EP0659559B1 - Method of controlling the ink supply in a printing machine - Google Patents

Abstract

A method for controlling the ink supply in a printing machine, in particular a sheet-fed offset printing machine, is described. Ink density spectrums are recorded at several points on the image of the master and of the printed copy by means of a spectral photometer and the differential ink density spectrums are determined therefrom. These differential ink density spectrums are then depicted as a linear combination of the individual ink density spectrums of the colours involved in the combined printing. Setting commands for the ink supply members of the printing machine are then derived from the proportionality factors of this linear combination.

Description

The invention relates to a method for controlling the color guidance in a printing press according to the preamble of claim 1.

The visual color impression of offset printing products is created in known way through an interaction of subtractive and additive color mixing. The individual grid points of the different Printing inks come in different sizes both side by side as well as overlapping one another more or less printed. The printing inks used are glazed, i.e. the effect corresponds to that on the white substrate lying filter. The color direction of the overprint of the Halftone dots are determined both by the layer thickness of the applied Printing ink as well as by the size of the halftone dots (geometric area coverage) determined. By adjusting the Color guide elements in the individual printing units can thus Color location of a print image point can be changed. Usually will for color printing the three chromatic colors cyan, magenta, yellow and a fourth ink black (contrast enhancement) printed.

It has long been known to apply ink to a printed product to detect photoelectrically on extra printed measuring elements and derive a measure for the amount of paint applied. Most of time this is done using densitometers, because between color density value and layer thickness of the color and thus also the position the color guide members, for example, designed as a color slide there is a relatively simple connection. A densitometric Capturing the color guide leaves no numerical statements regarding visual color perception to. The measuring elements which are also printed in addition have the disadvantage that that only the target color application of these measuring fields is controlled becomes. The color impression in the printed image itself remains completely disregard and is accordingly only changed indirectly.

EP 0 143 744 A1 describes a method for assessing the Known print quality and to regulate the color guide. On Image elements in the subject are made with one or more measuring heads Remissions measured in four spectral ranges. For the three The spectral ranges are selected in such a way that chromatic colors the usual color density values result. For the printing ink The spectral reflectance in the infrared range is determined in black. Using the Neugebauer equations, these become Remissions or color density values the corresponding area coverings determined (unmasked). This will be in the same picture areas copies printed on the machine as well as on one Debit template performed. From a target-actual comparison of the area coverings are then commands for the color control of the Printing machine derived.

DE 4 311 132 A1 describes a method for color regulation / control known in a printing press, which is characterized by that the density spectra of test prints of the individual at the combined printing involved printing inks with specified area coverage as well of paper white are recorded and stored that the density spectra a measuring point of the template and a measuring point of the Printed copies are recorded that these measured density spectra the template and the printed copy each as a linear combination the density spectra of the individual printing inks weighted by factors as well as the paper white, with the aim of Density spectra of the compression of the measuring point from the template and The best possible print copy through this linear combination to represent. The color supply depends on the difference of the individual interpreted as area coverage Factors of the respective linear combination (template / printed copy) changed.

A disadvantage of this known method is that the Factors in the approach of the linear combination to represent the entire density spectrum (compression) from the density spectra of the Individual colors can be applied as area coverage. Such one The approach with regard to the factors is taken at high areas of coverage a color in the image area problematic and completely fails when printing one or more colors in the Full tone. The reason is, in an easily recognizable way, that if, for example, a color of the template as a solid area the corresponding color in the printed copy is also printed only be printed as a full tone and not beyond that can. A regulation of colors in the area of full tone is therefore not possible.

The object of the present invention is therefore a method to expand in accordance with the preamble of claim 1 so that a control of the color guide with extended application possibilities is possible with great accuracy.

This task is solved by the characteristic features of Claim 1. Further developments of the invention result from the Subclaims.

According to the invention it is provided that at corresponding image locations of template and printed copy on a variety of spectral Areas determined the remission values and these in known Can be converted into color density values by logarithmization. So it will be at the image points of the template and printed copy Color density spectra determined. The color density spectrum of an image spot the template provides the so-called target color density spectrum represents and the spectrum of an image spot in the print copy the actual color density spectrum.

By forming the difference between the target and the actual color density spectrum is now the difference color density spectrum for each image point certainly. The difference color density spectrum determined in this way each image point is then in the form of a linear combination by those color density spectra of the printing inks, which on Compression of the image point involved are shown. The Proportionality factors of this linear combination, i.e. the Factors with which the corresponding color density spectra of the Individual colors to be multiplied then result in a direct and reliable measure of how much the color of each Printing ink is to be changed.

The color density spectra of the individual colors on the printed copy will be thereby also printed on the printed copy or based on previously produced test prints of existing measuring fields determined. In the first-mentioned procedure, this is known per se Way printed a print control strip next to the subject, which in a measuring field of each color metering zone for each printed color Contains full tones. The print control strip extends especially parallel to the sheet edge of the start of printing.

According to the invention it is provided that in the representation of the Difference color density spectrum of an image spot those colors to be disregarded, which is due to the compression Image passage are not involved. The associated proportionality factors are therefore set to zero. Can advantageously be provided here that colors their share of the printing area is below a certain percentage (e.g. <10%) the linear combination representation of the difference color density spectrum an image point are also disregarded. This advance information the proportion of printing area of an ink certain image location can either be directly from the prepress (Film or electronic prepress) or from the offset printing plate be determined. It is also possible in principle, the share printing area of a certain printing ink by a colorimetric Analysis of the corresponding image point to determine. It can then be exploited that with a certain set of inks and a certain type of substrate certain color locations Exclude involvement of one or more inks. Likewise it is possible for certain areas in the colorimetric color space the proportion of the printing area of one or more inks below a certain percentage of printing area. Basically, a visual (measuring magnifier) or video technology Analysis of the intended image points possible.

With the original, with respect to which the image locations of the printed copies with regard to their color or spectral appearance can be adjusted, it can be an OK sheet, a proof or a proof. In the method according to the invention it does not matter that the template with the same Printing inks and the same type of substrate was produced, which is particularly advantageous. The reason is here in that ultimately the spectral differences, that is Difference color density spectrum, between original and printed copy be minimized and this is done by having the appropriate Proportionality factors in the representation as a linear combination are to be determined in such a way that when printing the printed copies used printing inks or their solid colors Color density spectra are amplified or weakened in such a way that the difference in the color density spectrum becomes minimal.

A preferred development of the method according to the invention provides before that a weighting of the color density spectra of the combined printing printing inks involved in an image position according to their proportion Area is made. For example, it can be provided be that, for example, if cyan only with 5 to 10% area coverage is represented in the linear combination representation of the Difference color density spectrum the color density spectrum of the color cyan (as it is used when printing the printed copies) perfectly remains unconsidered. Within such a small percentage printing area is therefore zero-weighted for the corresponding area Color density spectrum. The degree of area coverage can then be adjusted accordingly those involved in the printing together of an image section of the original Inks in the lower, middle and upper tonal range certain weighting factors can be assigned. The corresponding The assignment of the weighting factors is empirically based on pressure tests certainly. To determine the area coverage can also here again the electronic prepress, the film, the printing plate or a visual or video analysis of the corresponding Serve images.

To get information about the area coverage of an ink, which is involved in the printing together of an image passage, can either be the image location of the template or the image location of the printed copy itself.

Black is a particularly advantageous and simple way to determine the area coverage possible. For this purpose, the infrared color density is in an area of the infrared certainly. In a known manner, an empirical context can be used then from the measured infrared color density to the area coverage the printing ink black. In particular is thereby it can be determined whether this image passage is printed together the printing ink black is involved.

The method according to the invention is particularly advantageous then, especially if the image locations of the originals are not only in the spectral range of visible light but beyond can still be measured in a range of infrared and from which Measured value in infrared the infrared color density is determined. How was explained above, is then at each image point determines the difference color density spectrum. By knowledge the infrared color density can now this difference color density spectrum according to an empirically determinable relationship to influence the printing ink black can be reduced. You get one Difference color density spectrum of the image area, which by the Only the colored colors were created together. The representation of the reduced difference color density spectrum (without black component) is then also in the form of a linear combination through the color density spectra of the manufacturing of the printing inks involved.

A brief explanation of an exemplary embodiment is also given of the invention in connection with the corresponding mathematical Approaches. It is believed that on a sheetfed offset press Print copies are to be made in the form of sheets and on these sheets a print control strip with individual Measuring fields of the colors involved in the printing is also printed. This Print control strips run parallel to the edge of the start of printing. A template on the same machine serves as a template manufactured bow, with regard to its color appearance was judged to be optimal (OK sheet).

In the subject of the printed sheet, a certain number of image areas, for example, one or more in each color metering zone. The image points are particularly special important parts of the subject. The selection of those to be considered Images are advantageously made on the printed sheet the template.

To further explain the exemplary embodiment of the invention noted that the determination and measurement of the Images on both the original and the printed copies a so-called X-Y coordinate measuring table, which is a in Spectrophotometer movable over the entire level of the printing sheet having. Such a spectrophotometer can then automatically via appropriate positioning drives at the intended positions of the selected image points. The positions of the selected image areas can be saved and then automatically approachable through program sequence. After each start one The spectral reflectance is then determined in the image area and the conversion into the color density spectrum in a downstream computer. Saving the position of the selected one Images are made in a one-time run Spin.

The intended image areas were used for the arch as a template selected and their positions with the above described measuring device stored. In the further explanation of the exemplary embodiment is only the process route a single place in the picture as well as in the sheet of the Print copy described. Using the spectrophotometer at an image point of the template at a total of 35 support points of the visible spectrum determines the reflectance values R-Soll (i; Bunt), with i = 1, ...., 35. So there are a total of 35 remission values R-target determines which, for example, spectrally around each 10 nm apart should be selected (350 - 700 nm).

At the same position in the image of a first printed sheet (printed copy) are in the same wavelength range using the Spectrophotometer determines the reflectance values R-Ist (i; Bunt) with i = 1, ...., 35.

In a known manner, logarithmization of the individual The respective reflectance values R-Soll (i; Bunt) and R-Ist (i; Bunt) Target or actual color density spectrum of the image areas of the original or Print copy determined. So there are the target and actual color density spectra generated at 35 points of the visible spectrum. Man receives the values as the target color density spectrum in the template D-Soll (i; Bunt) and as the actual color density spectrum in the printed copy the values D-actual (i; colored), each with i = 1, ...., 35.

Then the difference for each spectral range (i = 1, ..., 35) formed so that the difference color density spectrum Delta-D (i; Bunt) is obtained.

This differential color density spectrum Delta-D (i; Bunt) described above is now shown as follows in the form of a linear combination: Delta-D (i; colored) = α · a · D (i; C) + β · b · D (i; M) + ▿ · c · D (i; Y) + Δ · d · D (i; K).

Since i assumes the values 1, 2, ..., 35 there are a total of 35 equations in front. In the linear approach mentioned above, D (i; C) mean D (i; M), D (i; Y) and D (i; K) indicate the respective color density spectra Full-tone measuring fields of the sheet of the printed copy. The index C stands for the printing ink cyan, the index M for the magenta ink, the Y index for the yellow ink and the index K for the printing ink black. Of course you can other colors (house or special colors) are also taken into account.

In the exemplary embodiment, this is done in exactly that color metering zone, in which the measured image point lies, the corresponding ones Measuring fields of the full tones also spectrally measured and these remissions obtained in this way in the corresponding color density spectra the individual colors converted (D (i; C), D (i; M), D (i; Y), D (i; K).

Those in the linear approach to represent the difference color density spectrum Delta-D (i; Bunt) used weighting factors α, β, ▿, Δ are chosen as weighting factors such that the corresponding values lie between 0 and 1 and furthermore one all the more have larger amount, the higher the area coverage of the The printing ink involved is printed together.

The system of equations above, which consists of a total of 35 individual Equations now exist according to the well-known linear method Regression (minimizing the sum of squares) according to the proportionality factors a, b, c, d resolved. To be determined Factors a, b, c, d thus indicate how large the proportion of the printing ink is Cyan, magenta, yellow and Printing ink black in the generation of the difference color density spectrum Delta-D (i; colored) with regard to the respective spectral Share of the inks mentioned. From those so determined Factors a, b, c, d are then the necessary changes regarding the color guide (in the considered ink metering zone) can be derived. Vividly means, for example, a positive and amount-based great value of a that the ink supply for the printing ink cyan must be increased significantly in the corresponding printing unit. Corresponding clearly means a small and negative amount Value for a that the color metering in the corresponding color metering zone for the cyan ink must be reduced somewhat. Regarding the interpretation when deriving control commands for the color dosage applies to the magenta, yellow and black inks the same for the factors b, c, d.

Are not all of the images in the picture viewed together? Printing inks cyan, magenta, yellow, black but involved for example, only the printing inks cyan and magenta, see above are presented according to the linear approach given above of the difference color density spectrum Delta-D (i; Bunt) die Weighting factors α, β, ▿ and Δ are each set to zero. This can also be provided if it turns out that these colors mentioned here as examples only with one very small proportion of printing area are represented.

Above was the linear approach to represent the difference color density spectrum Delta-D (i; colored) represented such that the Black printing ink with its color density spectrum in the visible Area D (i; K) was taken into account. According to the invention, however, it can also be advantageous for the printing ink black in the range of the infrared, the infrared color density D (IR) is determined and via a relationship empirically determined in pressure tests from the Difference color density spectrum Delta-D (i; colorful) to the influence of Printing ink black reduced difference color density spectrum Delta-D (i; colored black) is calculated. In a simple and the Illustrative example, this can mean that the difference color density spectrum Delta-D (i; colorful) in all spectral ranges i = 1, ..., 35 by a certain amount of color density units is reduced. It takes advantage of that in the first approximation the printing ink black is almost constant in the visible range Has history.

In the above approach, taking into account the infrared color density D (IR) for the printing ink black, the reduced difference color density spectrum is then linearly displayed Delta-D (i; colored black) = α · a · D (i; C) + β · b · D (i; M) + ▿ · c · D (i; Y).

This difference in color density spectrum reduced by the influence of the printing ink black is therefore only from the color density spectra of the chromatic colors involved in the overprint. The determination is then carried out in accordance with this system of equations of the proportionality factors a, b, c in a known manner linear least squares regression. The factors a, b, c in turn represent the measure to how much the respective color guide for the printing ink cyan, magenta, Yellow to achieve a predetermined spectral course in the Image location must be changed.

Claims (5)

1. Process for controlling the ink feed in a printing press, particularly an offset printing press, in which the ink density spectra of at least one image position in master and printed sample are sensed and a determination of control commands for the ink feed elements of the printing press taking into account the ink density spectra of the inks taking part in the composite printing of the image position takes place starting from the fact that the greatest possible agreement of the ink density spectra of the image positions of master and printed sample is achieved, characterised in that from the ink density spectra of the image position of master and printed sample, the difference ink density spectrum is determined, that the difference ink density spectrum is represented as a linear combination of the ink density spectra of the individual printing inks taking part in the composite printing, and that the control commands for the ink feeding elements of the printing press are determined from the proportionality factors of this linear combination.
2. Process according to Claim 1, characterised in that a weighting of the ink density spectra of the individual printing inks taking part in the composition printing of the image positions takes place corresponding to their proportion of surface to be printed.
3. Process according to Claim 1 or 2, characterised in that the determination of the proportion of surface to be printed for the printing ink black takes place via a determination of the infrared ink density D(IR).
4. Process according to one of Claims 1 to 3, characterised in that printing inks with a small proportion of surface to be printed remain ignored in the representation of the difference ink density spectrum.
5. Process according to one of Claims 1 to 4, characterised in that the sensing of the ink density spectra of the individual printing inks taking part in the composite printing of the image position takes place on full tone measuring fields co-printed on the printed sample.