device interacting with the electronic system for area-wise photoelectric scanning of printing plates and for determining area coverage in picture elements of these printing plates. 17. Offset printing machine with an automatic control device for its ink guide elements, characterized in that the control device is designed according to one of claims 15 to 16.

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