DE4204980C1 - Auto-type colour image print shade intensifying - superposes printing forms of three basic colours in precise, punctiform raster with fine micro-raster - Google Patents

Abstract

The press forms for the three basic colours superpose the honeycomb of rasters for the colours in precise alignment so that the three possible two-component pairs give the permissible subtractive mixed colours red, green, and violent, (AC, AB). These act as additive colour mixes to form an additive mixed colour in conjunction with their original basic colour as a function of their limited area ratio. Each raster area not occupied by the basic colour has an extremely fine micro-raster tone for printing in the same pass with identical ink as reference surface so as to prevent a whitening effect. USE/ADVANTAGE - Colour printing, autotype. Optimum colour mixing obtained by basic additives on area basis, using micro-raster to avoid blank spaces.

Description

The invention relates to a method for multiplying the autotypical Color shades achievable in color printing.

The autotypical printing works according to the scheme that screened printing forms are used which, depending on the tonal value to be achieved, transfer printing ink to the material to be printed in a larger or smaller proportion of the area within each of the mostly square, sometimes also hexagonal, unit grid elements, while the remaining area remains unprinted . The greater the so-called area coverage (FD), the more intense the color appears, the lower the area coverage (FD) is, the weaker the resulting color appears. Fig. 1 illustrates a raster element in the form of a ring honeycomb, with a) 75% FD, b) 50% FD and c) 25% FD.

A closer look reveals that such grid tones are correct must be described as whitened: yours over the visible Spectrum distributed reflectance is to be calculated as for each wavelength Sum of the spectral reflectance of the color print on the one hand and the paper left unprinted on the other hand; a curve calculated in this way leaves a clear degeneration of the original color by the Recognize the whitening effect, actually in color image printing unacceptable impairment of the color effect.

But there is another serious drawback: becoming two of the three chromatic colors of a three-color set using printing forms printed on top of each other, which have a halftone tone with 50% FD, have a cross grid structure according to DIN 16 600 and according to DIN 16 547 are angled, so in one and the same grid unit element occur: color A solo, color B solo (side by side, d. H. cooperative additive), alongside mutual partial coverage (subtractive), as well as unprinted paper white. In addition, two additional additive mixed colors are created, namely formed from the resultant of the subtractive mixture b over A on the one hand and one of the solo colors A and B on the other.  

The applicant had already shown in DE-PS 32 20 711 that the problem of the chaotic coverage ratio, the occurs in the combination of cross line grid structure + screen angle, can be overcome by changing to a honeycomb grid + system Micro-shift of the grid network.

With this system, ordered coverage ratios were autotypical multi-color printing created, but the problem of whitening in the case of halftones of individual chromatic colors by unprinted permanent paper remained for the time being. This problem now takes place a complete solution through the present invention:

In the method according to the invention, a relatively light reference color tone is displayed in the same printing process and with the same printing color as the printing main screen element in order to switch off the whitening effect emanating from the unprinted paper white in the previously unprinted remaining areas of screen tones of each color tone. To achieve this, a micro-grid is integrated in the areas of the screen elements that are normally not printing, with a fineness that cannot be resolved by the eye - this makes the reference color tone appear as a colored halftone - and with an area coverage of between 10% and 60% depending on the main screen tone. Round points in a hexagonal configuration are preferably suitable as printing elements of the micro raster, but other shapes, such as lines, node lines, dashed lines or oval points, would also be suitable. One can also assume a fairly fine micro raster with a higher area coverage and, when generating a micro raster structure with a lower area coverage, some of the micro-printing elements fail. FIG. 2 shows ring honeycombs with area coverage portions corresponding to FIG. 1, but with an integrated micro grid.

The inventive introduction of a bright reference color for switching off the bleaching effect in autotypical color image printing increases the degree of color of colored halftone tones, but without bring out additional color nuances. The solution to this task is of particular importance because of the high technical Status of color photography ever more nuanced reproduction templates arise, the true color reproduction of the client of printing understandably asking.  

The applicant is based on an in-depth analysis of the result arrives that a noteworthy increase in autotypical color printing achievable color shades only through systematic use both subtractive and particularly additive color mixing processes (also seen as an "optical mixture", narrow at the same time neighboring colored areas defined) can be achieved. Important is first of all the knowledge that in the case of purely physical reasons subtractive color mixing - one color overprint corresponds the effect of color filter layers connected in series - only three pairings can be derived from the three primary colors, the three certain colors and three mathematically clearly calculable spectral density curves result, while in this process the perish the two single primary colors creating the mixed color. Analogue in addition to the three primary colors, the three have two primary colors subtractive mixed colors created by overprinting their permanent place in the color wheel. In the following they are called "sub-primary colors" designated. From correctly coordinated basic colors, here are simply called yellow, pink, blue, the sub-basic colors are created by pairwise subtractive mixing Red, green, purple.

It follows that all color transitions from one basic color to the neighboring Sub-base color and from this back to the next base color can only come about through additive color mixing, namely in a practical implementation of the autotypical principle of color determination by defining area ratios. Example: A large proportion of green areas and a small proportion of yellow areas provide yellowish Green; if the areas of green and yellow are the same, the middle area is created Yellow-green; with a large amount of yellow and a small amount of green, greenish results Yellow.

Fig. 3 explains on a color hexagon a scheme of color generation that is only valid for the autotypical three-color printing with the targeted use of the subtractive and additive color mixing (the former only for the formation of the three sub-basic colors, the latter for the entirety of the color transitions in the six sextants of the color hexagon). In Fig. 3 is designated with A: yellow, with B: pink, with C: blue, with AC: the sub-base color green, with AB: the sub-base color red, with BC: the sub-base color violet. The percentages (except 100%) indicate the additively interacting colored areas.

The raster-technical implementation of this principle, based on the hexagon of colors, of the nuanced, autotypical color representation can be optimally solved on the basis of honeycomb grids, with the proviso that the raster elements of all three bright primary colors are printed concentrically on top of each other - of course, no more than two primary colors take on the structure of all pure image tones part. This is explained with reference to FIG. 4, where honeycomb a is a yellowish green (75% AC + 25% A), honeycomb b is a medium yellow green (50% AC + 50% A), honeycomb C is a greenish yellow (25% AC + 75% A).

FIG. 5 shows that even in this system, lightened tones can be represented with the greatest possible degree of color in both primary colors involved, with the cooperation of the integrated micro grid.

Fig. 6 shows two known honeycomb models, namely a) a Ringwabe, b is an Hexagonal honeycomb); c) represents a honeycomb with a rotated hexagon, which significantly reduces the conspicuousness of the honeycomb grid structure.

In today's method of autotypical multi-color printing, it is customary to add the third basic color in each case to reproduce tarnished colors, a method of operation which is incompatible with the system according to the invention of producing subtractive and additive color mixing processes because the safety of the color generation would be lost. Consequently, the printing ink set according to the invention was supplemented by a highly glazing, low density neutral gray color. The printing form for this neutral gray color is produced with the same honeycomb grid structure as the printing forms for the chromatic colors, but without an integrated micro-grid and in a grid-net position shifted from the common grid position of the chromatic colors in accordance with DE-PS 32 20 711, Fig. 2, 4, 5.

Since the cloudiness only logically relates to colored shades can, but, as explained above, by additive color mixing come about, the respective primary and sub-primary colors involved with its maximum remission comes into effect, an extremely suffices low neutral gray overpressure to achieve the desired cloudiness bring forth. The representability itself made possible in this way minor cloudiness is of great importance for adequate reproduction the light / shadow gradients that allow the spatial shape to be recorded Recognize objects and their classification in the image space.  

The printing ink set for executing the described method, in which both the three basic colors and the ones to be formed from them Sub-primary colors, so to speak, deliver the "color coordinate system" within which creates the numerous additive mixed color nuances of course, differs significantly from those common today Ink sets: This is not just a generally higher purity required, but it also has to meet some specific criteria will:

• 1. The yellow component "golden yellow" must not be greenish (i.e. after lemon yellow) still tend to orange or even reddish;
• 2. The pink component "pink" may be little to the reddish, but never tend to violet;
• 3. The blue component "sapphire blue" must not be cyan or Turquoise blue still tend to purple.

Further information is given on the basis of the spectral density curves ( FIG. 7).

In Fig. 7, the spectral density curves of the three basic printing colors "golden yellow" (- - - - -), "pink" (.........) and "sapphire blue" () are shown schematically.

It is particularly important under the symmetry conditions to be observed the spectral position of the intersection of the golden yellow with the sapphire blue curve in the spectral range 520 nm-530 nm; those are still important Curve drops of the gold yellow curve after short-wave and the sapphire blue curve according to Langwellig.

The applicant has already used a non-covering neutral gray color in DE-PS 32 16 360, A.2 u. 3, proposed, however as a mixture of at least three colors. At the time, the involvement was additive mixed colors not yet considered so that higher densities had to be sought. According to the invention on the other hand, the new highly glazed neutral gray color must be unusual have low density, which is why this color only two very low pigment inks is mixed, namely from a pink and a light green color, with the latter in excess (e.g. 1 part pink, 2-4 parts light green).

Claims (1)

A process for increasing the color nuances that can be achieved in autotypical color image printing by combining a special structure of the screening with a set of printing base colors consisting of a yellow, pink and blue component, which is occasionally supplemented by an additional achromatic color, characterized in that

• a) the printing forms for the printing of the three brightly colored primary colors ensure a precise, concentric overprinting of the honeycomb raster elements of these three colors and therefore also the three possible two-component pairings deliver, which in turn appear as additive color mixing components and, together with the first or the second of the two basic colors from which they originated, form an additive mixed color that is dependent on the mutual area ratio;
• b) in each unit grid element, the area of which is not completely, but only partially, taken up by the printed basic colors, in the remaining area a microraster tone of great fineness, but little to medium integral area coverage in the same printing process with the same printing ink as color reference area and to avoid a whitening effect becomes;
• c) the spectral density curves of the basic printing inks to be used meet particular conditions with regard to the formation of both the subtractive and the additive mixed colors, namely in such a way that
  • 1. The yellow component has the hue of golden yellow, the pink component has a slightly reddish pink hue and the blue component has a clear sapphire blue tone which neither tends towards cyan or turquoise nor towards violet;
  • 2. the intersection of the gold yellow density curve with the sapphire blue curve bwi 520 nm + 10 nm lies in order to ensure that the subtractive mixed color to be formed from both components has the hue medium green;
  • 3. The overall course of the sapphire blue spectral density curve is approximately similar to a sine curve, with the special characteristic of a steep curve drop in the spectral section 620 nm-690 nm, without which bright reddish violet tones would not be attainable; What is important is the similarity to a sine curve, above all so that the clear blue hue will certainly remain constant even when there are fluctuations in the coloring of the print run;
• d) the printing ink set consisting of three brightly colored primary printing colors is complemented by an unusually bright, high-glazing neutral gray color, which is mixed from only two components, namely the low-pigmented printing inks pink and light green, for example in a ratio of 1: 2 to, when performing more demanding color image printing 1: 4.