EP0143744B1 - 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

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 by means of e.g. B. an automatic scanning densitometer, the control loop to the inking units in both cases open (quality assessment) or closed (machine control). 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. B. for format reasons very often before 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 by 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 (O.K. 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 which differ from the corresponding picture elements of the reference by more than a certain degree of tolerance, the differences in the sampled values summed up over selected image areas to the corresponding sampled values of the reference and the differences between the sampled values summed up via certain scan tracks 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 connected to the machine densitometer 3, 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 given, inter alia, by the documents 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). Picture element sizes that are reasonable in practice are 0.5 x 0.5 mm ² to approximately 20 x 20 mm ² , preferably approximately 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 emission detection mg can rather also be distributed over several printed sheets (less expenditure 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", i.e. the associated area coverings for the printing inks are calculated from the four remissions of each image element. This calculation is carried out in a manner to be explained in more detail 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 print sheets created at this point in time and immediately afterwards can be used as a reference (OK sheet). 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 measure of quality can be determined.

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

• F_i,j F.*. : Flächenbedeckung des Bildelements i bezüglich Druckfarbe j für Referenz bzw. Fortdruck
• G_i,j : Gewichtsfaktor des Bildelements i bezüglich Druckfarbe j
• : Summation über alle Bildelemente i einer Zone z
• Δ F_relj : genormte zonale Abweichung der Flächenbedeckung für die Druckfarbe j

Where:

• F _{i, j} F. *. : Area coverage of the image element i with respect to 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
• Δ F _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 e.g. article "Theoretical foundations of multicolour book printing" in "Journal for Scientific Photography, Photophysics and Photochemie ", Volume 36, Issue 4, April 1937). Expanded to four colors j = infrared, red, green, blue:

• ß. : Remissionen gemessen mit Filter der Farbe j
• W_j : Weissremission mit Filter j
• B_j, C_j, M., Y_i : Remission von Vollton Black, Cyan, Magenta, Yellow gemessen mit Filter j
• BC_j, 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., N_j : Remission von Vollton-Uebereinanderdruck B+C+M, B+C+Y, B+M+Y, C+M+Y (Noir) gemessen mit Filter j
• BN. : Remission von Vollton-Uebereinanderdruck 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 _j : white remission with filter j
• B _j , C _j , M., Y _i : Remission of solid black, cyan, magenta, yellow measured with filter j
• BC _j , 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., N _j : Remission of full tone overprint B + C + M, B + C + Y, B + M + Y, C + M + Y (Noir) measured with filter j
• BN. : Remission of full tone overprint B + C + M + Y measured with filter j
• b, c, m, y: area coverage of the printing inks B, C, M, Y

B _j -BN _j are constants that depend on the printing 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:

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 _j mean the measured remissions, are solved 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. At 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. Are at If several colors are printed at the same point, the individual color can be isolated less precisely. In order to take these factors into account, three partial weights are defined for each picture element and / or printing ink, namely a partial weight G ₁ dependent on the area coverage, a partial weight G ₂ depending on the environment and a partial weight G ₃ 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 _i -g ₃ mean the influence weights of the three partial weights. These influencing weights are in the range from 0 to 1. Usually, G receives the strongest and G ₂ the weakest influencing 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 an effect at approx. 50-70%, surface coverage has the greatest impact. With medium area coverings, deviations can thus be determined with greater certainty. Correspondingly, the area weight-dependent partial weight G is chosen such that it is maximum for medium area coverings, but drops towards smaller and larger area coverings. Suitable courses (partial weight G, as a function of the area coverage) are, for example, parabolas, triangles, trapezoids, the maximum value 1 of the partial weight being odet around 50% area coverage. 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.

• 010
• 1-41 1
• 010

In practice, the lapalace operator can look like this:

• 010
• 1-41 1
• 010

For finer gradations, the environment taken into account can be enlarged as desired. The diagonal coefficients can also be taken into account (≠ 0).

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:

Praktische Werte sind beispielsweise:

In it, ß _B , ßc, ß _M and ßy are the remissions in the colors B, C, M. And Y and a _jl 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} has the focus at approx. 50%, area coverage. G ₁ thus causes a dynamic compression of the deviations with larger and smaller area coverings. Is z. B. the trapezoidal function of G ₁ chosen wide enough, however, 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. Exceeds z. B. G _z or G ₃ 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 ₁ 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. Such an alternative is e.g. B. in using 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 (empirical, tables) and then directly used 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 as described above and thereby transformed into the target area coverings. 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 a conventional type (for example 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 pre-calculation of the remissions or the determination of the surface densities from the printing plates can be found in the 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 last underlying raster films or the like). 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. Process for evaluating printing quality and/or regulating ink feed controls in an offset printing machine (1) in which a printed product (4) which is to be evaluated and a reference, divided in the same manner into a plurality of image elements, are measured photoelectrically by image elements, and compared by image elements, and in which on the basis of the comparison result a quality measure or respectively a setting value is determined for the ink feed control elements (2) of the printing machine (1), characterized in that for each image element of the reference in the form of a printed product (4) which is deemed to be good, or printing plates upon which the printing process is based, reference surface coverages are determined for the individual printing colours, that a weighting factor indicating a measure of the assurance with which the respective surface coverage may be determined is assigned to each image element for each printing colour, that for each image element of the printed product (4) which is to be evaluated the reflectances are measured and therefrom actual surface coverages for the individual printing colours are determined by calculation, that for each image element and the individual printing colours the deviations of the actual surface coverages from the reference surface coverages are determined and weighted with the respectively assigned weighting factors, and that the quality measure or respectively the setting values for the ink feed control elements (2) are determined from the thus weighted deviations.

2. Process according to Claim 1, characterized in that the weighted deviations are combined to zonal weighted deviations preferably by summation and standardization in print zones.

3. Process according to Claim 2, characterized in that the zonal weighted deviations are used as setting values for the ink feed control elements (2).

4. Process according to one of Claims 1 to 3, characterized in that the determination by way of calculation of the surface coverages from the reflectances takes place through preferably iterative solution of the Neugebauer equations.

5. Process according to one of Claims 1 to 4, characterized in that the extent of the image elements is selected to be approximately 0.25 to 400 mm², preferably approximately 1 to 100 mm².

6. Process according to one of Claims 1 to 5, characterized in that the weighting factor of each image element for each printing colour is established from the surface coverage, the foreign colour component and as a function of the environment of the respective image element.

7. Process according to Claim 6, characterized in that the weighting factor of each image element is established for each printing colour as a combination of three partial weights, in which a first partial weight is determined from the surface coverage, a second partial weight from the foreign component and a third partial weight from the environment of the respective image element.

8. Process according to Claim 7, characterized in that the partial weight dependent on the surface coverage is selected such that it has its maximum value in the case of medium surface coverages and has lower values in the cases of smaller and greater surface coverages.

9. Process according to Claim 7 or 8, characterized in that the partial weight dependent on the environment is selected such that it increases in value with increasing homogeneity of the surface coverage in the environment of the respective image element.

10. Process according to one of Claims 7 to 9, characterized in that the partial weight depending on the foreign colour component is selected such that its value increases as the surface coverages in the foreign colours decreases.

11. Process according to one of Claims 1 to 10, characterized in that for each image element the coordinates are determined in the colour space from the surface coverages, the deviations of the coordinates of the reference are determined from those of the printed product (4) to be evaluated and are weighted with weights, established individually for each image element, which weights indicate the importance for the visual appearance, and that from the thus weighted deviations a quality measure is determined for the alteration of the visual image appearance.

12. Process according to one of Claims 1 to 11, characterized in that in the case of a printed product (4) found to be good as reference for each image element of same the reflectances in the printing colours are measured and from these reflectances the reference surface coverages are determined by calculation in the same way as for the printed products (4) to be evaluated.

13. Process according to one of Claims 1 to 11, characterized in that in the case of printing plates as reference for each image element of same the surface coverages are measured and by means of the printing characteristic of the printing machine (1) are converted into surface coverages in print, and that these surface coverages in print are utilised directly as reference surface coverages.

14. Process according to one of Claims 1 to 11, characterized in that in the case of printing plates as reference for each image element of same the surface coverages are measured and by means of the printing characteristic are converted into reference reflectances to be expected in print, and that from these reference reflectances the reference surface coverages are determined by calculation in the same manner as for the printed products (4) to be evaluated.

15. Apparatus for evaluating printing quality and/or regulating ink feed controls in an offset printing machine, having a photoelectric scanning device (3) for the printed products (4) while the printing machine (1) is running and also having an electronic system (5) for controlling the scanning device (3) and for evaluating the measured data produced by the scanning device, taking into account a quality measure or setting values for the ink feed control elements (2) of the printing machine, characterized in that the electronic system (5) has means (51) for converting the reflectances detected by the scanning device into surface coverages, means (53) for determining weighting factors by means of the calculated surface coverages, means (56) for determining the deviations of the surface coverages from printed products to be examined with respect to the surface coverages of a reference printed product, means (54, 55, 57-59) for the weighting of these deviations with the weighting factors and means for summing the weighted deviations, by zones, to zone-specific quality measures or setting values to influence the ink feed control elements of the printing machine.

16. Apparatus according to Claim 15, characterized in that it has a device (6) cooperating with the electronic system (5) for photoelectrically scanning the printing plates by areas, and for determining surface coverages, in image elements, of these printing plates.

17. Offset printing machine having an automatic regulating device for its ink feed control elements, characterized in that the regulating device is constructed in accordance with one of Claims 15 to 16.

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