EP0659559A2 - 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 a known manner through the interplay of subtractive and additive color mixing. The individual halftone dots of the different printing inks are printed in different sizes both side by side and overlapping one another to a greater or lesser extent. The printing inks used are glazed, i.e. the effect corresponds to a filter lying on the white substrate. The color direction of the printing of the halftone dots is determined both by the layer thickness of the applied printing ink and by the size of the halftone dots (geometric area coverage). By adjusting the color guide elements in the individual printing units, the color location of a print image point can thus be changed. As a rule, the three chromatic colors cyan, magenta, yellow and a fourth printing color black (increase in contrast) are printed for color printing.

It has been known for a long time to photoelectrically detect the ink application on a printed product on measuring elements which have also been printed and to derive a measure for the amount of ink applied. This is usually carried out by means of densitometers, since there is a relatively simple relationship between the color density value and the layer thickness of the color and thus also the position of the color guide elements, for example in the form of a color slide. A densitometric detection of the color guide does not allow numerical statements regarding visual color perception. The measuring elements which are also printed in addition have the disadvantage that only the target color application of these measuring fields is controlled. The color impression in the printed image itself is completely disregarded and is accordingly only changed indirectly.

EP 0 143 744 A1 discloses a method for assessing the print quality and for regulating the color guidance. The reflectances in four spectral ranges are measured on image elements in the subject using one or more measuring heads. The spectral ranges are selected for the three chromatic colors in such a way that the usual color density values result. The spectral reflectance in the infrared range is determined for the printing ink black. Using the Neugebauer equations, the corresponding area coverings are determined (unmasked) from these remissions or color density values. This is done on the same image points on the machine printed copies as well as on a target template. From a target-actual comparison of the surface coverings, setting commands for the ink guide of the printing press are then derived.

From DE 4 311 132 A1 a method for color regulation / control in a printing press is known, which is characterized in that the density spectra of sample prints of the individual printing inks involved in the overprinting with predetermined area coverage and of the paper white are recorded and stored in such a way that the density spectra a measuring point of the original and a measuring point of the printed copy are recorded, that these measured density spectra of the original and the printed copy are each represented as a linear combination of the factor-weighted density spectra of the individual printing inks and the paper white, with the aim of determining the density spectra of the printing of the measuring point from the original and to represent the printed copy in the best possible way using this linear combination. The ink supply is interpreted depending on the difference between the individual areas of coverage Factors of the respective linear combination (template / printed copy) changed.

A disadvantage of this previously known method is that the factors in the approach of the linear combination for representing the entire density spectrum (compression) from the density spectra of the individual colors are used as degrees of area coverage. Such an approach to the factors becomes problematic at high areas of coverage of a color in the image area and fails completely when printing one or more colors in full tone. The reason for this is, in an easily recognizable manner, that if, for example, a color of the template is printed as a solid area, the corresponding color in the printed copy can also only be printed as a full tone and not beyond that. It is therefore not possible to regulate colors in the full tone range.

It is therefore the object of the present invention to extend a method according to the preamble of claim 1 in such a way that control of the color guidance is possible with great accuracy with extended use.

This object is achieved by the characterizing features of claim 1. Further developments of the invention result from the subclaims.

According to the invention it is provided that the reflectance values are determined in a plurality of spectral regions at corresponding image locations of the original and printed copy and these are converted in a known manner by logarithming into color density values. The color density spectra are therefore determined at the image locations of the original and the printed copy. The color density spectrum of an image area of the original represents the so-called target color density spectrum and the spectrum of an image area in the printed copy represents the actual color density spectrum.

By forming the difference between the target and the actual color density spectrum, the difference color density spectrum is now determined for each image point. The difference in color density spectrum of each image point determined in this way is then represented in the form of a linear combination by those color density spectra of the printing inks which are involved in the printing together of the image point. The proportionality factors of this linear combination, i.e. the factors by which the corresponding color density spectra of the individual colors are to be multiplied, then give a direct and reliable measure of how much the color guide of the respective printing ink is to be changed.

The color density spectra of the individual colors on the print copy are determined on the basis of measurement fields which are also printed on the print copy or on the basis of previously produced test prints. In the first-mentioned procedure, a print control strip is also printed in a manner known per se, which contains a measuring field of the full tone for each printed color in each color metering zone. The print control strip extends in particular parallel to the sheet edge of the start of printing.

According to the invention it is provided that those colors which are not involved in the overprinting of this particular image location are disregarded in the representation of the difference color density spectrum of an image location. The associated proportionality factors are therefore set to zero. Advantageously, it can be provided here that colors whose portion of the printing area is below a certain percentage (eg <10%) are also not taken into account in the linear combination representation of the difference color density spectrum of an image point. This preliminary information on the proportion of printing area of a printing ink of a specific image location can be determined either directly from the preliminary stage (film or electronic preliminary stage) or from the offset printing plate. It is also possible in principle to determine the proportion of the printing area of a specific printing ink by a colorimetric Analysis of the corresponding image point to determine. It can then be exploited that certain color locations exclude the participation of one or more printing inks in a certain set of printing inks and a certain type of printing material. It is also possible to assume the portion of the printing area of one or more printing inks below a certain percentage of the printing area in certain areas in the colorimetric color space. In principle, a visual (measuring magnifier) or video-technical analysis of the intended image points is also possible.

The template for which the image areas of the printed copies are to be reproduced with regard to their color or spectral appearance can be an OK sheet, a proof or a proof. In the method according to the invention, it is not important that the original was produced with the same printing inks and the same type of printing material, which is to be regarded as particularly advantageous. The reason for this is that ultimately the spectral differences, i.e. the difference color density spectrum, between the original and the printed copy are minimized, and this is done by determining the corresponding proportionality factors in the representation as a linear combination in such a way that those when the printed copies are printed used printing inks or their color density spectra obtained from full tones are amplified or weakened in such a way that the difference in the color density spectrum is minimal.

A preferred development of the method according to the invention provides that the color density spectra of the printing inks involved in the printing together of an image area are weighted according to their proportion of the printing area. For example, it can be provided that, for example, if cyan is only represented with 5 to 10% area coverage, the color density spectrum of the color cyan (as it is used when printing the printed copies) is completely disregarded in the linear combination representation of the difference color density spectrum. Within such a small percentage printing area is therefore zero weighted for the corresponding color density spectrum. Correspondingly, certain weighting factors can then be assigned to the degrees of area coverage of the printing inks involved in the printing together of an image point of the original in the lower, middle and upper tonal range. The corresponding assignment of the weighting factors is determined empirically using pressure tests. To determine the area coverage, the electronic pre-press, the film, the printing plate or a visual or video-technical analysis of the corresponding image areas can also be used here.

In order to obtain information about the area coverage of a printing ink which is involved in the printing together of an image area, either the image area of the original or also the image area of the printed copy itself can be used.

A particularly advantageous and simple way of determining the area coverage is possible with the printing ink black. For this purpose, the infrared color density is determined in a region of the infrared. In a known manner, the area coverage of the printing ink black can then be deduced from the measured infrared color density via an empirical relationship. In particular, this makes it possible to determine whether the printing ink black is involved in the printing together of this image point.

The method according to the invention is particularly advantageous when, in particular, the image locations of the originals are measured not only in the spectral range of visible light but also in a range of infrared and the infrared color density is determined from the measured value in infrared. Then, as explained above, the difference color density spectrum is determined at each image point. By knowing the infrared color density, this difference color density spectrum can now be reduced according to an empirically determinable relationship by the influence of the printing ink black. You get one Difference color density spectrum of the image area, which is the result of printing together only the chromatic colors. The representation of the reduced color density spectrum (without black component) reduced in this way is then likewise carried out in the form of a linear combination by the color density spectra of the printing inks involved in the production of the printed copies.

Furthermore, a brief explanation of an embodiment of the invention is given in connection with the corresponding mathematical approaches. It is assumed that printed copies in the form of sheets are to be produced on a sheetfed offset printing press and that a print control strip with individual measuring fields of the colors involved in the printing is also printed on these sheets. This print control strip runs parallel to the edge of the start of printing. A sheet produced on the same machine serves as a template and was judged to be optimal in terms of its color appearance (OK sheet).

In the subject of the printed sheet, a certain number of image locations, for example one or more in each ink metering zone, is defined. The image areas are particularly important parts of the subject. The selection of the image points to be taken into account advantageously takes place on the printed sheet of the template.

In order to explain the exemplary embodiment of the invention, it should also be pointed out that the determination and measurement of the image locations both on the original and on the printed copies takes place on a so-called XY coordinate measuring table, which has a spectrophotometer which can be moved in the entire plane of the printed sheet. Such a spectrophotometer can then be automatically moved to the intended positions of the selected image locations via corresponding positioning drives. The positions of the selected image points can be saved and then automatically approached by the program sequence. After each start one The image point is then determined and the spectral reflectance is converted in a downstream computer into the color density spectrum. The position of the selected image locations is stored in a single measurement run.

In the case of the sheet serving as a template, the intended image locations were selected and their positions saved with the measuring device described above. In the further explanation of the exemplary embodiment, only the process path at a single image location is described both in the template and in the sheet of the printed copy. Using the spectrophotometer, the reflectance values R-Soll (i; Bunt) are determined at an image point of the original at a total of 35 support points of the visible spectrum, with i = 1, ..., 35. A total of 35 remission values R-Soll are thus determined, which are selected, for example, spectrally spaced 10 nm apart (350-700 nm).

At the same image point of a first printed sheet (printed copy), the reflectance values R-actual (i; color) are determined in the same wavelength range by means of the spectrophotometer, with i = 1, ..., 35.

In a known manner, the respective target or actual color density spectrum of the image locations of the original or printed copy is now determined by logarithmizing the individual reflectance values R-target (i; color) and R-actual (i; color). The target or actual color density spectra are generated at 35 points of the visible spectrum. The target color density spectrum in the template is given the values D target (i; color) and the actual color density spectrum in the printed copy is the value D actual (i; color), each with i = 1, ..., 35.

The difference is then formed for each spectral range (i = 1,..., 35), so that the difference color density spectrum delta-D (i; colored) 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:

$\text{Delta-D (i; colored) = α · a · D (i; C) + β · b · D (i; M) + ∇ · c · D (i; Y) + Δ · d · D (i; K).}$

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

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

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

The system of equations above, which consists of a total of 35 individual equations, is now solved according to the known method of linear regression (minimizing the sum of the squares of error) according to the proportionality factors a, b, c, d. These factors a, b, c, d to be determined thus indicate how large the proportion of the printing ink cyan, the printing ink magenta, the printing ink yellow and the Printing ink black when generating the difference color density spectrum Delta-D (i; Bunt) with regard to the respective spectral components of the printing inks mentioned. From the factors a, b, c, d determined in this way, the necessary changes with regard to the color guidance (in the color metering zone under consideration) can then be derived. Clearly, for example, a positive and large value of a means that the ink supply for the printing ink cyan must be increased significantly in the corresponding printing unit. Correspondingly, a small and negative value for a clearly means that the color metering in the corresponding color metering zone for the printing ink cyan has to be reduced somewhat. The same applies to the factors b, c, d with regard to the interpretation in the derivation of setting commands for the color metering for the printing inks magenta, yellow, black.

If not all of the cyan, magenta, yellow, and black printing inks are involved in the combined printing of the image areas under consideration, but only the cyan and magenta printing inks, for example, then according to the linear approach made above to represent the difference color density spectrum Delta-D (i; Bunt) Weighting factors α, β, ∇ and Δ are each set to zero. This can also be provided if it turns out that these colors exemplified here are only represented with a very small proportion of the printing area.

Above, the linear approach for the representation of the difference color density spectrum Delta-D (i; Bunt) was represented in such a way that the printing ink black with its color density spectrum in the visible range D (i; K) was taken into account. According to the invention, however, it can also be advantageous that the infrared color density D (IR) is determined for the printing ink black in the region of the infrared and by means of an empirically determined relationship from the difference color density spectrum Delta-D (i; Bunt) in printing experiments the influence of the 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; Bunt) is reduced in all spectral ranges i = 1, ..., 35 by a certain amount of color density units. This takes advantage of the fact that, as a first approximation, the printing ink black has an almost constant course in the visible area.

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 $\text{Delta-D (i; colored black) = α · a · D (i; C) + β · b · D (i; M) + ∇ · c · D (i; Y).}$

This difference in color density spectrum, reduced by the influence of the printing ink black, is therefore only shown from the color density spectra of the chromatic colors involved in the combined printing. According to this system of equations, the proportionality factors a, b, c are then again determined in a known manner by linear regression using the method of least squares. The factors a, b, c in turn represent the amount by which the respective color guide for the printing ink cyan, magenta, yellow has to be changed in order to achieve a predetermined spectral course in the image area.

Claims (5)

Method for controlling the color guidance in a printing press, in particular offset printing press, in which the color density spectra are recorded in at least one image location in the original and print copy and a determination of setting commands for the color guidance organs of the printing press taking into account the color density spectra of the colors involved in the printing together of the image location in such a way that the greatest possible agreement of the color density spectra of the image areas of the original and printed copy is achieved,
characterized by
that the difference / color density spectrum is determined from the color density spectra of the image locations from the original and printed copy, that the difference color density spectrum is represented as a linear combination of the color density spectra of the individual printing inks involved in the co-printing, and that the control commands for the ink guide elements of the printing press from Proportionality factors of this linear combination can be determined. Method according to claim 1,
characterized by
that the color density spectra of the individual printing inks involved in the overprinting of the image areas are weighted according to their share of the printing area. The method of claim 1 or 2,
characterized by
that the proportion of printing area for the printing ink black is determined by determining the infrared color density D (IR). Method according to one of claims 1 to 3,
characterized by
that printing inks with a small portion of the printing area are not taken into account in the representation of the difference color density spectrum. Method according to one of claims 1 to 4,
characterized by
that the recording of the color density spectra of the individual printing inks involved in the overprinting of the image locations takes place on measuring fields of the full tone printed on the printed copy.