EP0505769B1 - Method for the determination of area coincidence of a master, in particular a printing plate, as well as a device for carrying out the method - Google Patents

Abstract

The invention relates to a method and a device for the determination of area coincidence of a master to be printed, in particular a printing forme of a printing machine, preferably an offset printing machine, wherein the local remission of a measurement area recorded is determined by optical scanning of the master and the master has an inhomogeneity which is independent of the area coincidence, dependent on the location and influences the measurement result of the scanning. To reduce measurement errors, provision is made for the printing areas to have a different colour (colour difference) in relation to the non-printing areas of the master, for at least two remission values, which differ from one another in spectral terms in accordance with the colour difference, to be determined by each measurement area (12), and for the two remission values to be evaluated to separate a proportion of the measurement result influenced by the area coincidence (fD) from a proportion of the measurement result influenced by the inhomogeneity ( gamma ). <IMAGE>

Description

The invention relates to a method for determining the area coverage of a printing template, in particular a printing form of a printing press, preferably an offset printing machine, wherein the local reflectance of a detected measuring field is determined by optical scanning of the template, the printing surfaces differ from the non-printing surfaces of the template Have color (color difference) and the original has a location-dependent inhomogeneity which is independent of the area coverage and influences the measurement result of the scanning.

The method according to the invention is suitable for determining the area coverage, that is to say for determining the percentage of a printing area in relation to the total area under consideration. It can be used in different technical fields. It can be used, for example, to determine the area coverage of a print template. However, provision is preferably made to determine the area coverage on a printing form of a printing press, in particular on the printing plate of an offset printing press, before the printing process, in order to obtain ink presetting values for ink metering zones of the printing unit or the inking units. The more precisely the area coverage and thus the ink presetting values can be determined, the faster the production status can be achieved, which reduces waste and makeready times. Under these conditions, even small print runs can be printed economically.

It is known to measure area coverage on printing plates by means of optical remission. This is preferably done zonally according to the ink metering zones to be set on the inking unit of the printing press. For this purpose, each zone of the printing plate is suitably illuminated and the light scattered back from the printing plate surface is detected by a measuring head. The measuring head preferably has a photodiode for detecting the remission. The measured intensities are compared with previously measured reference intensities. A reference intensity comes from a so-called full-tone area, that is, an area that has an area coverage of 100%. A further reference intensity is formed by a so-called zero percent area that does not carry ink when printing; their area coverage is therefore 0%. The full tone area and the zero percent area form two extreme values which serve to calibrate the measuring head. Signals emitted by the measuring head, which are based on an area coverage lying between the extreme values, can be classified as a percentage on the basis of the calibration, that is to say the percentage area coverage corresponding to these signals can thus be determined. In the known method, it is therefore necessary, for example, to measure the local remission for a full-tone area and a zero percent area at the plate edge in the subject-free area. If the area coverage is then determined in the subject, the reference areas located at the edge of the plate are used to determine the area coverage. It is disadvantageous that, in particular, subject-free printing plate surfaces (zero percent surfaces) have a locally different intensity characteristic — hereinafter referred to as inhomogeneity — so that the same reference cannot be assumed at every point on the printing plate. It would be ideal if the reference could be determined in the same measuring field in which the area coverage is also to be determined. However, since the subject lies in this measuring field, there can be no full-tone or zero percent area with some exceptions. If these were to be created there, the printed image would have a color spot or a color-free area at that point. This is nonsensical not only because it damages the printed image, but also leads to a falsification of the associated zonal area coverage.

Due to the locally different reference intensities, the area coverage can only be determined approximately, namely within a relatively wide tolerance band. The zero percent area reference is particularly critical because it varies considerably more locally than a full tone reference and, with the same absolute size of the error, leads to larger relative errors.

From DE-OS 36 40 956 a method for determining an average zonal area coverage is known, wherein a zonal scanning of the printing form of a printing press is carried out with a sensor and a zero percent reference from the plate edge or at a measuring point with maximum remission is determined. Then the zero percent reference is measured again with additional filtering. Then the subject of the printing plate is scanned zonally by the sensor and the measured values determined are normalized to the filter transmission curve. The degree of area coverage is then calculated by averaging all standardized measurement values for the respective ink zone and ink presetting values for the printing press are obtained therefrom. Errors that occur due to inhomogeneities in the printing plate surface have a falsifying effect on the measurement result.

The invention is therefore based on the object of providing a method and a device in which inhomogeneities in the original, in particular the printing form, are taken into account and the accuracy of the measurement result is thus improved. In particular, it is provided that these inhomogeneities are essentially Chen subject-free printing plate surface areas to be taken into account, so that the critical measurement of small surface areas is decisively improved.

This object is achieved according to the invention in that at least two reflectance values which differ spectrally from one another in accordance with the color difference are determined from each measurement field and in that the two reflectance values are evaluated for separating a portion of the measurement result which is influenced by the area coverage and a portion which is influenced by the inhomogeneity.

The printing form can be designed such that the printing and / or the non-printing areas are colored in such a way that the printing or the non-printing areas are given a different color. On the basis of the different colored areas and the spectral evaluation of the reflectance, it can be distinguished at each measuring field under consideration whether the measurement result has been influenced by an inhomogeneity. If this is the case, then there is an inhomogeneity, this can be determined and the measurement result can be corrected accordingly, so that finally the actually existing area coverage of the present measurement field can be determined. The measurement result is therefore much more precise, so that essentially error-free ink presetting values for the inking unit or units of an offset printing press can be determined. This means that the production status can be reached more quickly after setting up the printing press.

The result is short makeready times and minimal waste. The printing form is now more or less colored as standard to make the subject visible, and is done, for example, by the coloring of the photoresist that forms the color-guiding surfaces of the printing form. This coloring is now used specifically according to the invention.

In particular, as mentioned, the coloring can be carried out using a diazo varnish already used today by the printing plate manufacturers. This photoresist currently used, among other things, to make the subject visible, is thus also used according to the invention.

It is essential to the invention, however, that there is a color difference due to the coloring, that is to say not only a color gradation (for example light gray-dark gray).

However, while it was irrelevant in the prior art which color the photoresist had relative to a non-printing zero percent area, according to the invention there must be a color difference between the areas mentioned. In the prior art, it was sufficient if, for example, the zero percent areas were light gray and the printing areas with photoresist were dark gray, since the subject was recognizable on account of this color difference and the aforementioned intensity measurement for determining the area coverage was also possible. However, a colorimetric measurement cannot then be carried out. However, this is an essential element of the present invention so that inhomogeneities can be recognized. In the known method, inhomogeneities, for example a color-darker zero percent area lying opposite the plate edge in the area of the subject, were considered to be a measuring field with area coverage, that is to say the present inhomogeneity was misinterpreted, so that measurement errors were unavoidable.

• - der Remission der Volltonfläche gewichtet mit der zugehörigen Flächendeckung und
• - der Remission der freien, also nicht druckenden beziehungsweise druckfreien so genannten Nullprozentfläche gewichtet mit dem restlichen Flächenanteil des Meßfelds und gewichtet mit einem Faktor, der die Inhomogenität beschreibt.

According to the invention, the remission of each measuring field is composed of the following components for evaluation:

• - The remission of the full tone area weighted with the associated area coverage and
• - The reflectance of the free, i.e. non-printing or non-printing, so-called zero percent area weighted with the remaining area portion of the measuring field and weighted with a factor that describes the inhomogeneity.

zusammen, wobei

• - S ein dem Meßergebnis entsprechendes Signal,
• - V ein der Volltondichte entsprechendes Signal,
• - fα die Flächendeckung,
• - die Inhomogenität und
• - H ein der Nullprozentfläche entsprechendes Signal ist.

The measurement result determined during the optical scanning preferably exposes itself

together, being

• S a signal corresponding to the measurement result,
• V a signal corresponding to the solid tone density,
• - fα the area coverage,
• - inhomogeneity and
• - H is a signal corresponding to the zero percent area.

As already mentioned, it is advantageous if the area coverage is determined zonally and if color presetting values for ink metering zones of an inking unit of the printing press are determined from the zonal area coverage values.

According to a development of the invention, an additional, third, spectrally deviating reflectance value is determined from each measuring field, which takes into account a local change in the reflectance of a printing, that is to say a printing ink-carrying or printed surface, in particular a full-tone surface. In this way, inhomogeneities within the full-tone areas can be determined and eliminated during the measurement. However, these errors, which are based on inhomogeneities of full-tone areas, are very much smaller than with zero percent areas, so that a further improvement in the accuracy of the measurement result is achieved, but this is not as serious as with zero percent areas or areas with low area coverage.

Particularly good results can be achieved if the subject has a relatively low global area coverage, since then the elimination of the inhomogeneity errors becomes correspondingly clear. In the case of templates with a globally high area coverage, it can therefore be advantageous for the measurement result of a spectrally independent optical detection of the area coverage to be taken into account. This means that both the method according to the invention and the known method of the prior art determine the area coverage and the results of both methods are used in the final determination of the area coverage. If the printing form has no color difference, but only color gradations (for example gray in gray), the device according to the invention can still be used according to the known, so-called one-filter method mentioned.

To improve the determination of the area coverage, it may be advantageous to use the inhomogeneities of adjacent measurement areas and primarily determined area coverage (according to the so-called two-filter method described above) for smoothing when determining the inhomogeneity of a measurement field. This takes into account the fact that the inhomogeneities mostly do not change abruptly, but rather continuously, between adjacent measuring points, so that "outliers" have no serious effects due to measuring errors or the like. In this respect, it is advantageous if a local inhomogeneity distribution is first determined by determining the inhomogeneities of the entire original (in particular printing plate). From this, a preliminary pseudo zero percent reference can be determined at each point. "Pseudo" because this zero percent reference was only determined indirectly, since the subject cannot be "removed" and "provisionally" because the pseudo zero percent references obtained in this way are then corrected by smoothing, weighting or evaluating each neighboring point of inhomogeneity, so that finally there is a final pseudo zero percent reference for each measuring field. The final determination of the respective local area coverage can then take place.

The invention further relates to a device for determining the area coverage, in particular for carrying out the method described, with at least one measuring head optically scanning the template, which has a reflective light receiver with a filter arrangement, so that due to different filtering of each optically scanned measuring field, several spectrally different ones Measurement results can be achieved. The filter arrangement can have a plurality of filters, so that a different filter can be used for each measurement. However, it can also be done in such a way that one of the measurements takes place without a filter and another or several others take place with a filter. It is also possible for the remission light receiver to have a plurality of light-sensitive elements, to which the remission is fed via the corresponding filters. This has the advantage that several measurements can be carried out simultaneously. Alternatively, it is also conceivable that the remission light receiver has only one light-sensitive element and that the filters can be pivoted into its beam path. In the latter case, however, the different measurements of each measuring field can only be carried out one after the other.

It is preferably provided that the measuring head has a beam splitter which feeds the remission of a first photodiode directly, that is to say without additional filtering, and to a second photodiode via a filter forming the filter arrangement. At the same time, the reflectance of a measuring field can be measured in a spectrally different way.

According to a development of the invention, it is provided that the measuring head has a further beam splitter which feeds the remission of a third photodiode via a further filter. The first photodiode thus receives the remission unfiltered, the second photodiode via a filter and the third photodiode via the further filter, which differs from the first filter in its filter characteristic.

In order to be able to comprehensively cover the entire original, in particular the subject of the printing form, in a short time, a plurality of measuring heads are preferably arranged next to one another, the measuring heads being movable relative to the original. Alternatively, the measuring heads can also be arranged in a stationary manner and the template can be moved. The row of measuring heads is preferably so long that the subject length or the subject width is completely recorded. Either the measuring heads can be moved in the printing direction of the printing form or transversely to the printing direction. Alternatively, however, it is also possible that, for example, one or more measuring heads for optical scanning on a meandering path over the printing form or in the case of a return movement by moving the sensor arrangement, different partial areas chen of the printing form.

The filter or filters can preferably be designed as an edge filter or as a tristimulus filter with special attention to their mutual course.

Alternatively, however, it is also possible to carry out the filter function by spectroscopic detection of the remission by means of, for example, a spectrophotometer and to form a downstream, computer-aided summary of adjacent wavelength intervals.

According to a further development of the invention, it is also possible to recognize on the basis of the reference signals for the full-tone and zero percent areas which plate type (that is, from which manufacturer or from which material) is used. In this respect, a printing plate identification can also be carried out with the aid of the device according to the invention. It is also possible to take into account the expected inhomogeneities from the outset, that is, the characteristic data about these inhomogeneities are saved and used when these plates are used again. This enables, for example, a plate-specific evaluation of the measurement result using a simpler algorithm.

• Figur 1 eine Vorrichtung zur Ermittlung der Flächendeckung einer Druckplatte für eine Offset-Druckmaschine,
• Figur 2 eine Draufsicht auf die Vorrichtung gemäß Figur 1,
• Figur 3 eine Draufsicht auf eine Variante entsprechend der Darstellung in Figur 2,
• Figur 4 ein mit Remissions-Lichtempfänger versehener Meßbalken der Vorrichtung gemäß Figur 1,
• Figur 5 eine Prinzipzeichnung zur Verdeutlichung der Remission,
• Figur 6 einen Querschnitt durch den Meßbalken der Figur 4 mit zwei Remissionslichtempfängern,
• Figur 7 ebenfalls einen Querschnitt durch den Meßbalken nach einem anderen Ausführungsbeispiel,
• Figur 8 den Remissions-Lichtempfänger in perspektivischer, geöffneter Darstellung,
• Figur 9 einen Längsschnitt durch den Remissions-Lichtempfänger,
• Figur 10 ein Beispiel der spektralen Transmission der beiden in dem Meßkopf der Figur 9 verwendeten Filter,
• Figur 11 ein Diagramm der Remissionen verschiedener Flächendeckungen einer Druckplatte einer Offset-Druckmaschine in Abhängigkeit der Flächendeckung,
• Figur 12 ein Diagramm der Signale eines Zwei-Filter-Meßkopfes, wobei das Diagramm den mathematischen Hintergrund des erfindungsgemäßen Verfahrens verdeutlicht und
• Figur 13 mehrere Diagramme zur Verdeutlichung des k_f-Kriteriums.

The figures illustrate the invention using exemplary embodiments and show:

• FIG. 1 shows a device for determining the area coverage of a printing plate for an offset printing machine,
• FIG. 2 shows a top view of the device according to FIG. 1,
• FIG. 3 shows a top view of a variant corresponding to the illustration in FIG. 2,
• FIG. 4 shows a measuring bar of the device according to FIG. 1 provided with a reflectance light receiver,
• FIG. 5 shows a basic drawing to illustrate the remission,
• FIG. 6 shows a cross section through the measuring bar of FIG. 4 with two reflectance light receivers,
• FIG. 7 also shows a cross section through the measuring bar according to another exemplary embodiment,
• FIG. 8 the remission light receiver in a perspective, open representation,
• FIG. 9 shows a longitudinal section through the remission light receiver,
• FIG. 10 shows an example of the spectral transmission of the two filters used in the measuring head of FIG. 9,
• FIG. 11 shows a diagram of the remissions of different area coverings of a printing plate of an offset printing press as a function of the area coverage,
• FIG. 12 shows a diagram of the signals of a two-filter measuring head, the diagram illustrating the mathematical background of the method according to the invention and
• FIG. 13 several diagrams to clarify the k _f criterion.

FIG. 1 shows a device with which the zonal area coverage of a template, in particular a printing plate of an offset printing machine, can be determined.

The device has a desk-shaped measuring table 1. A pressure plate 2 to be measured is placed on the measuring table 1 and is preferably held pneumatically by negative pressure. For this purpose, corresponding suction channels are provided in the measuring table 1. A measuring bar 3 is movably mounted on the measuring table 1. If one looks at FIGS. 2 and 3, it can be seen that the measuring bar can be moved in the directions of the double arrow 4. Assuming that the arrow 5 indicates the printing direction of the printing plate 2 held on the measuring table 1, the measuring bar 3 can thus be displaced transversely to the printing direction.

According to another embodiment, not shown, it is also possible that the measuring bar 3 is arranged offset by 90 compared to the embodiment of Figures 1 to 3, so that it can be moved in or against the printing direction.

Operating and display fields 6, not shown, are also provided on the measuring table. Furthermore, a calibration strip 7 (FIG. 2) or a calibration field 8 (FIG. 3) can be provided on the measuring table or the printing plate.

As mentioned, the full-tone reference surface required for the calibration can be on the edge of the plate, and it is possible to provide the full-tone reference surface, for example, by sliding on a calibration field mask; under certain circumstances this would simplify the manufacture of the printing plate.

Figure 4 shows an example of the measuring bar 3 in a schematic representation. This has two light sources 9, which are preferably designed as fluorescent lamps. In the longitudinal direction of the measuring bar 3, a plurality of measuring heads 10 are arranged in rows, for example between the two fluorescent lamps. Only one measuring head is shown in detail in FIG. If only one measuring head is used, it is arranged to be displaceable in the longitudinal direction of the measuring bar, so that the pressure plate can be completely scanned, for example in a meandering shape. A total of 32 measuring heads, for example, can also be arranged in rows next to one another, the optical field of view of which is reduced to 32.5 by means of a screen grille 11. 32.5 mm ^{2 is} limited. Assuming that this field of view length corresponds to the width of a color zone of the (not shown) offset printing press, a position of the measuring bar 3, a zone of the pressure plate 2 can be detected. If the measuring bar is displaced by the dimension of a zone after this zone has been detected, the adjacent zone can then be optically scanned. Each individual zone is divided into a corresponding number of measuring fields 12, which correspond to the openings of the screen grille 11. In the exemplary embodiment mentioned, 32 measuring heads and thus also 32 measuring fields 12 are provided for each measuring bar position.

Before the more precise structural design of the measuring bar 3 is to be dealt with, FIG. 5 illustrates the reflectance measurement which can be carried out with the measuring table 1. The light 13 incident from the light sources 9 shown in FIG. 4 reaches the surface of the printing plate 2, which — depending on the area coverage — is provided with a corresponding number of raster points or full-surface portions 14 of a certain size. In accordance with the available area coverage, the incident light 13 is reflected from the surface of the printing plate 2 in a spectrally different manner. This reflected light 15 optionally passes through a filter 16 (this will be discussed in more detail below) and then reaches a remission light receiver 17, which is located in the associated measuring head 10.

FIG. 6 illustrates the structural design of the measuring bar 3. This has a housing 18 in which the measuring heads 10 are accommodated. The two light sources 9 are also located in the housing 18 and are shielded from the measuring heads 10 with opaque walls 19. In the direction of the measuring fields 12, light exit openings 20 are provided, which are provided, for example, with diffusing screens 21. A diffuse light is radiated onto the original to be scanned by the diffusing disks 21.

The two exemplary embodiments of the measuring bar 3 in FIGS. 6 and 7 differ in that the measuring heads 10 are designed differently. First, reference is made to the measuring head 10 of the exemplary embodiment in FIG. It has a housing 22 which is provided at its lower end with a light entry opening 23. If necessary, optics can also be provided there and / or in front of the photodiodes 24, 25, 26. Each measuring head 10 has a remission light receiver 17, which in the exemplary embodiment in FIG. 7 consists of three photodiodes 24, 25 and 26. Two beam splitters 27 and 28 are arranged within the housing 22. The design is such that the reflected light incident into the light entry opening 23 first hits the beam splitter 27 and is divided there in such a way that a portion reaches the photodiode 24. The remaining part passes along the optical axis 29 through the beam splitter 27 and arrives at the beam splitter 28. Here, a division takes place in such a way that a portion reaches the photodiode 25 and a portion penetrating the beam splitter 28 reaches the photodiode 26. The photodiode 25 is preceded by a filter 30 and the filter 31 is preceded by a filter 31. The light supplied by the beam splitter 27 to the photodiode 24 does not pass a filter. However, an embodiment is also possible in which a filter is also provided there, in particular also when the signal level is to be adjusted. Regardless of whether two filters 30, 31 and no further or a third filter are provided, the measuring head 10 of FIG. 7 is by definition a three-filter measuring head (if no third filter is provided, the spectral sensitivity can be the photodiode 24 can be regarded as a filter).

The embodiment of FIG. 6 differs from the aforementioned embodiment with regard to the measuring head 10 in that only two photodiodes, namely the photodiode 24 and the photodiode 25, are provided. The photodiode 25 is no longer on the side of the housing 22, but at the head end. Only one beam splitter 27 is also provided. The light incident through the light entry opening 23 passes unfiltered to the photodiode 24 and, due to the beam splitter 27, also to a portion of the photodiode 25, the filter 30 being passed through in the process. In accordance with the previously mentioned exemplary embodiment, a filter can also be connected upstream of the photodiode 24. The exemplary embodiment in FIG. 6 is a two-filter measuring head (even if only one filter 30 is provided; according to the terminology used, the spectral sensitivity of the photodiode 24 can also be regarded as a filter).

It is essential that the spectral transmission of the individual filters 30, 31 (or of the third filter assigned to the photodiode 24) is different. This can be seen in particular from FIG. 10, which shows the filter characteristic of the filter 30 or 31 (the corresponding reference symbols are assigned to the associated characteristic curves).

FIGS. 8 and 9 again illustrate the structure of the three-filter measuring head 10.

Another embodiment, not shown, is that the measuring head has only one photodiode with a filter wheel provided with several different filters.

Before the invention is discussed in more detail, the known method for determining the area coverage of a printing plate is first explained, since the differences compared to the invention then become clearer.

• - der Remission des lokalen Volltonflächenanteils gewichtet mit der Flächendeckung und
• - der Remission des lokalen nichtdruckenden sogenannten Nullprozentflächenanteils gewichtet mit dem Komplement der Flächendeckung.

As already explained, the area coverings or the zonal area coverings on printing plates are measured via the optical reflectance. It is used here to make the areas of the ink that are in the printing process colored by the printing plate manufacturer by means of a photoresist or that the color of the surfaces differing in color to make the subject visible. The remission of a measuring point (measuring field 12) with a certain area coverage is composed of two components:

• - the remission of the local full tone area weighted with the area coverage and
• - The remission of the local non-printing so-called zero percentage area weighted with the complement of the area coverage.

The signal received at the remission light receiver 17 of FIG. 5 is then

Φ _{0 is} the spectrum of the incident light, β the reflectance of the measuring field 12, ₇ the transmission of a filter, S _E the spectral sensitivity of the photodiode and the wavelength. The integration limits λ ₁ and X ₂ are typically in the visible range or are adapted to the spectral profiles of the individual terms. Particularly in the case of low area coverage, the known method has the disadvantage that measurement errors occur. This is mainly due to the fact that the free printing plate surface is optically inhomogeneous: the reflectance measured on a zero percent surface can differ locally, that is to say that it may not match the zero percent reference remission measured at the plate edge.

The above equation shows that the received signal S is dependent on several parameters. From this it is clear that the spectral sensitivity by using different filters, that is, ₇ variable, Φ and S _E constant or else by differently incident light, that is, Φ variable τ and S _E constant or finally by a different spectral sensitivity of used photodiodes of the reflectance light receiver, that is, S _E variable, τ and Φ constant can be achieved.

The method with different filters τ is discussed below.

mit S als gemessenem Signal, H als Nullprozentreferenz, V als Volltonreferenz sowie f_o als Flächendekkung.The signal model of the known method, which is also referred to as a one-filter method (with a single-filter measuring head) (even if there is no filter, the photodiode used for evaluation can be regarded as a filter due to spectral sensitivity):

with S as the measured signal, H as the zero percent reference, V as the full tone reference and f _o as the area coverage.

In the known method, it is assumed that the measured remission is influenced only by the halftone dots or solid areas; the signal S is therefore only dependent on the area coverage f _o . The inhomogeneities already mentioned are therefore not taken into account and are incorrectly included as area coverage.

Eine Berücksichtigung einer Inhomogenität kann bei dem bekannten Verfahren jedoch dann erfolgen, falls S größer als H gemessen wird, da daraus eine negative Flächendeckung resultiert, was physikalisch nicht möglich ist. Insofern kann hier eine, wenn auch nur unvollkommene Korrektur angebracht werden. Es gibt jedoch keine Möglichkeit, zuverlässig die lokale Nullprozentreferenz im Meßfeld 12 des Sujets selbst zu bestimmen. Vielmehr wird die der entsprechenden Zone zugeordnete Nullprozentreferenz am Rande der Druckplatte gemessen und dann für die gesamte Zone verwendet. Für sämtliche Zonen werden also die entsprechenden zugehörigen Referenzen am Plattenrand gemessen; sie können dann nur global innerhalb der zugehörigen Zone benutzt werden. Die lokale Nullprozentreferenz des jeweils zugehörigen Meßfeldes 12 läßt sich nach der bekannten Methode nicht näherungsweise ermitteln.The area coverage f _o is then:

However, inhomogeneity can be taken into account in the known method if S is measured to be greater than H, since this results in a negative area coverage, which is not physically possible. In this respect, a correction, if only imperfect, can be made here. However, there is no way to reliably determine the local zero percent reference in the measurement field 12 of the subject itself. Rather, the zero percent reference assigned to the corresponding zone is measured at the edge of the printing plate and then used for the entire zone. So for all zones corresponding corresponding references measured at the plate edge; they can then only be used globally within the associated zone. The local zero percent reference of the respectively associated measuring field 12 cannot be determined approximately using the known method.

dabei ist s die Sensornummer (Nummer des entsprechendes Meßkopfes 10) und z die Zonennummer. Tatsächlich benutzt man im Stand der Technik in Ermangelung einer lokalen Referenz jedoch:

• s = 0 bedeutet die zonale Referenz.
• V(0,0) bedeutet eine für alle Zonen global gültige einzige Meßstelle.

The main deficiency of the known one-filter method becomes clear in the foregoing; the correct formula for local area coverage is:

where s is the sensor number (number of the corresponding measuring head 10) and z is the zone number. In fact, in the absence of a local reference, the prior art uses:

• s = 0 means the zonal reference.
• V (0.0) means a single measuring point that is globally valid for all zones.

While the missing local references can still be accepted for the full tone reference, since only slight inhomogeneities occur with full tone areas, this does not apply to the zero percent reference. The following applies:

This means that the local reference H (s, z) i.a. does not match the zonal reference H (0, z).

According to the invention, for an improved measurement, it is provided that the local references are determined, that is, one does not work with a plate edge reference and assigns them to different measurement fields of the associated zone.

wobei mit y die Inhomogenität bezeichnet wird. Ferner kann eine sogenannte Pseudoreferenz H^* definiert werden. Sie ergibt sich zu:

In the two-filter method according to the invention (which is carried out with a two-filter measuring head 10), the local zero percent reference is determined approximately within the measuring fields 12 of the subject of the printing plate 2. This is done based on a model. The basic assumption is that the spectral change of the local zero percent reference relative to the zonal zero percent reference can be described by a scalar 1 - y. In relation to the actual conditions, this approach means that the local reference may be lighter or darker than the zonal reference, but must be of the same color. According to the invention, the signal model is:

where y denotes the inhomogeneity. A so-called pseudo reference H ^* can also be defined. It results in:

The pseudo reference H ^* (s, z) can be calculated for each measuring point (for each measuring field 12). That makes it local. The reference is called "pseudo" because it is not the actual reference, since the subject cannot be "removed" for measurement purposes, but is (only) spectrally similar to the zonal reference. The following therefore applies:

For the two unknowns f _o and γ, two signals must be measured per measuring field 12. This is possible with the two photodiodes 24 and 25 and because of the spectral differentiation by the filter 30. The calculation of the area coverage is then analogous to the formula known from the prior art:

The method according to the invention is to be illustrated by means of FIG. 12 by means of a two-dimensional signal space. A prerequisite for the practical measurement is that the printing surfaces of the printing plate 2 differ in color from the non-printing surfaces. For example, assume that it is an aluminum printing plate, the non-printing surfaces (anodized aluminum) of which are gray, and that a blue photoresist (diazo lacquer) is used, which is located on the printing surfaces. Since the measuring head 10 has two photodiodes 24 and 25, two signals are recorded per measuring field, which are shown on the ordinate or abscissa of the coordinate system of FIG. This is the signal of a filter 1 - for example, permeable to short-wave range (this is the signal of the photodiode 24, which - as already explained - can either have a filter or not) and the signal of the filter 2 which, for example, advantageously passes light complementary to filter 1, which is received by the photodiode 25. With V ₁ and V ₂ , the signals of the photodiodes 24 and 25 are designated, which have been taken from a full tone area (full tone reference). The zonal zero percent reference is identified by the signals H ₁ and H ₂ . The calibration of the photodiode pair is discussed in more detail below. S ₁ and S ₂ denote the signal detected by the measuring head 10 at the currently locally measured measuring field 12. The recorded signals lead to the vectors V, S and H in the two-dimensional signal space. According to the invention, the vector H ^* , that is to say that the vector taking into account the inhomogeneities, must have the same direction as the vector H. If the vector H is extended so far that it intersects the degrees of extension of the end points of the vectors V and S, the end point of the vector H ^* results ^. This can be broken down into H ₁ and H ₂ ^* . The distance between the end points of the vectors H and H ^* thus indicates the correction variable that takes into account the inhomogeneities. According to the signal model in FIG. 12, the vectors H *, V and S lie on a straight line.

The exemplary embodiment in FIG. 12 can be regarded as a 2-dimensional color space, the angle of, for example, a vector S formed from the signals "filter 1 or" filter 2 "with respect to the axes being interpreted as color and the length of the vector S as intensity. The signals "filter 1 and" filter 2 "arise from the spectrally different photodiodes 24 and 25. If filter 1 were to measure in the short-wave spectral range, for example, and if the measuring surface 12 had a higher short-wave blue component, the associated signal vector would be above that in FIG displayed vector S because the intensity behind the shorter-wave filter would be higher.

It can be clearly seen from FIG. 12 that the zero percent reference is scalable. This means that the vector H must be lengthened for inhomogeneities γ <0 or shortened for inhomogeneities γ> 0.

Dabei gibt z die Zonennummer und i = j einen Signalindex an. Das k_f-Kriterium ist umso mehr von Eins verschieden, je mehr sich die Volltonreferenz farblich von der Nullprozentreferenz unterscheidet (immer bezogen auf die benutzten Filter). Das k_f-Kriterium wird zunächst zonal berechnet und dann der Mittelwert benutzt. Die Signale V_i und H_i müssen so unterschiedlich sein, daß für eine tolerierbare Fehlerempfindlichkeit der erfindungsgemäßen Zwei-Filter-Methode ein k_f von (erfahrungsgemäß) mindestens 1,1 erreicht werden sollte. Wird dies nicht erreicht, so wird ausschließlich nach der bekannten Ein-Filter-Methode ausgewertet.With the so-called k _f criterion, it can be checked whether the present printing plate can be measured "spectrally" in the manner of the method according to the invention. The k _f criterion is defined as:

Z indicates the zone number and i = j a signal index. The k _f criterion is all the more different from one, the more the color of the full tone reference differs from the zero percent reference (always based on the filters used). The k _f criterion is first calculated zonally and then the mean is used. The signals V _i and H _i must be so different that a k _f of (based on experience) at least 1.1 should be achieved for a tolerable error sensitivity of the two-filter method according to the invention. If this is not achieved, only the known one-filter method is used for evaluation.

This k _f criterion is illustrated geometrically on the basis of FIG. For the three possible combinations, the products H _i · Vj and H _j · V _{i are shown} as hatched areas in the signal space. The value of the k _f criterion corresponds to the maximum quotient of these area pairs. The dynamic and spectral measurability (embodied by the difference vector H - V or the Angle between the two vectors). When using three diodes and two filters, the combination of the pair of filters with the largest k _f value is selected.

According to the invention it is therefore provided that the inhomogeneity can be distinguished from a change caused by the area coverage according to the spectral effect.

To calibrate (calibrate) the arrangement, proceed as follows:

The measuring bar 3 is moved over a calibration surface, which is either separate from the pressure plate 2 also on the measuring table 1 (but then must be of exactly the same type of plate as the pressure plate 2 used), or is advantageously integrated into the pressure plate 2. For each zone, this calibration area consists, for example, half of a full tone area and the other half of a zero percent area, each of which is large enough to completely fill the optical field of view of the photodiodes 24 and 25. The intensity of the remitted light is then measured on each of the two reference surfaces. This provides the data H (O, z) for the zero percent area and V (0, z) for the full tone area, which are stored for later evaluation.

The measuring run is then carried out, the local area coverage f _o (s, z) and the local inhomogeneity y (s, z) being calculated on the basis of the signal model for each measuring field (measuring point).

In the final evaluation, it is taken into account according to the invention that the inhomogeneities γ (s, z) define so-called pseudo zero percent references H ^* (s, z) on the spectral basis of the zonal zero percent references H (0, z) within the printing plate. These pseudo zero percent references H ^* indicate how the printing plate 2 would look without a subject if the remission of subject-free areas within the printing plate 2 would result from the zero percentage emission of the printing plate edge. The inhomogeneities present can then be recognized locally from the determination of the zero-plate, so-called zero-subject.

In order to obtain a particularly reliable measurement result, it can be provided according to a further development that the zero percent plate thus determined is subjected to smoothing, weighting or evaluation, i.e. the locally determined inhomogeneities are compared with neighboring inhomogeneities and abrupt changes are reduced. Different methods of mathematics known per se can be used for this smoothing.

The smoothing can be weighted in such a way that the signals of a measuring location (s, z) are given a high weighting if the area coverage initially determined at this point (s, z) is low, because precisely there the inhomogeneity of the subject-free area is easier to grasp.

If, according to another exemplary embodiment, a measuring head 10 according to FIG. 7 (three-filter measuring head) is used, it is possible to take into account not only the inhomogeneity of zero percent areas but also of full tone areas. However, the influence of the inhomogeneity of full-tone areas compared to the inhomogeneity of zero percent areas on the measurement result is significantly smaller.

Damit ist eine Skalierung in der Art von Inhomogenitäten nicht nur bei einer Nullprozentfläche (mit γ bezeichnet), sondern auch bei Volltonflächen (mit S bezeichnet) einführbar.If you add another filter to the two-filter model, you have more freedom for the signal model (in addition to the area coverage f _o and the inhomogeneity y) with which you can simulate the actually existing reflectance spectrum of a measuring field using known reference remissions. The signal model then looks like this:

Scaling in the manner of inhomogeneities can thus be introduced not only in the case of a zero percent area (denoted by γ), but also in the case of full tone areas (denoted by S).

oder als dreidimensionaler Vektor geschrieben:

Hierbei gilt:

The result is:

or written as a three-dimensional vector:

The following applies:

This means that spectral changes in all signal-determining variables are recorded in a first approximation and not only as in the signal model described in more detail for zero percent emission.

FIG. 11 shows the spectral reflectance of a full tone area V and a zero percent area H. It can clearly be seen that there is a spectral course due to the colored (blue) full tone area. The non-printing zero percent area H (0%), on the other hand (dark gray), has an almost uniform spectrum. In addition, remissions of area coverage of 4.10 and 20% are recorded. The larger the area coverage, the more the curve-shaped course of the full tone area V (100%) is assumed.

According to another development of the invention, it is also possible, instead of using filters, to measure the reflectance spectroscopically, for example with a spectrophotometer, which breaks down the visible range of light, for example into 32 intervals of 10 nm. With a downstream computer, neighboring wavelength intervals can then be combined into an optimal two-filter combination or else into a three-filter combination.

Claims (21)

1. Process for determining the surface coverage of a printing original, especially of a printing forme of a printing machine, preferably an offset printing machine, in which the spatial reflectance of a registered measurement field is determined by optical scanning of the original, the printing surfaces exhibit differing colour (colour difference) in relation to the non-printing surfaces of the original and the original exhibits an inhomogeneity which is independent of the surface coverage, which is space-dependent and which influences the measurement result of the scanning, characterised in that of each measurement field (12) at least two reflectance values deviating from one another spectrally in accordance with the colour difference are determined, and in that the two reflectance values are evaluated for the separation of a component of the measurement result which is influenced by the surface coverage (f_D) and a component of the measurement result which is influenced by the inhomogeneity (y).

2. Process according to Claim 1, characterised in that for evaluation the reflectance of each measurement field (12) is composed of the following components:

- the reflectance of the full-tone surface (V) weighted by the pertinent surface coverage (f_D) and

- the reflectance of the free, i.e. non-printing or print-free so called zero percent surface (H) weighted by the remaining surface component (1 - f_D) and weighted by a factor which describes the homogeneity (y).

3. Process according to Claim 1 or 2, characterised in that the measurement result determined in the course of the optical scanning is composed of:

- where S is a signal corresponding to the measurement result,

- V is a signal corresponding to the full-tone surface

- f_o is the surface coverage

- y is the inhomogeneity, and

- H is a signal corresponding to the zero percent surface.

4. Process according to Claim 1, characterised in that an additional, third, spectrally deviating reflectance value of each measurement field (12) is determined, which takes into account a spatial change of the reflectance of a printing surface, i.e. a surface carrying printing ink or a printed surface, especially a full-tone surface.

5. Process according to Claim 4, characterised in that the measurement result determined in the course of the optical scanning is composed of:

- where S is a signal corresponding to a measurement result,

- V is a signal corresponding to the full-tone surface,

- fo is the surface coverage

- is the inhomogeneity of the zero percent surface

- is the inhomogeneity of the full-tone surface

- H is a signal corresponding to the zero percent surface.

6. Process according to at least one of the preceding claims, characterised in that the surface coverage (f_D) is determined zonally, and in that ink preset values for ink metering zones of an inking device of the printing machine are determined from the zonal surface coverage values.

7. Process according to at least one of the preceding claims, characterised in that in the case of originals with a globally high surface coverage (f_D) the measurement result of a spectrally independent optical detection of the surface coverage, i.e. the measurement by means of the single filter method known per se, is also taken into consideration.

8. Process according to at least one of the preceding claims, characterised in that to determine the inhomogeneity (y) of a measurement field (12) the inhomogeneities of adjacent measurement fields (12) are utilised for smoothing.

9. Process according to at least one of the preceding claims, characterised in that for a determination of the local surface coverage (f_D) pseudo zero percent references are formed, and are adapted by smoothing, weighting or evaluation, to the determined inhomogeneities of adjacent measurement fields.

10. Device for determining the surface coverage of originals, especially of printing forms of a printing machine, preferably for carrying out the process according to one or more of the preceding claims, characterised by at least one measuring head (10), which optically scans the original and which exhibits a spectrally operating reflectance light receiver (17), so that a plurality of spectrally differing measurement results can be determined on account of differing spectral evaluation of each optically scanned measurement field (12).

11. Device according to Claim 10, characterised by a filter arrangement (filters 30, 31) for carrying out the differing spectral evaluation.

12. Device according to Claim 10 or 11, characterised by an illumination device, emitting spectrally differing light, for carrying out the spectral evaluation.

13. Device according to at least one of the preceding Claims 10 to 12, characterised in that the reflectance light receiver exhibits reception elements of spectrally differing sensitivity, especially photodiodes, for carrying out the spectral evaluation.

14. Device according to at least one of the preceding Claims 10 to 13, characterised in that the reflectance light receiver (17) exhibits at least one photodiode (24, 25, 26).

15. Device according to at least one of the preceding Claims 10 to 14, characterised in that the measuring head (10) exhibits a beam splitter (27), which passes the reflectance directly to a first photodiode (24) and to a second photodiode (25) via a filter (30) forming the filter arrangement, or passes the reflectance to a first photodiode (24) via a filter and to a second photodiode (25) via a further filter (30), the two filters exhibiting a spectrally differing characteristic and forming the filter arrangement.

16. Device according to at least one of the preceding Claims 10 to 15, characterised in that the measuring head (10) exhibits a further beam splitter (28) which passes the reflectance to a third photodiode (26) via a further spectrally differing filter (31).

17. Device according to at least one of the preceding Claims 10 to 16, characterised in that a plurality of measuring heads (10) are disposed side by side, and in that the measuring heads (10) are displaceable relative to the original.

18. Device according to at least one of the preceding Claims 10 to 17, characterised in that the measuring heads (10) are displaceable in the printing direction of the printing forme.

19. Device according to at least one of the preceding Claims 10 to 18, characterised in that the measuring heads (10) are displaceable transverse to the printing direction of the printing forme.

20. Device according to at least one of the preceding Claims 10 to 19, characterised in that the filter or filters (30, 31) is or are designed as cut-off filters or tristimulus filters.

21. Device according to at least one of the preceding Claims 10 to 20, characterised in that a filtering is carried out by a spectroscopic registration of the reflectance, preferably by means of a spectrophotometer and with a downstream computer, by combination and weighting of adjacent wavelength ranges.

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