EP1744884B1 - Method for determining colour values and/or density values, and printing device for implementing said method - Google Patents

Abstract

In a method for determining color and/or density values for use in monitoring and regulating a printing process in a printing apparatus, specifically in a sheet-fed offset printing press, measuring areas of a printed sheet are measured photoelectrically during the printing process, directly in or on the running printing apparatus. From the measured values obtained in the process, the color and/or density values for the relevant measuring areas are formed. From the measurement, measured value deviations caused directly in the printing process with respect to a measurement outside the printing process can be corrected computationally.

Description

The invention relates to a method for determining color and / or density values for monitoring and / or regulating the printing process in a printing device according to the preamble of independent claim 1. The invention also relates to a trained for the method printing device according to the preamble of the independent claim 27th

In such a generic method, the measured values are detected directly during the printing process with a measuring arrangement which is located within the printing device - e.g. a sheetfed offset press or generally a printer - is installed. This type of measured value acquisition or measurement is referred to below as "inline". In contrast, "externally" refers to a measured value acquisition outside the printing device in a stable state of the printed product.

An inline measured value acquisition is eg in US-A-4,660,159 disclosed.

At the time of the inline measurement, ie during the printing process, the inking is not yet stable. The disturbing effects of inking are caused by various parameters of the printing process. In addition, the appearance of the printed product by subsequent processing steps, for. B. painting the surface, to be changed. Both effects lead to differences between the measured values measured inline and the corresponding measured values determined externally in a stable state of the printed product. Inline and external measured values are therefore not directly comparable.

The most general object of the invention is the correction of these measurement differences. This object is achieved by the measures cited in the characterizing part of independent claim 1. Further developments and particularly advantageous embodiments of the invention are the subject of claims dependent on claim 1.

Another general object of the invention is to provide a printing device in which the inventive method can be used. This object is achieved by the cited in the characterizing part of the independent claim 27 training the printing device. Trainings and Particularly advantageous embodiments of the inventive printing device are the subject of claims dependent on claim 27.

According to the most general idea of the invention, said correction of the measurement differences is achieved by computational correction measures and preferably in conjunction with a special design of the measuring arrangement (measurement technique). In the following the invention will be described using the example of sheet-fed offset printing. However, the approaches according to the invention are generally valid and can also be used for other printing methods and devices.

The colorimetry, as described for example by the Commission Internationale de l'éclairage (CIE) in the CIE publication 15.2 "Colorimetry", and the standards for the color and density measurement technique to be used (eg DIN 5033, ISO 5) allow a absolute description of a color value. This standard forms the basis for color communication in modern digital workflow and color management systems. CIE compliant color values (XYZ or L * a * b *) are used to transfer the color information of the subject from the input stage (original, camera, scanner, monitor), digital proof, prepress, to the press. For efficient conversion of the absolute CIE color values into machine control parameters (e.g., color separation into the primary colors C, M, Y and K), process standards have been defined. A process standard for offset printing technology is defined in the standard DIN / ISO 12647-2. The application of a process standard enables flexible processing of a print job with different printing presses. However, it requires characterization, adjustment and stable operation of the printing press according to the specifications of the process standard.

The measuring technology used must be able to output standard-compliant color and density values for these tasks. This can e.g. be achieved by a combination of a tri-colorimeter and a densitometer. Ideally, however, a spectrophotometer is used as the measurement technique because it supports both measurement modes and allows flexibility in the selection of density filters.

• tragbare Handmessgeräte, wie zum Beispiel das Spektralphotometer SpectroEye und das Densitometer D 19 der Firma Gretag-Macbeth AG, und
• halb-automatisierte Messsysteme wie AxisControl und ImageControl mit spektralphotometrischen Messköpfen der Firma Heidelberger Druckmaschinen AG.

The current state of color measurement in the printing sector is represented by two types of measuring systems:

• portable hand-held instruments, such as the spectrophotometer SpectroEye and the densitometer D 19 from Gretag-Macbeth AG, and
• Semi-automated measuring systems such as AxisControl and ImageControl with spectrophotometric measuring heads from Heidelberger Druckmaschinen AG.

These measuring devices and systems are used externally, ie outside the printing press. With the hand-held measuring device, the printer can control individual measuring fields in the print control strip or in the image. With semi-automated systems, the printer can manually load a single sheet. Depending on the system, the complete print control strip (AxisControl) or the entire sheet (ImageControl) is automatically measured. These measuring systems use standard measuring geometries. The template is a finished printed end product in a stable state. The resulting readings conform to CIE-compliant color readings and can be used directly to control and monitor or control the printing process, for color communication, or for display.

To be able to print jobs more efficiently and cost-effectively, the trend is towards automated printing machines. For color measurement, this means that the measurements are no longer performed manually by the printer outside the press, but fully automatically directly in the press. This inline measurement technology offers great advantages. By integrating the inline measuring technology into a closed control loop with the individual printing units, the printing press can be automatically and quickly color-coded. In addition, the color can be constantly checked and tracked during the printing process, which allows a continuous quality control.

However, inline measurement technology is much more complex than conventional external color measurement technology. The inline measurement must be carried out shortly after the paint application. At this time, the color layer is not yet stable. It is influenced by various printing process parameters and color properties, which decay with different time constants. Depending on the situation, this can result in large differences between the inline measured values and corresponding external measured values on stable dry samples. In addition, the process dependence complicates the interpretation of the measurement data. It is not clear whether a measured variation was caused by a change in the color application or by a change in the process parameters. A similar problem arises if the printed product is further processed after the inline measurement. A typical example is the application of a lacquer layer in a subsequent coating plant.

The present invention is particularly concerned with inline measurement in sheetfed offset presses, but is also suitable for other printing methods and devices. As already mentioned, the invention essentially comprises a special design of the measuring technology and measuring geometry as well as correction methods for the inline measured values, which enable a conversion into standard-compliant color and density measured values for corresponding stable external samples (printed products).

Inline measuring systems are available for web offset printing presses, eg For example, QuadTech's ColorControlSystem (CCS) system. However, these systems are installed after the drying systems at the end of the web offset printing press. At the time of measurement, the print material is already dry and in a stable state. A process-dependent correction of the measured values is not necessary here.

On the other hand, so-called "Webinspection" systems for color measurement and control are used in flexo, gravure and web offset printing presses. One example is the Print-Vision 9000 NT system from Advanced Vision Technology (AVT). These systems use imaging measurement techniques that map the artwork on two-dimensional or one-dimensional CCD sensors. The color values were determined with non-standard filter function and correspond to camera-specific RGB values. These measurements are transformed into CIE color values. The conversion of the measured values does not correspond to a printing process-dependent correction, but a colorimetric characterization of the measuring system, as it is also used in common color management systems for screen, camera and scanner profiling. A general description of this technique can be found in the publication "Digital Color Management, Encoding Solutions" by E. Giorgianni.

Fig. 1

eine schematische Darstellung eines Ausführungsbeispiels der erfindungsgemässen Druckeinrichtung,

Fig. 2

ein Prinzipschema eines spektral arbeitenden, zum Einsatz in der Druckeinrichtung nach Fig.1 geeigneten Messanordnung,

Fig. 3a,3b

zwei Skizzen zur Erläuterung einer erfindungsgemässen Messgeometrie,

Fig. 4

ein Diagramm zur Erläuterung der Messgeometrie der Fig. 3,

Fig. 5

ein allgemeines Blockschema des erfindungsgemässen Verfahrens,

Fig. 6

ein Blockschema eines speziellen Ausführungsbeispiels des erfindungsgemässen Verfahrens und

Fig. 7a,7b

zwei Diagramme zur Erläuterung von nach dem erfindungsgemässen Verfahren vorgenommenen rechnerischen Messwertkorrekturen.

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

Fig. 1

a schematic representation of an embodiment of the inventive printing device,

Fig. 2

a schematic diagram of a spectrally operating, for use in the printing device according to Fig.1 suitable measuring arrangement,

Fig. 3a, 3b

two sketches for explaining a measuring geometry according to the invention,

Fig. 4

a diagram for explaining the measuring geometry of Fig. 3 .

Fig. 5

a general block diagram of the inventive method,

Fig. 6

a block diagram of a specific embodiment of the inventive method and

Fig. 7a, 7b

two diagrams for explaining made by the inventive method arithmetic measured value corrections.

In the FIG. 1 a sheetfed offset printing machine is designated as a whole by 1. The printing press has four (or possibly also more) printing units 11-14 and prints sheets which are provided on a so-called feeder 15. The sheets are first printed in the first printing unit 11 with a first color, then passed on to the second printing unit 12 until finally finished with all the colors printed leave the last printing unit 14. At the last printing unit 14, a measuring arrangement 20 is provided, which measures the sheets (at the measuring points provided for this purpose) immediately after printing. Subsequently, the printed sheets are fed to further processing stages, for example a dryer unit and a coating unit 16, and finally output in a so-called boom 17. Apart from the measurement during the printing process or immediately thereafter, the printing press as far as the prior art, so that the expert requires no further explanation.

The inline measuring arrangement 20 comprises, in a manner known per se, one or more simultaneously measuring measuring heads. The measuring heads can also be installed in different printing units. For reasons of cost, however, it makes sense to combine the measurement of the colors of all participating printing units at a common location after the last printing unit. The measuring heads are preferably arranged in a row at right angles to the printing direction. The measuring unit 20 further includes an automated linear movement device perpendicular to the printing direction, so that each point can be approached and measured across the sheet width. The mechanical design of an automated measuring arrangement with several measuring heads is known per se and requires no further explanation.

In the Fig. 1 Furthermore, a correction computer 40 is shown, which receives the measured values detected by the measuring arrangement and, after the correction, feeds them to a control computer 50, which finally controls the printing units 11-14 of the printing press 1 in a manner known per se. On the correction computer 40 and its functions will be discussed below.

At high printing speeds, the measurement takes place immediately after the ink application at the last printing unit. The time difference between measurement and ink application is only a fraction of a second. The Fogra Research Report No. 52.023 contains images showing the state of the ink layer immediately after the ink splitting at the printing nip. In these pictures the emergence of threads, the so-called microstripes, between rubber blanket and printed sheet is visible. These threads have a diameter of 30 to 60 microns and tear off after a certain distance from the nip. The result is a color layer with a macroscopic surface modulation in relation to the layer thickness, which has not decayed at the time of the inline measurement. When measuring the color of the last printing unit, the surface modulation is directly caused by the threading of the color fission. When measuring colors of front printing units, a reduced effect occurs which is caused by the interaction of the fresh color on the printing sheet with the blanket of the last printing unit. An emulsion of paint residues and dampening solution is transferred to the paint layer.

The surface modulations of the color layer influence the measured values. They depend on a variety of printing process parameters, such as printing speed, printing unit, substrate and color type. In addition, differences between measured values determined inline and externally are also caused by the drying behavior of the ink on the substrate, which has a significantly longer time constant.

The differences between measured values determined inline and externally must be corrected for the practical utilization of the measured values. The method according to the invention uses for this correction a metrological component (special design of the measuring arrangement 20) as well as computational components, which are executed in the correction computer 40.

The aim of the metrological component is to maximally reduce the influence of the process-dependent disruptive effects and to provide as unambiguous as possible measured values. In addition, additional boundary conditions often have to be taken into account for the design of the measurement technique, such as space limitations in the printing press or varying measurement distance, which boundary conditions according to a further aspect of the invention by deviations from the normalized 0 ° / 45 ° measurement geometry can be taken into account. The remaining measured value deviations from externally determined, standard-conforming measured values are then compensated by numerical correction measures or models in the correction computer 40.

The arrows in FIG. 1 represent the data flow of the measured values. The measured values may be density values, color values or reflectance spectra depending on the measuring technique of the measuring arrangement used. In fact, the data flow between the components is bidirectional. The measurement data acquired by the measuring arrangement 20 are transmitted in digital form to the correction computer 40. This corrects the measurement data and forwards it to the control computer 50 of the printing press 1. The corrected measurement data can be displayed by the control computer 50 for the printer, stored, or used for the color control of the printing press. In this case, the (corrected) measured data for the color control are compared with desired values 51 in a manner known per se, and the settings of the printing units 11-14 are determined therefrom and transmitted electronically to them.

The correction computer 40 requires for the conversion of the measured values process-specific correction parameters, which are made available in a correction database 41. The correction computer 40 requires for the selection of the correction parameters from the database 41 information 42 about the current printing process. These necessary information 42, for example substrate type, color type and printing unit assignment, are selected or entered by the printer at the (not shown) control panel of the printing machine 1 and transmitted in practice via the control computer 50 to the correction computer 40.

In the following, the special inventive design of the measuring arrangement 20 with reference to Figures 2 and 3 explained in more detail.

As already mentioned, the measuring unit 20 consists of a bar in which there are several measuring heads 21 mounted in a row transversely to the direction of travel of the paper, the bar being installed at the end of the last printing unit of an offset printing press. The measuring heads themselves are mounted on a motor-driven slide, which can be moved electronically controlled transversely to the paper direction within the beam. In this way it is possible to detect any measuring locations on the paper.

In order to be able to make optimal use of the scarce installation space in the printing press, the measuring arrangement 20, in addition to the measuring heads 21, also has separate measuring heads for determining the paper and register position. In addition, the measuring arrangement is connected to a rotary encoder of the printing unit, so that the measuring sequence can be synchronized with the rotational movement of the printing cylinder.

A typical measuring head 21 is in FIG. 2 shown schematically. The measurement geometry corresponds to the color measurement standard 0/45 ° in accordance with DIN 5033. The illumination from a light source 22 takes place below 0 ° and is imaged by means of an optical system 23 in the measurement plane 24. As a light source, a central flash light source is preferably used, the light is passed with a fiber optic multiple distributor to the individual measuring heads. The measuring light reflected by the measuring point on the printed sheet is recorded at 45 °. An optical system 25 images the measuring spot in the measuring plane onto an analyzer 26. The analyzer 26 is shown as a photodiode array grating spectrometer with a fiber coupling 27. The measuring head 21 in this design corresponds to a spectrophotometer. The design of such a measuring head corresponds to the known state of the art and therefore requires no further explanation. In principle, all known techniques for the spectral analysis of the reflected light from the sample can be used. Alternatively, the inverted 45 ° / 0 ° measurement geometry can be used with reversed illumination and receiver channels.

The following is the case for a spectral measurement technique over the entire visible range. The measured values are a reflectance spectrum which corresponds to the spectral reflectance of the sample of typically 400 to 700 nm with a spectral resolution of 10 or 20 nm. Density and tristimulus color measuring heads use only part of this spectrum. However, the metrological aspects and the correction models for these spectral subareas are identical to the general case and can be derived directly from the spectral case.

As already mentioned, inline measurement technology must be able to supply compatible measured values to an external reference. The external reference is defined by measured values on stable samples with a standard-compliant spectrophotometer with 0 ° / 45 ° measuring geometry. Stable sample in this context means that the effects of color separation have subsided and that the sample is finished. In addition, the color layer must be in a defined external state.

For this reason, the inline measuring arrangement must suppress the effects of the varying surface structure. For this purpose, 21 polarizing filters 28 and 29 are used according to an aspect of the invention in the illumination and receiver channel of the measuring head. The polarization filters consist of linear polarizers and are installed with mutually perpendicular polarization axes in the illumination and receiver channel. The use of polarizing filters is known per se for density measurement in hand-held measuring devices. A description of this technique is contained in the publication "Color and Quality" of Heidelberger Druckmaschinen AG. The use according to the invention of polarization filters in inline measurement for the purpose of eliminating or suppressing the surface effect, i. However, the suppression of that component of the measuring light, which is reflected directly on the structured surface of the ink layer, is not yet described in the literature.

Another special design of the measurement technique is that in addition to the polarization filter in the illumination channel, a UV filter 30 is installed, which suppresses the ultraviolet (UV) portion of the illumination spectrum below 400 nm. This UV barrier filter 30 can be realized, for example, with a type GG420 filter glass from Schott. The UV blocking filter prevents the fluorescence of the brightener additives in the paper from being excited. As a result, a better reproducibility of the measured data from sheet to sheet and, above all, order to order is achieved for the inline measurement since the brightener components in the paper can fluctuate. In addition, the UV cutoff filter 30 improves the match with the external reference values because the external meter can use a different illumination source.

Further boundary conditions in the printing press can influence the design of the measuring arrangement 20, for example limited installation space in the printing press or unclean paper support in the measuring plane. According to another important aspect of the invention, these boundary conditions can be taken into account by measuring geometry deviating from the normalized 0 ° / 45 ° measuring geometry.

The FIG. 2 shows that the distance 31 from the lower edge of the measuring head 21 to the measuring plane 24 has a significant influence on the size of the measuring assembly 20. Namely, in the standard geometry, it determines the distance between the illumination and receiver channels at the lower edge of the measuring arrangement. In addition, it can be seen that the receiver and illumination channel move laterally in the measurement plane (arrow 32) when the measurement distance 31 changes. The mutual displacement limits the working range of the measuring optics.

An improvement for the installation space and the working area is achieved if the lighting and receiver channel are arranged on the same side from the vertical on the measuring plane. This inventive configuration is in the Fig. 3b shown. Fig. 3a shows in comparison the standard geometry 0 ° / 45 °. If the measuring distance is changed, the lateral offset between the illumination and the receiver is reduced. The measurement angles correspond to Fig. 3b no longer the standard geometry. Since any deviation from the standard geometry inevitably entails measured value deviations, the new measurement angles must be chosen so that the smallest possible deviations result from the measurement with standard geometry. Since measurement is carried out using polarizing filters, this requirement corresponds to the condition that the path lengths of the light beams in the color layer are identical for the different measuring geometries. This corresponds to the same absorption behavior. The condition for equal absorption paths in the color layer can be described in a first approximation by the following equation [1]: $$\frac{1}{\cos \left({\beta }_B\right)}+\frac{1}{\cos \left({\beta }_e\right)}=1+\frac{1}{\cos \left({\alpha }_e\right)}=2.13$$

β_B :

mittlerer Beleuchtungswinkel in der Farbschicht mit Brechungsindex n

β_E :

mittlerer Empfängerwinkel in der Farbschicht mit Brechungsindex n

α_E :

Empfängerwinkel in Normgeometrie in der Farbschicht (n sin(α_E )=sin45°)

n :

Brechungsindex der Farbschicht n = 1.5

In it are:

β _B :

average illumination angle in the color layer with refractive index n

β _E :

average receiver angle in the color layer with refractive index n

α _E :

Receiver angle in standard geometry in the color layer (n sin (α _E ) = sin45 °)

n:

Refractive index of the color layer n = 1.5

The corresponding illumination angles and receiver angles in air can be calculated from the angles in the color layer with the known refraction law (H. Haferkorn, Optik, p. 40).

The combinations of illumination and receiver angles in air satisfying equation [1] are in Fig. 4 represented in the form of a diagram. The coordinate axes denote the illumination angle and the receiver angle in air, the points on the curve 33 each correspond to an angle pair for the measurement geometry. Particularly useful and advantageous for inline measurement are illumination angles greater than 10 ° with the corresponding receiver angles less than 45 °.

The measurement geometry according to the invention explained above is also of interest for a measurement technique without a polarization filter. The crossed polarizing filters cause a large signal loss and can not be used when, for example, a weak light source has to be used. Also in this case, it is necessary to reduce the reflection component from the modulated surface. This is achieved according to a further aspect of the invention by tilting the illumination channel in the direction of the receiver channel. In FIG. 3b As can be seen, this increases the angular separation between the directed reflection at the surface and the receiver angle. The measurement angles should in this case also satisfy the equation [1]. Advantageous measuring geometries are illumination angles in the range of 10 ° to 15 ° and receiver angles in the range of 40 ° to 45 °.

In the following, the mathematical correction measures for the measured values and the underlying correction models are explained in more detail.

The goal of all corrective measures, both metrological and computational, is to make the inline measurements compatible with corresponding external reference values. Reference values are understood to mean those measured values which are obtained with a standard-compliant color measuring device on finished printed sheets outside the printing press. For the correction of the measured values There are three different states, which are defined in more detail below.

State 1 corresponds to the inline measurement in the printing machine with the measuring arrangement 20. At the time of measurement, the ink layer on the substrate is still wet. In addition, the surface of the ink layer is greatly disturbed by the effects of color splitting at the last printing unit.

State 2 corresponds to the situation when a sheet is taken out of the boom 17 directly after the printing process and a color measurement is made thereon. In this condition 2 the color layer is still wet. The effects of color splitting have already subsided. The surface of the color layer can be assumed to be smooth and glossy with maximum gloss, only a minimal surface effect occurs.

State 3 corresponds to the situation when the color measurement is performed on a printed sheet of completely dried ink. The drying process typically takes several hours. In this state, the ink film has assumed the microscopic surface roughness of the substrate. In coated papers, the ink layer remains on the substrate during the drying process, the thickness of the ink layer on the substrate is maintained. For uncoated papers, part or even all of the color pigments penetrate the substrate during the drying process. This effect alters the density and color measurements and must be corrected.

The correction models according to the invention described below enable the conversion of the measured values between these three states. The conversion is possible in both directions.

According to the invention, a sequential sequence is advantageously selected for the practical implementation, ie the measured values obtained in accordance with condition 1 from the measuring arrangement 20 are first transformed into measured values corresponding to condition 2 in state 2 (external measurement wet) and then these measured values corresponding to state 2 are measured state 3 (external measurement dry) corresponding measured values transformed. This sequential correction process is in FIG. 5 shown schematically. The correction of the measured values of state 1 (Block 401) to state 2 (block 402) mainly involves correcting the effects of color splitting (block 404). The correction from state 2 (block 402) to state 3 (block 403) corresponds to the correction of the drying behavior of the ink layer on the particular substrate type (block 405). In this implementation, there is exactly one external reference state (state 2, block 402) into which all inline metrics (block 401) are transformed. Starting from this state 2, the measured data are then further processed for all applications. Typical applications are displaying the measurements (block 406), storing the measurements as setpoints for the print job (block 407), communicating the setpoints to another press (block 406), and using as the current actual color control value (block 407). ,

For the determination of reference values in the states 2 and 3, it makes sense that an external measuring device is used together with the inline measuring arrangement 20. The corrected measurement values in states 2 and 3 must correspond to the reference values which correspond to the measurement with a standard-compliant spectrophotometer, colorimetric or density meter. In order to minimize the metrological differences between the inline and the external measurement, the external reference values are performed with a measuring device equipped with the same measuring filters as in the inline measuring device 20. This means that in the preferred realization of the method, the external reference values are determined with a measuring device which is equipped with polarization filters and a UV blocking filter.

If the inline measuring device 20 and the external measuring device do not use the same bandwidth, for example spectrophotometers with 10 nm or 20 nm spectral resolution, a numerical bandpass correction is performed. The bandpass correction may be performed as described in standard ISO 13655 (ISO 13655, Graphic Technology - Spectral measurement and colorimetric computation for graphic arts images, Annex A, 1996).

Furthermore, it makes sense that, together with the inline measuring arrangement 20, an external measuring device is used, which has exchangeable measuring filters in the illumination and receiver channels. The meter should support the metering modes without filters, with UV cut filter and with polarizing filters. An exemplary embodiment of such a measuring device is the spectrophotometer SpectroEye from Gretag-Macbeth AG. This functionality allows for takeover or delivery of readings from or to other measuring systems using other measuring filters. The external measuring device can measure a printed reference sheet in all measuring modes. The measured values with the corresponding measuring filter can then be forwarded to the inline measuring arrangement 20 or to another external system. In particular, this allows the adoption of setpoints for color control, which were measured with other measuring filters.

If the measured density values on the reference sheet do not correspond to the required target density, the transformed measured values can be adjusted with a correction model which changes the layer thickness. This transformation can be carried out with the layer thickness modulation model which will be described below.

The following sections describe the theoretical basis for the inventive computational correction measures (correction algorithms). The first section describes the correction of the inline measurement error, while the second section describes the correction of the drying behavior. The practical application of the correction algorithms and the concrete implementation of the entire correction system are described below.

The starting point for the correction or compensation of the inline measurement errors is the color layer at the time of the inline measurement with a modulated surface. The result of the correction must be a compatible external state 2 measurement, which corresponds to a homogeneous color layer.

The necessary correction parameters and freedom line and their influence are derived from a color model that simulates the metrological behavior of the ink layer.

mit

c₀

Anteil der Oberflächenreflektion, welcher unter 0° gemessen wird

R₀

Oberflächenreflektionskoeffizient für 45° Einfallswinkel in Luft

α

Absorptionskoeffizient der Farbschicht

d

Schichtdicke der Farbschicht

θ₂

Einfallswinkel in Medium 2 (Farbfilm) mit Brechungsindex n₂: n₂sin(θ₂) = n₁sin(θ₁)

θ₁

Einfallswinkel in Luft unter 45° mit Brechungsindex n1

ρ_p

diffuser Reflexionsgrad des Substrats

sin²(α₁)

Normalisierungsfaktor für Absolutweiss für Messgeometrie mit Erfassungswinkel α₁ = 5°

I_A

Integral für den gemessener Anteil des ausgekoppelten diffusen Strahlflusses aus der Farbschicht

I_P

Integral für den rückreflektierten diffusen Strahlfluss in der Farbschicht

R₂₁

interner Reflektionskoeffizient in der Farbschicht gegenüber Luft (von Medium 2 zu Medium 1)

The color model is based on Hoffmann's theory, which allows a precise physical description of the reflection factor of a single, homogeneous, non-diffusing color layer on a diffusely reflecting substrate. The Hoffmann theory is designed for a diffuse measuring geometry. The fit for the reflection factor in the 0/45 ° measurement geometry is described in Equation [2]: $$R=c_0{R}_0+\frac{\left(1-R_0\right)e^{-\mathit{.alpha..sub.d}/\mathrm{<figure-callout\; id="2"\; label="cos\; \theta "\; filenames="imgf0007.png"\; state="\{\{state\}\}">cos\; </figure-callout>}{\theta }_{}{}_2}{\rho }_p{I}_A}{{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0007.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2\left({\mathrm{<figure-callout\; id="1"\; label="\alpha "\; filenames="imgf0004.png,imgf0007.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}_1\right)\left(1-{\rho }_p{I}_p\right)}.$$

With

c ₀

Proportion of surface reflection measured below 0 °

R ₀

Surface reflection coefficient for 45 ° angle of incidence in air

α

Absorption coefficient of the paint layer

d

Layer thickness of the color layer

θ ₂

Angle of incidence in medium 2 (color film) with refractive index n ₂ : n ₂ sin (θ ₂ ) = n ₁ sin (θ ₁ )

θ ₁

Angle of incidence in air below 45 ° with refractive index n1

_p

diffuse reflectance of the substrate

sin ² (α ₁ )

Normalization factor for absolute white for measuring geometry with detection angle α ₁ = 5 °

I _A

Integral for the measured proportion of decoupled diffuse beam flux from the color layer

I _P

Integral for the back-reflected diffuse beam flow in the color layer

R ₂₁

Internal reflection coefficient in the ink layer with respect to air (from medium 2 to medium 1)

$$I_p=2\underset{0}{\overset{\pi /2}{\int }}{R}_{21}{e}^{-2\mathit{\alpha d}/\mathrm{cos\; \theta }}\mathrm{cos\; \theta \; sin\; \theta }d\theta$$

R ₀ and R ₂₁ are calculated using the Fresnel formulas (H. Haferkorn, Optik, p.50): $$I_A=2\underset{0}{\overset{{\mathrm{<figure-callout\; id="2"\; label="\alpha "\; filenames="imgf0007.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}_2}{\int }}\left(1-{\mathrm{<figure-callout\; id="21"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{21}\right)e^{-\mathit{.alpha..sub.d}/\cos }\cos \theta \sin \mathit{\theta }d\theta$$

$$I_p=2\underset{0}{\overset{\pi /2}{\int }}{\mathrm{<figure-callout\; id="21"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{21}{e}^{-2\mathit{.alpha..sub.d}/\mathrm{cos\; \theta }}\mathrm{cos\; \theta \; sin\; \theta }d\theta$$

The correction models for solid-color fields macroscopically surface-modulated by the color splitting are explained below. The adaptation for grids can be done with the well-known theory of Neugebauer.

From equation [2] it can be seen that the reflection factor R consists of two additive components. The first component corresponds to the surface effect and can be written as a remission difference: $$\mathrm{\Delta }{\mathrm{R}}_{\mathrm{0}}\mathrm{=}{\mathrm{c}}_{\mathrm{0}}{\mathrm{R}}_{\mathrm{0}}$$

In equation [3], c _{0 is} a correction function dependent on the relevant printing process parameters.

As described above, the surface effect is preferably eliminated by metrological means, ie by the use of polarization filters in the measuring arrangement 20. In this case c ₀ = 0 can be assumed. If polarizing filters can not be used, the surface effect must be corrected numerically. The amplitude of the surface effect is influenced by the relevant printing process parameters. The correction function c ₀ or the dependence on the printing process parameters is determined experimentally. The general procedure is explained below.

The second component in equation [2] includes the absorption by the ink as well as the multiple reflections at the interfaces of the ink layer. The multiple reflections are referred to in the literature as Lichtfang.

The modulated surface of the color layer after the color splitting influences the absorption behavior and light trapping. The behavior and influence of both effects can be deduced as follows.

wobei αd_c anstelle von αd in die Gleichung [2] einzusetzen ist. c₁ ist eine von den massgeblichen Druckprozessparametern abhängige Korrekturfunktion, die, wie weiter unten noch ausgeführt ist, experimentell mit Charakterisierungsmessungen bestimmt werden kann.The modulation of the surface causes the thickness of the ink layer at certain points is smaller than the corresponding layer thickness without modulation. By this effect, the average absorption capacity of the ink layer is reduced. The effect can therefore be described in equation [2] by adapting the product of absorption coefficient α and layer thickness d. One possibility for the implementation is multiplication by a process-dependent correction factor c ₁ , which assumes values less than or equal to 1 as a function of the layer thickness modulation. The values after the correction of the film thickness modulation are described by equation [4] $$\mathrm{\alpha }{\mathrm{d}}_{\mathrm{c}}\mathrm{=}\mathrm{.alpha..sub.d}{\mathrm{c}}_{\mathrm{1}}\mathrm{,}$$

where αd _{c is} substituted for αd in equation [2]. c ₁ is a correction function dependent on the relevant printing process parameters, which, as explained below, can be determined experimentally with characterization measurements.

wobei n_2c der Brechungsindex nach der Korrektur und c₂ eine multiplikative Korrekturfunktion ist, welche wie die Korrekturfunktionen c₀ und c₁ prozessabhängig ist und experimentell charakterisiert werden muss.The modulated surface also affects the light trapping of the color layer, since the modulation affects the angles of incidence of the light rays and thus the critical angle for the total reflection at the surface. An elegant implementation of this dependence in equation [2] is achieved according to the invention by varying the refractive index of the ink layer n ₂ in the calculation. The surface modulation reduces the mean critical angle for the total reflection, leaving more light trapped in the ink layer. This behavior corresponds to an increase of the refractive index n ₂ . One way to correct light trapping is described in Equation [5]: $${\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="imgf0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_{\mathrm{2}\mathrm{c}}\mathrm{=}{\mathrm{n}}_{\mathrm{2}}{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="imgf0007.png"\; state="\{\{state\}\}">c</figure-callout>}}_{\mathrm{2}}\mathrm{.}$$

where n _{2c is} the refractive index after the correction and c _{2 is} a multiplicative correction function which, like the correction functions c ₀ and c _{1, is} process-dependent and must be characterized experimentally.

The correction of the inline measurement errors can thus be implemented with three different types of errors, namely surface effect, layer thickness modulation and light capture according to the equations [2] to [5]. For the correction, the three correction functions c ₀ , c ₁ , c _{2 are} used, which are parameterized as a function of the printing process parameters and whose corresponding values are stored in the correction database 41 already mentioned.

Although the described correction of the inline errors on the basis of the exact color model according to equation [2] is readily possible, the numerical implementation is relatively complicated.

A more efficient numerical implementation can be obtained by using the Kubelka-Munk theory with the surface phenomena (Saunderson correction) for the color model. This model corresponds to the state of the art. A detailed description of this theory is in the Dissertation "Modeles de prédiction de couleurs appliquées à l'impression jet d'encre "by P. Emmel (thesis No. 1857, 1998 , Ecole polytechnique féderale de Lausanne).

The theory of Kubelka-Munk applies to a diffuse measuring geometry and scattering color layers. Nevertheless, it can be used for the phenomenological explanation of the effects of inline measurement errors in the 45/0 ° measurement geometry and their correction.

The reflection factor of an absorbing ink layer on a diffusely scattering substrate can be described by the following equation. $$R=c_O{R}_O+\frac{(1-R_0)\left(1-{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0007.png"\; state="\{\{state\}\}">R</figure-callout>}}_2\right){\rho }_p{e}^{-\mathit{kd}}}{1-{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0007.png"\; state="\{\{state\}\}">R</figure-callout>}}_2{\rho }_p{e}^{-\mathit{kd}}}.$$

R₂

diffuser Reflexionskoeffizient in der Farbschicht (R₂= 0.6)

K

diffuser Absorptionskoeffizient

ρ_p

diffuser Reflexionsgrad des Substrats

Mean in it

R ₂

diffuse reflection coefficient in the color layer (R ₂ = 0.6)

K

diffuse absorption coefficient

_p

diffuse reflectance of the substrate

The first additive component c ₀ R ₀ again corresponds to the surface effect and is identical to equation [2].

In equation [6] a diffuse absorption coefficient K is introduced. It does not correspond to the material absorption α in equation [2]. For a diffuse flow, K = 2α can be assumed as an approximation.

The advantage of the Kubelka-Munk approach is that the equation [6] is simply invertible, ie from the remission measurement, the absorption spectrum (extinction E) can be determined directly. This relationship is shown in equation [7]. $$e^{-e}=e^{-\mathit{kd}}=\frac{R-c_O{R}_O}{{\rho }_p\left(\left(1-R_0\right)(1-R_2)+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0007.png"\; state="\{\{state\}\}">R</figure-callout>}}_2\left(R-c_{O}R\right)\right)}\mathrm{,}$$

A comparison of equations [2] and [6] shows that the multiple reflections and the absorption are rated differently. In this application, a color does not have to be absolutely described. A relative measured value correction must be carried out become. Therefore, the spectral absorbance E of the color can be determined from equation [7] and used as a model parameter.

The three error types for the correction of the inline measurement errors from the equations [2] to [5] can be assigned equivalent errors in the Kubelka-Munk description:

The surface effect is identical to equation [3]. $$\mathrm{\Delta }{\mathrm{R}}_{\mathrm{0}}\mathrm{=}{\mathrm{c}}_{\mathrm{0}}{\mathrm{R}}_{\mathrm{0}}$$

The layer thickness modulation according to equation [4] is implemented as a multiplicative correction of the extinction: $${\mathrm{e}}_{\mathrm{c}}\mathrm{=}\mathrm{e}{\mathrm{<figure-callout\; id="1"\; label="c"\; filenames="imgf0004.png,imgf0007.png"\; state="\{\{state\}\}">c</figure-callout>}}_{\mathrm{1}}\mathrm{,}$$

The correction of the light capture is implemented in the Kubelka-Munk model as a scaling of the diffuse internal reflection coefficient R. $${\mathrm{R}}_{}$$$${\mathrm{}}_{\mathrm{2}\mathrm{c}}\mathrm{=}{\mathrm{R}}_{\mathrm{2}}{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="imgf0007.png"\; state="\{\{state\}\}">c</figure-callout>}}_{\mathrm{2}}\mathrm{.}$$

c ₀ , c ₁ and c ₂ are again process parameter-dependent correction functions.

The application of the algorithm for the correction of inline measurement errors with a color model is shown schematically in FIG. 6 shown. The illustrated sequence corresponds to the correction of a spectral measured value of the remission spectrum. The correction of the entire reflectance spectrum is achieved by performing the correction cycle for each support point of the spectrum.

As a first step of the correction cycle, the diffuse reflectance ρ _{p of} the substrate is determined from the measured absolute remission value of the substrate (paper white measurement, block 411) (block 413). The reflectance ρ _p can be calculated with the Hoffmann model (H) from equation [2] or the Kubelka-Munk-Saunderson model (KMS) from equation [6] for a color layer without absorption and without surface effect (block 412). This corresponds to the following parameter values in equations [2] and [3]: c ₀ = K = α = 0, R ₀ = 0.04.

From the measured inline reflectance spectrum in state 1 (block 421), the extinction spectrum is calculated using the inverse KMS model according to equation [7] (block 422) E (block 423) of state 1 is calculated. The fixed model parameters are R ₀ = 0.04, R ₂ = 0.60, and the diffuse reflectance of the substrate ρ _p calculated in step 1. For the correction of the surface value, the correction function c ₀ valid for the specific print job and for the concrete printing process parameters is read from the correction database 41 and applied.

At the extinction value E (block 423), the correction of the film thickness modulation according to equation [8] is carried out (block 424). The corresponding correction function c ₁ is again read from the correction database 41. The result of this operation is the extinction value according to external state 2 (block 425).

As the next step, the absorbance value according to state 2 (block 425) is transformed into the remission value of state 2 (block 427). For this the direct KMS model in equation [6] is used. During this operation (block 426) the correction of the light capture is performed. The internal reflection factor R ₂ is multiplied by the corresponding correction function c ₂ , which is also read from the correction database 41. The surface effect is set equal to zero in this transformation.

Alternatively, the correction of the inline measured values can also be carried out without a color model. In this case, the correction is advantageously carried out directly on the measured remission value R or the corresponding density value D. The density value D is calculated from the remission value R according to the known formula: $$\mathrm{D}\mathrm{=}\mathrm{-}{\log }_{\mathrm{10}}\left(\mathrm{R}\right)\mathrm{,}$$

It makes sense that in this case too, the measured value deviation is considered to be composed of the three error types surface effect, layer thickness modulation and light capture and is corrected accordingly.

The surface effect is added to equation (3) as an additive component to the reflection factor R.

The with the Hofmann model gem. Equation [2] simulated behavior of the correction of the layer thickness modulation acc. Equation [4] and the correction of the light capture acc. Equation [5] is in the FIGS. 7a and 7b shown. The diagram of Fig. 7a represents the behavior of the relative density error Dc / D as a function of the density value D for the two correction types Fig. 7b shows the behavior of the relative remission error Rc / R as a function of the remission value R for the two correction types.

wobei c₁ wieder eine prozessabhängige Korrekturfunktion ist, die aus der Korrekturdatenbank eingelesen wird.The behavior of the correction of the film thickness modulation shows a constant relative density error as a function of the density. For the direct correction method without color model, it therefore makes sense to implement the layer thickness modulation error as a multiplicative correction of the measured density value D according to equation [11]: $${\mathrm{D}}_{\mathrm{c}}\mathrm{=}\mathrm{<figure-callout\; id="1"\; label="D\; c"\; filenames="imgf0004.png,imgf0007.png"\; state="\{\{state\}\}">D\; </figure-callout>}{\mathrm{c}}_{}{\mathrm{}}_{\mathrm{1}}\mathrm{.}$$

where c _{1 is} again a process-dependent correction function that is read in from the correction database.

wobei c₂ wieder eine prozessabhängige Korrekturfunktion ist, die aus der Korrekturdatenbank eingelesen wird.Analog shows the behavior of the light-catching error in Fig. 7a and 7b in that this error type for the direct correction without color model is best implemented as a scaling factor of the remission value R: $${\mathrm{R}}_{\mathrm{c}}\mathrm{=}\mathrm{<figure-callout\; id="2"\; label="R\; c"\; filenames="imgf0007.png"\; state="\{\{state\}\}">R\; </figure-callout>}{\mathrm{c}}_{}{\mathrm{}}_{\mathrm{2}}\mathrm{.}$$

where c _{2 is} again a process-dependent correction function, which is read from the correction database.

The FIGS. 7a and 7b also show that the film thickness modulation error and the light tracing error have different signs and can compensate each other. This behavior can cause numerical instabilities in the correction. For this reason, according to a further aspect of the invention for the correction with and without a color model, a threshold value D _{s is} introduced. For high densities, mainly the film thickness modulation error is dominant. For low densities, the error caused by light trapping is dominant. The distinction between high and low densities is made by the threshold, which is preferably chosen in the range of about 1.0.

According to an advantageous and particularly expedient development of the method according to the invention, for a density value D greater than D _s, only the Layer thickness modulation error according to the corresponding equations [4], [8], or [11] for a correction with or without a color model. Conversely, for a density value D less than D _s, only the error of the light trap according to the equations [5], [9] or [12] is carried out for a correction with or without a color model.

The correction of the drying effect enables the transformation of the measured values of state 2 (externally wet) into measured values of state 3 (externally dry).

It is known that on coated paper the drying process mainly corresponds to a change in the microscopic surface structure. Using polarizing filters for colorimetry eliminates this effect. On coated papers, therefore, no correction of the drying effect is needed with polarizing filters. Without a polarization filter, the surface effect according to equation [3] must be considered as an additive remission component.

On uncoated papers, part of the ink layer penetrates into the substrate. This behavior requires additional correction parameters. The application of a color model for this case is possible in principle. It requires an approach that can simulate two color layers overlying the paper. A layer corresponds to the color fraction that has penetrated the paper. The upper layer corresponds to the remaining amount of ink remaining on the paper. One possibility for the implementation is the application of the multilayer Kubelka-Munk model from the already mentioned dissertation by P. Emmel. However, the correction with a multi-layer color model and the determination of the model parameters becomes complex. Therefore, according to a further aspect of the invention, a direct correction of the measured values, as described above in connection with equations [11] and [12], is undertaken.

The drying behavior on coated and uncoated papers is also characterized according to the invention with the three error types surface effect, layer thickness modulation and light capture and corrected accordingly. The required correction function c ₀ , c ₁ , c ₂ are (after their determination) also stored in the correction database 41 and correspond to a second data set in addition to the correction functions for the correction of the inline error.

In the following, the method according to the invention is again clearly summarized by means of a preferred exemplary embodiment.

The correction computer 40 is connected to the measuring arrangement 20 and receives therefrom for each scanned measuring field the data of the acquired spectra. In addition, the control computer 50 transmits to the correction computer 40 the environmental parameters suitable for each measured field, ie machine, process and measuring field parameters. These parameters are in detail: printing speed, number of the printing unit in which the measuring arrangement 20 is located, paper class (eg glossy paper, matte paper, natural paper), color type class (eg scale color cyan), measuring field type (eg solid tone, grid 70%, gray) and the Number of the printing unit in which the measuring field was printed. The correction is case specific, with a single case defining a particular combination of environment parameters. For example, the combination of "glossy paper", "scale color magenta", "solid field" and "printed on the last printing unit" is a case. Corresponding correction parameters which define the already mentioned sets of (parameterized) correction functions c ₀ , c ₁ and c ₂ are assigned to each correction database 40 present in the correction computer 40 for each case occurring in practice.

The correction database 41 is realized as a table in which a correction case is treated in each line. A single line comprises a set of conditional parameters (corresponding to the environmental parameters) and a set of correction parameters. The correction computer 40 compares the relevant environmental parameters with the condition parameters in the correction database 41 for each measurement. For this purpose, the table is processed line by line until a first match is found. In this way, the appropriate case and thus the appropriate correction parameters are found. The table is traversed from the top (beginning of the table) downwards (end of the table). The cases are ordered in the table according to the degree of specificity, the table starting with very specific cases and ending with very generic cases. So it always tries first to perform a specific correction. If no cases are defined for this purpose, the correction gradually becomes more generic.

In correction computer 40 it is decided for each individual value of the uncorrected reflectance spectrum, with each measurement, whether this is in the absorption, transmission or transitional range of the color. These are the remission values of the individual wavelengths (spectral values) are compared with defined threshold values D _s (see above). Spectral values in the transmission range (D <D _s ) are multiplied by the correction function c ₂ (see equation [12]), which is defined by the correction parameters in the respective table line. Spectral values in the transition range (D ~ D _s ) are not corrected. Spectral values in the absorption range (D> D _s ) are logarithmized, multiplied by a density-dependent correction function c ₁ (see equation [11]) and then delogarithmated again, the correction function c ₁ typically being a second-degree polynomial of density and its coefficients also being part the correction parameters are. As measured with polarizing filters, there is no surface effect and thus c ₀ can be set equal to zero. The corrected spectrum is then forwarded to the control computer (50).

It is clear that first of all the correction database 41 has to be created before the actual inline correction. In order to determine the individual correction parameters, prints with defined fields are made for all cases of interest (see definition above) and measured both with the inline measuring arrangement 20 and with an external measuring device. Since the correction parameters depend strongly on the layer thickness, prints for each case of interest are made and measured at at least 3 different layer thicknesses. From the totality of these measurement data, a set of correction parameters is then calculated for each individual case, whereby, of course, this preferably takes place computer-aided.

To determine the correction parameters for a case, the spectra of the inline measurements and the externally acquired measurements are offset against each other. In a first step, it is determined for each part of the spectrum, based on a defined threshold value, whether this is in the absorption, transmission or transition range of the color. In a second step, the correction parameters required for these areas are determined, which define the correction functions c ₁ and c ₂ (c ₀ is not required in the measurement with polarization filters). The correction function c ₂ is obtained by dividing the spectral values of the transmission ranges of the measurements recorded inline and externally by one another and then averaging them. In order to obtain the density-dependent correction function c ₁ for the absorption range selected as polynomial of degree 2, the density values of the measurements recorded in-line and externally are divided by each other. With the thus obtained density - dependent quotients are calculated by the method of least squares the coefficients of the correction polynomial and thus the correction function c ₁ determined. The correction functions c ₁ and c ₂ or their parameters are then stored in the correction database 41 according to cases structured.

The method according to the invention also allows the corrected values to be provided only after a mean value formation or another method for compensating fluctuations of the measured values. This fluctuation may be due to metrological reasons, but in particular comes from the printing process itself. Especially in offset printing, it has been known for a long time (for example, "Offsetdrucktechnik", Helmut Teschner), that the printing process is subject to both systematic and random fluctuations, these fluctuations also being of very short-term nature, i. especially from bow to bow, can be. Conventional procedure is taken to measure a single sheet after printing the printing press and measured. The measured values obtained from this are then used, for example, for process control or displayed. It would be conceivable to measure several consecutive sheets here as well and to calculate the measured values with each other; for reasons of time, this is not the case in practice. The consequence of this is that with conventional procedure, the measured values also reflect the short-term fluctuations of the printing process. It is now an advantage of the method according to the invention that a billing of the measured values of a plurality of measuring times, in particular the measured values of a plurality of successive paper sheets measured in the machine, is possible without great expenditure of time and thus the measured values subject to short-term fluctuations can be adjusted and consequently process parameters can be better estimated. In particular, the process control can thus work more accurately.

It is also within the meaning of the inventive method that the corrected measured values are not only provided as described above directly after a correction of the inline error, but can also be subjected to further computational processing steps. One such processing step is, for example, the conversion between different measurement conditions. A particularly relevant case in practice is the conversion of measurements with different filters. If, for example, the corrected measured values initially exist as values measured with polarization filters, then it may be necessary to compare these values for tuning with specifications from the preliminary stage with values measured without polarization filters. A computational component for the conversion of polarization filters measured values in values measured without polarization filter then fulfills this task.

Claims (31)

1. Method of determining colour and/or density values for the monitoring and/or closed-loop control of the printing process in a printing apparatus, in particular in a sheet-fed offset printing press, wherein measuring areas of a printed sheet are measured photoelectrically directly in or on the running printing apparatus and the obtained measured values are used to form the colour and/or density values for the relevant measuring areas, characterized by the fact that measured value deviations that are caused by the fact that the measurement is taken directly in the printing process in comparison to a measurement that is taken on printed sheets outside the printing process are corrected computationally.
2. Method according to Claim 1, characterized by the fact that the measured value deviations are partly corrected by means of a suitable measurement technique.
3. Method according to Claim 2, characterized by the fact that effects of the ink splitting phenomenon at the printing nip and of the surface changes caused thereby are at least partly eliminated due to the use of polarization filters (28, 29) in the measurement.
4. Method according to Claim 2 or 3, characterized by the fact that to improve reproducibility of the measured values UV elimination filters are used in the measurement.
5. Method according to Claim 2, characterized by the fact that effects of the ink splitting phenomenon at the printing nip and of the surface changes caused thereby are at least partially eliminated due to the use of a measurement geometry that has an angle separation between a directed reflection of the illumination and a receiver of greater than 45°.
6. Method according to any one of the preceding claims, characterized by the fact that the computational correction of the measured values is carried out in such a way that the measured values of a first state corresponding to the printed sheet directly in the printing process are converted into measured values of a second state corresponding to a printed sheet that is still wet but outside the printing apparatus, and the measured values of the second state are converted into measured values of a third state corresponding to a dry printed sheet outside the printing machine.
7. Method according to Claim 6, characterized by the fact that the computational correction of the measured values is carried out in such a way that the measured values of the first, second, and third states can be converted mutually into one another.
8. Method according to Claim 6 or 7, characterized by the fact that the computational correction of the measured values of the measuring areas is carried out in dependence on the environmental parameters relevant to each measuring area with the aid of correction parameters, using appropriate correction parameters for each set of environmental parameters in question.
9. Method according to Claim 8, characterized by the fact that the correction parameters are stored in a database together with the environmental parameters and can be retrieved selectively from the database by using the environmental parameters.
10. Method according to any one of the preceding claims, characterized by the fact that the correction of the measured values is carried out by using three error types, which represent contributions to a measured value deviation from a surface effect, layer thickness modulation, and light capture.
11. Method according to Claim 10, characterized by the fact that the contributions to the measured value deviation of the surface effect, the layer thickness modulation, and the light capture are calculated with the aid of a respective corrective function, the corrective functions being defined by the correction parameters.
12. Method according to Claim 11, characterized by the fact that correction of the measured values is carried out on the basis of a ink model.
13. Method according to Claim 12, characterized by the fact that the contribution to the measured value deviation from the layer thickness modulation is calculated as a multiplicative factor of a product of layer thickness and absorption coefficient or extinction.
14. Method according to Claim 12, characterized by the fact that the contribution to the measured value deviation from the light capture is calculated by means of a modification of a refractive index of an ink layer.
15. Method according to Claim 12, characterized by the fact that the contribution to the measured value deviation from the light capture is calculated by means of a multiplicative change to the internal integral refraction coefficient of an interface between an ink layer and air.
16. Method according to any one of Claims 12-15, characterized by the fact that the correction of the measured value is carried out in a sequential correction cycle, wherein firstly the level of diffuse reflection of the sheet is calculated using a paper white measurement, then the surface effect is corrected, then the extinction is calculated using an ink model that is inverse of the selected ink model, then the contribution to the measured value error from the layer thickness modulation is corrected based on the extinction, and then the contribution to the measured value error from the light capture is corrected using the selected ink model, and finally a corrected reflectance value is calculated.
17. Method according to any one of the preceding claims, characterized by the fact that the correction of the measured value is carried out directly on the measured values, the contribution to a measured value error from the layer thickness modulation being applied as a scaling error of the measured density value, and the contribution to the measured value error from the light capture being applied as a scaling error of the reflection factor.
18. Method according to any one of the preceding claims, characterized by the fact that the correction of the contribution to the measured value error from the layer thickness modulation and the correction of the contribution to the measured value error from the light capture are applied separately to different regions of a measured reflectance value, correcting only the contribution to the measured value error from the layer thickness modulation for reflectance values whose calculated density values are above a density threshold and correcting only the contribution to the measured value error from the light capture for all other reflectance values.
19. Method according to any one of the preceding claims, characterized by the fact that the measured value correction from the second state to the third state is likewise carried out on the basis of three measured value error contributions from the surface effect, the layer thickness modulation, and the light capture, a second set of correction parameters analogous to those for the correction from the first to the second state being used, and the second set of correction parameters likewise being provided in the correction database.
20. Method according to any one of the preceding claims, characterized by the fact that generic correction parameters that are configured for typical paper grades and standard process inks are stored in the correction database.
21. Method according to Claim 20, characterized by the fact that in addition, specific correction parameters that are configured for specific cases in which the generic correction parameters are inapplicable or inaccurate are stored in the correction database.
22. Method according to any one of the preceding claims, characterized by the fact that the correction parameters are calculated from measured values from prints produced with systematically varied environmental parameters in the first state and from reference measured values from the prints in the second and/or third state.
23. Method according to Claim 22, characterized by the fact that the reference values are measured using an external measuring device that is equipped with the same measurement filters as the internal measuring configuration within the printing apparatus.
24. Method according to Claim 23, characterized by the fact that the differences in the spectral resolution between the external measuring device and the internal measuring configuration are eliminated by using a numerical band pass correction.
25. Method according to Claim 23, characterized by the fact that an external measuring apparatus having multiple changeable measuring filters is used to measure the reference values and that reference measurements are carried out in different measuring modes of the external measuring device, the measuring data being interchangeable between the internal measuring configuration and other measuring systems having other measuring filters.
26. Method according to Claim 23, characterized by the fact that, in a case in which the measured density on the reference sheet does not correspond to a required desired density, the measured values transformed for the required measuring filter are adapted by using a correction step.
27. Printing apparatus, in particular sheet-fed offset printing press, characterized by an in-line measuring configuration (20) for the photoelectrical measuring of measuring areas of a printed sheet directly during a printing process, and means (40) for forming the colour and/or density values for the relevant measuring areas from the measured values obtained by the measurement, and a correction computer (40) for computationally correcting measured value deviations caused by the fact that the measurement was taken directly during the printing process in comparison to a measurement on printed sheets outside the printing process.
28. Printing apparatus according to Claim 27, characterized by the fact that the measuring configuration (20) is designed at least partly to suppress that proportion of the measured value deviation that is caused by the surface effect.
29. Printing apparatus according to Claim 28, characterized by the fact that the measuring configuration (20) includes polarization filters (28, 29) and preferably a UV elimination filter (30).
30. Printing apparatus according to any one of Claims 27-29, characterized by the fact that the measuring configuration (20) has a measuring geometry that deviates from the standardized measuring geometry 0/45°, the measuring angle of illumination and receiver being selected such that they are located on the same side of the normal to the measuring plane and the corresponding path length of the main beams of receiver and illumination in the ink layer being identical to the standardized measuring geometry.
31. Printing apparatus according to any one of Claims 27-30, characterized by the fact that the correction computer (40) is designed to implement the method according to any one of Claims 1-20.

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