EP0142470B1 - Method and device for judging the printing quality of a printed object, preferably printed by an offset printing machine, and offset printing machine provided with such a device - Google Patents

Abstract

Printed products and their respective corresponding printing plates are divided into a plurality of image elements. For each image element the surface coverage is determined by photoelectric measurements; a reference reflectance value is then calculated from these measurements, taking into consideration such parameters as the printing characteristic. These reference reflectance values are compared with the actual reflectance values measured on the printed products and the results of the comparison are evaluated to form a quality measure and to calculate control values for the ink feed devices of the printing machine. In this manner, the use of special color measuring strips may be eliminated.

Description

The invention relates to a method and a device for assessing the print quality of a printed product, preferably produced on an offset printing machine, according to the preamble of claim 1 and 16, and to an offset printing machine equipped with a corresponding device, according to the preamble of claim 21.

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 dimension of the color control strips can be 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 u. a. in U.S. Patent Nos. 4200932 and 3835777.

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 done visually.

Recently, a system has also become known (see, for example, the published UK patent application 2115145) which enables machine evaluation of printed products even without color control strips. With this system, the printed products are scanned photoelectrically over the entire image area, so the measurement is carried out “in the image”. The measurement is carried out on the running printing press 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) that 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.

Although this new system already makes work easier, it can still be improved in some ways. It is therefore an object of the invention to refine and improve the mechanical quality assessment of printed products produced in particular on offset printing presses, the reliability of the quality statements in particular being to be increased.

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, 16 and 21. Preferred embodiments and further developments result from the dependent claims.

According to the invention, therefore, "in the picture" is measured. Measurement in the picture is also from US-A-3958509, EP-Publ. No. 29561 and EP publ. No. 69572 known. The surface coverage is determined as men zone of printing plates and evaluated for the manual or mechanical presetting of the ink control elements of the printing press _- in the case described therein Syste. However, this is a one-time presetting, there is no quality assessment of the printed products.

GB-A-2115145 also does not disclose any method comparable to the invention, since the differences formed from the target / actual value comparison are subjected to a threshold value comparison immediately thereafter, whether the differences are still acceptable or not. These differences are not multiplied for conversion into other color spaces, not because there are only fluctuations in brightness.

As far as the second alternative, as claimed in the current claim 1, regarding multiplication by a so-called full tone weight factor is concerned, it is known to print out a color measuring strip with the printed image, from which the full tone density present in this printed product is determined directly by means of a densitometer and with a reference value (OK sheet) can be compared to assess the quality of the print. In contrast to this, according to the invention no color measuring strip is required, but the full-tone density deviations from a reference are formed from the measured reflectance differences between the reference and the printed product by multiplication by the mentioned full-tone weight factor, without the printed product having to have full-tone density fields in the image. The full tone weight factor is therefore a new, specially created multiplication factor for assessing the print quality.

• Fig.1 eine blockschematische Übersicht über eine erfindungsgemäss ausgestattete Offset-Druckmaschine und
• Fig.2 und 3 zwei Skizzen zur Erläuterung der erfindungsgemässen Messmethode.

In the following, the invention will be explained in more detail with reference to the drawing. Show it:

• 1 shows a block schematic overview of an offset printing machine equipped according to the invention and
• 2 and 3 show two sketches to explain the measurement method according to the invention.

The overall system shown in Fig. 1 comprises a ^four-f arben offset printing machine 100, three fotoeleldrische scanning devices 120, 220, 320, three computers 150, 250 and 350 and four optical display devices 171, 172, 270 and 370.

The offset printing press 100 is of conventional design, its ink guide elements 111-114 (ink fountain keys, etc.) are only indicated symbolically in the drawing.

The scanner 120 is a so-called machine densitometer built into the printing press 100 with four scanning channels 121-124 for each printing ink. With this scanning device, printed products can be measured densitometrically on the running printing press. Examples of suitable machine densitometers are u. a. in U.S. Patent Nos. 2968988, 3376426, 3835777, 3890048 and 4003660. In the following, the scanning device 120 is briefly referred to as “machine densitometer 120”.

The scanning device 220 is used for the photoelectric measurement of printing plates or raster films on which they are based. The scanner 220 may e.g. B. a commercial scanner ("scanner"), as used for lithofilms, or any other suitable scanning device, for example according to US Pat. Nos. 3958509 and 4131879 or EP-Publ. Nos. 69572, 96227 and 29561, which allows printing plates or halftone films to be photoelectrically scanned with the resolution specified below. The scanning device 220 is referred to in the following, regardless of its type and the object actually scanned, as a "plate scanner 220".

The scanner 320 is used for photoelectric measurement of z. B. after visual assessment for qualitatively satisfactory printed products, so-called proof sheets or OK sheets. This scanning device scans the pressure sheets or OK sheets in exactly the same way as the machine densitometer 120 the printed products and is therefore constructed accordingly. In practice, OK sheets can be scanned directly and even advantageously directly from the machine densitometer 120 in the printing press 100. To facilitate understanding of the invention, however, this scanning device, referred to below as “OK sheet scanner 320”, is shown as a separate element.

The four optical display devices 171, 172, 270 and 370 are preferably color television monitors, which allow a visual representation of the measured values recorded by the scanning devices or the data determined therefrom by the computers. Of course, four separate display devices are not absolutely necessary, these are only drawn for ease of understanding. In the same way, the system could also have only a single computer instead of three, which would then have to operate all connected scanning devices, display devices, etc. On the other hand, the plate scanner 220 with its computer 250 and its display device 270 and the OK sheet scanner 320 with its computer 350 and its display device 370 can also form independent units, which then, for. B. would be connected via cable 251 or 351 to the computer 150. All of these possibilities are indicated in Fig. 1 by the dashed frames. They are completely irrelevant to the invention and the invention is in no way limited to them.

The general mode of operation of the arrangement shown in FIG. Is as follows:

The printed products D (sheets) and the printing plates D on which they are based are divided in the same way into a plurality of picture elements E (FIG. 2). By means of the plate scanner 220, each picture element E of the (here four) printing plates P is now measured photoelectrically and a remission target value R _{s is} calculated for each picture element E from the measured values to be explained, which the relevant picture element E of the printed products for should have the respective printing color if printing is carried out under normal conditions, ie with correctly adjusted inking, etc.

In an analogous manner, the printed products D are photoelectrically scanned on the running machine by means of the machine densitometer 120 (or also individual sheets off-line on a separate scanning device, for example the OK sheet scanner 320) and fine remissions for each printing ink and for each picture element Actual value R; certainly.

The individual remission target values R _s and the corresponding remission actual values R _; compared with each other, and a conclusion about the print quality (quality measure Q) is obtained from the comparison results. If desired, control variables (manipulated values) ST for influencing the ink guiding elements 111-114 of the printing press 100 can also be calculated and the ink guiding of the printing press can thus be regulated.

The display devices 270, 171, 172 and 370 can be used to display the sample values or the values calculated therefrom. Device 270 can, for example, cover the area or the brightness distribution of the individual printing plates P determined therefrom, device 370 the brightness distribution of the OK sheets, device 171 the remission target values R _s and device 172 the differences between remission target values R _s and remission actual values R; Show. Of course, the display devices can also be used to display all possible other data of interest.

The method according to the invention is thus based on the knowledge that under offset conditions it is possible, under certain conditions, to predict the remission target value of a picture element of the printed product for the individual printing inks from the area coverage of the picture element in question in the respective printing plate (or the corresponding raster film). These prerequisites include, on the one hand, knowledge of the characteristic of the printing press and the influence of the solid color density on the change in reflectance depending on the areas cover, and on the other hand, that the picture elements are sufficiently small to come to meaningful results.

The printing characteristic, which takes into account all influences such as paper quality, printing ink, dot gain, ink acceptance, overprinting, wet-on-wet printing, etc., can be determined empirically relatively easily. Tables are created for the remission target value of the printed product as a function of the area coverage of the printing plates. The table values are obtained by measuring standardized color tables, which are printed under representative conditions on the respective printing press. For the measurement of these color charts, that scanner is preferably used with which the printed products are later also measured during operation, in the present case the machine densitometer 120.

The influence of the solid tone density on the remission change due to the dot gain can also be recorded using tables. To create these tables, the above-mentioned color charts are used under appropriate printing conditions, i.e. H. printed with varying solid density of all printing inks.

In order to achieve the highest possible accuracy, the picture elements E must be chosen to be as small as possible. A natural lower limit is given by the grid fineness (e.g. 60 lines per cm). In practice, however, this lower limit cannot be reached for technical and, above all, economic reasons. This applies above all to the measurement of the printed products D on the running machine, since the amount of data that arises under the usual sheet formats in the short time available cannot be recorded and processed with economically justifiable effort. There would also be significant positioning difficulties.

Image elements E of approximately 25 to 400 mm ^{2 in} area are acceptable for remission measurements on the running printing press. Practically, a picture element E z. B. be square with about 1 cm ² area. With image elements E of this size, however, the predetermination of the remissions taking into account the overprinting based on the surface coverings of the printing plate becomes too imprecise.

According to an essential aspect of the invention, each individual element E of the printing plate P (or the raster films on which it is based) is therefore divided into a larger number, e.g. B. 100, sub-elements SE and determines the area coverage for each of these sub-elements. The determination of the area coverage for the image elements of the printing plates is therefore carried out with a higher resolution than the remission determination of the image elements of the printed products. This is both technically and economically justifiable, because the measurement on the printing plates can be carried out on the stationary object on the one hand and on the other hand only has to be measured once and there is practically enough time available for this. The size of the sub-elements SE can be approximately 0.25 to 25 mm ² , a practical example is approximately 1 mm ² based on a picture element size of approximately 1 cm ² .

The area coverage of each individual sub-element SE is determined by means of the plate scanner 220 in a manner known per se, e.g. B. by remission measurement integrally over the area of the sub-element or by means of television scanning or scanning by means of discrete photo sensor fields or the like a remission subset RS _{S is} calculated (intermediate table values can be found by interpolation). From the individual remission subsets RS _{S of} each picture element E is then. B. arithmetic averaging of the remission target value R _{s of} the relevant picture element E, which for comparison with the corresponding remission actual values R; the printed matter D is required.

The influence of the solid color density on the dot gain depends, as already mentioned, on the area coverage. According to a further important aspect of the invention, each sub-element SE is assigned a sub-full weight factor GS _e that takes this influence into account. These weighting factors contain the required change in full tone (change in layer thickness) for each printing ink for a desired change in reflectance, taking into account the overprinting and the local area coverage. The weighting factors can be determined from tables for the full tone change as a function of the reflectance change. These tables can be determined from the table values for the reflectance as a function of the solid density (see influence of solid density).

From the sub-full weight weight factors GS _{e of} the individual sub-elements SE of each picture element E, z. B. formed by arithmetic averaging a full tone weight factor G _e for the entire picture element E in question. These mean full tone weight factors G _e are now used to determine the weight with which any deviation of the actual remission value R; from the remission target value R _{s of} each individual picture element E in the calculation of the quality measure Q or the control variables ST for the color guidance. In the formation of the average full tone weight factor G _e z. B. the standard deviation in the sense of a weight reduction with a large standard deviation are also taken into account.

For the assessment of the print quality according to the senses, it is also possible to assign a sensory weight factor H _e (or sub-sensory weight factor HS _e ) to each individual image element E (or possibly also to each sub-element SE), which represents a sensory-metrical evaluation standard for deviations from the target value and actual value. This sensation ge weight factors can include B. according to CIELAB (Comite International de l'Eclairage) from the sensory metrics L ^* , a ^* , b ^{* defined} there.

For the assessment of the print quality, the deviations Δ _e between the measured actual remission values R _; and predicted remission target values R _s for each printing ink a quality measure Q is calculated and displayed in a suitable manner. This quality measure can be calculated, for example, in such a way that the deviations Δ _{e are} weighted with the respectively assigned full tone and / or perceptual weighting factor G _e or H _e and added (integrated) over one or more selected surface areas of the printed product. The surface areas can be adapted to the respective printed product. It is also possible to achieve several quality measures in this way.

The print zones Z (FIG. 2) specified by the printing press form a special role as surface areas. For the automatic control of the ink guide elements 111-114 of the printing press, the control variables ST are preferably determined individually for each individual printing zone by the deviations Δ _{e of} the actual remission values R _; weighted with the full tone weight factors G _{e .} of the remission nominal values R _{s of} the picture elements E are summed (integrated) over the entire respective printing zone Z. Of course, other evaluation and calculation methods are also possible here.

The actual adjustment of the ink guide members 111-114 due to the control variables ST is carried out in a manner known per se (see, for example, US Pat. No. 4,200,932) and is not the subject of the invention.

The area coverage determined by the plate scanner 220 can be integrated via the individual pressure zones Z and z. B. as described in US-PS 3185088 for presetting the color guide members.

As already mentioned, the remission target values R are calculated in advance; of the individual image elements E on the basis of the area coverage of the corresponding image elements of the individual printing plates P or, if the measurements on these cannot be carried out for any reason, the corresponding raster films on the basis of which the printing plates were produced.

This applies to the initial setup and startup of the press. To control the production run, a printed product that is found to be good, a so-called OK sheet OKB, can also be used as a basis for comparison. This would then no longer need to be scanned with the same resolution as the printing plate P, since only the remissions in the individual picture elements are of interest. These remissions can be determined either using the OK sheet scanner 320, or else using the plate scanner 220, if they are not already stored. The weight factors G _e and / or H _e assigned to the individual picture elements can be adopted from the earlier measurement of the printing plate P.

The densitometric measurement of the printed products D on the running machine can take place in various ways, as long as it is ensured that the remission or remission change of each image element is recorded for each color. It is not absolutely necessary for each individual printed product D to be measured completely, but a sequential measurement of different image elements on successive printed products is also sufficient. Furthermore, e.g. B. each individual color can be measured according to the respective inking unit or the remissions in the individual printing inks can be determined together on the finished printed product. A double measurement before and after each individual inking unit is particularly expedient, since in this way the influence of the respective color can be recorded particularly precisely.

Finally, it should also be mentioned that instead of scanning the printing plates or the raster films, it is also possible to use scanning data which are obtained in the production of the lithofilms or printing plates.

Claims (21)

1. Process for evaluating the printing quality of a printed product produced on a printing machine, wherein the printed product and a reference are each divided into image elements (E) and photoelectrically measured by image element, wherein for each image element (E) of the printed product and for each color a corresponding reference reflectance value (R_s) and an actual reflectance value (R;) are determined by measuring techniques and the reference reflectance and actual reflectance values of corresponding image elements are compared with each other, and wherein a quality measure (Q) is determined from said comparisons, characterized in that the determination of the reference reflectance and actual reflectance (R_s, R_i) by measuring techniques is carried out densitometrically for each printing color, that to every image element (E) of the printed product as a function of the reference and actual reflectance values for said element a perception weighting factor (He) expressing the relationship between the sensitometric and technically measured color deviations and/or as a function of its surface coverage and color a full tone factor (G_e) describing the effect of the full tone density on reflectance are assigned and that the differences (A_e) between the reference reflectance values (R_s) and the corresponding actual reflectance values (R;) are weighted with the perception weighting factor (He) and the full tone factor (G_e), in order to obtain on the one hand a conversion into a sensitometric color coordinate system and on the other, to determine full tone density deviations from the reference potentially also without the use of a color measuring strip, and that the quality measure (Q) is formed by the summation of the differences weighted in this manner over at least part of the image elements.

2. Process according to claim 1, characterized in that as the reference for the individual colors the corresponding printing plate (P) or the photographic master upon which it is based, are used, that the surface coverage is determined for every image element of said reference and that the reference reflectance value (R_s) is calculated for every image element (E) and each printed color of the printed product from the determined surface coverage of the corresponding image element of the reference.

3. Process according to claim 1, characterized in that a printed product (OKB) found to be satisfactory is used as the reference.

4. Process according to one of claims 1 to 3, characterized in that the determination of the reference reflectance values (R_s) for the image elements (E) of the reference (P) is carried out with a higher resolution than the determination of the actual reflectance values (R;) for the image elements (E) of the printed product (D).

5. Process according to claim 4, characterized in that the image elements (E) of the printed product (D) are measured integrally.

6. Process according to claim 4 or 5, characterized in that the resolution in the measuring of the reference (P) is at least 10 times higher than in the case of the printed product.

7. Process according to one of claims 4 to 6, characterized in that the area of the image elements is approximately 25 to 400 mm², preferably about 1 _cm ² _.

8. Process according to one of claims 4 to 7, characterized in that in the measuring of the reference (P) the image elements (E) are divided into subelements (SE) of approximately 0.25 to 25 cm², preferably about 1 m_m ².

9. Process according to claim 8, characterized in that for each individual subelement (SE) of an image element (E) with consideration of the parameters controlling the printing process a subreference reflectance value (RS_s) and from all of the subreference reflectance values of an image element the reference reflectance value (R_s) of the image element concerned, are formed.

10. Process according to claim 9, characterized in that the reference reflectance value (R_s) is calculated by an averaging method from the subreference reflectance values (RS_S).

11. Process according to one of claims 9 and 10, characterized in that to each subelement (SE) a sub-full tone weighting factor (GS_e) and/or a sub- perception factor (HS_e) are assigned, and that the full tone and the perception weighting factors (G_e, He) of an image element (E) are calculated by averaging from the sub-full tone and sub-perception factors (GS_e, HS_e) of the subelements (SE) of the corresponding image element.

12. Process according to one of claims 1 to 11, characterized in that the summation of the weighted differences (A_e) is carried out over the image elements (E) belonging to a common printing zone (Z) or a selected surface area, to obtain a zonal error value (O_Z) or an area error value, which represents the quality measure (Q).

13. Process according to claim 3, characterized in that for every ink color the reference reflectance values (R_s) and the corresponding full tone and/or perception weighting factors (G_e, He) of the image elements (E) are obtained from one and the same reference (P).

14. Process according to claim 3, characterized in that the reference reflectance values (R_s) are determined from a printed product (OK8) and the full tone and/or perception weighting factors from the corresponding printing plates (P) or photographic master.

15. Process according to one of the preceding claims, characterized in that the printed product (D) is measured densitometrically for the determination of the actual reflectance values (R;) before and after each printing mechanism.

16. Apparatus for the evaluation of the printing quality of a printed product produced on a printing machine by means (120, 220) for the photoelectric scanning by image elements of printed products and a reference, and with a computer (150,250) connected with said scanning means for the comparison by image elements of printed products and the reference, and for the formation of a quality measure (Q) from the results of said comparison, characterized in that for the determination by measuring techniques of at least the actual reflectance values for each printed color a densitometer is provided and the computer is equipped to store a plurality of perception weighting factors expressing the relationship between the sensitometric and technically measured color deviations for different reflectances and/or a plurality of full tone weighting factors stating the effect of full tone density on reflectance in the case of different surface coverages, to assign to every image element (E) of the printed product a perception weighting factor (He) selected as a function of the reflectance determined for the element concerned and/or as a function of its surface coverage and the color involved a full tone weighting factor (G_e), in order to obtain on the one hand a sensitometric color coordinate system and to determine, optionally without the use of a color measuring strip full tone, deviations from the reference, and on the other, to weight the differences by image element detected by the scanning means (120, 220) between the printed product and the reference with the associated perception weighting factor and full tone weighting factor, and from this to form the quality measure (Q).

17. Apparatus according to claim 16, characterized in that the scanning means (120, 220) are capable of scanning the reference (P) with a higher resolution than the printed products (D).

18. Apparatus according to claim 17, characterized in that the scanning means (120, 220) are equipped to scan a reference in the form of a printing plate (P) photoelectrically by image element.

19. Apparatus according to one of claims 16 to 18, characterized in that it is equipped to carry out the steps according to one of claims 2 to 15.

20. Apparatus according to one of claims 16 to 19, characterized in that it comprises a display (171, 172, 270, 370) for the graphical representation of the measured refelectances of the reference and/or the reference reflectance values calculated therefrom and/or the actual refelctance values of the printed products and/or the differences between the reference and actual reflectance values.

21. Offset printing machine characterized by an apparatus for the evaluation of the printing quality according to one of claims 16 to 20.

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