DE2903062A1 - Digital control raster scanner - has laser writing head with different spacings for each colour - Google Patents

Abstract

The electronic screen control uses a digital pattern with each colour processed separately and with a laser writing head for each colour. The colours are projected at different angles with different screen pitches to reduce Moire effects. The base raster has a set spacing w.r.t. the laser scan spacing while the other colours have spacings which differ from a set number of laser scan lines with a repeat pattern of about seven dots. The spacing is typically 60 lines/cm and the screen angles 0 deg. for yellow, 45 deg. for black, - 15 deg. for magenta and +15 deg. for cyan.

Description

Process for the production of grid excerpts

The invention relates to an electronic method of manufacture of raster extracts with raster scanners, in particular using digitally controlled Laser scanners.

The manufacture of raster scanners is a common practice. It are either raster images of color images or color slides or halftone color separations manufactured with a given constant screen width and screen angle transform the tonal values into corresponding point sizes. The tonal range of 0 up to 100% is achieved by using small, non-transparent ones at low tonal values Points are created on the film, while at high tonal values only small ones there are transparent holes (negative points). A well-known electronic one Manufacturing system is equipped with a digital computer that works with a given Screen ruling and screen angle the three essential parameters are calculated. These are: point size, point shape, point offset from normal position.

With the conventional method, the screen ruling is constant (e.g. 60 L / cm) the screen angles of the four basic colors cyan, magenta, yellow, black so angled to each other so that moiré effects only occur over a small area. Common are z. B. the angles 0 0 ° for yellow 45 ° for black 0 - 15 ° for magenta + 15 ° for cyan.

for rotogravure printing / in a well-known electronic raster process, that works with digitally controlled laser beams is for different screen angles a different looking grid is provided. This grid fulfills the task to fit into the system of engraving lines. You bet after each round a well in the gap between the two neighboring wells of the previous one Engraving line and gets a diagonal grid. By varying the point spacing In the engraving line as well as the engraving line spacing it is possible to compress as well as to achieve elongated cell configurations with which different colors then can be printed on top of each other.

Four screen angles are required to produce color separations. If the raster point is set exactly next to the neighboring one during one revolution of the platen Grid point of the previous writing line, this creates a 0 grid with vertical-horizontal Point arrangement. If you go with the grid points in the gap between the two Neighboring points of the previous writing line, then you win a 45 grid.

Thus, the task is to record the grid points in the system of the writing lines to fit, for 0 0 the screen angles 0 0 and 45 0 without difficulty achievable. However, there must be two more screen angles in this system of Writing lines inserted will. You can do this with the two angles +/- 18.4 to the direction of writing. This stands for this electronic raster process the angles 0 ° +/- 18.4 ° and 45 ° are available. The tangent of all these angles is rational. The term "rational grid" therefore also applies to these grids to. This deviates from the practice that has been practiced for many years, the angle of 30 between to use the supporting colors. Also the 15 angle, which is mostly yellow opposite Occupies cyan and magenta is no longer used.

Changed screen angles to each other are not the only special feature this new grid. They do not have exactly the same mesh size as the 0 conventional grids. The grid below 45 ° is about 6% coarser than the one below 0 °.

The two were recorded at 18.4 ° from the direction of writing Grids are about 6% finer than the grid below 0 0. That electronically generated grid is in a fixed relationship to the platen and cannot be turned in relation to the register pins.

Because of the division into discrete steps, the deviation is provided by the usual conventional angle, namely - 18.4 ° for Magenta and + 18.4 ° for cyan, with the same angles for yellow and black.

To enable these angles - as described - also the Screen rulings changed, namely z. B.

as follows: 60 L / cm for O 0 56 L / cm (or 80 L / cm) for 45 63 L / cm for +/- 18.4 * also for the offset process For a relatively long time this approach is considered an acceptable solution. In practice, especially in special color tones (printing two or more tone values of the basic colors together), z. For example, violet tones often result in instabilities in printing and their causes attributed to ink management, ink acceptance in wet-on-wet printing and similar things will.

The instabilities mainly occur when register shifts are present. For example, due to the system, there are two raster tones (60 L / cm) of the angles 45 0 and 18.4 ° fitted into one another in such a way that either in the X and Y directions exactly each second or fourth point of the two grids in the combination structure to cover come. This results in a certain pattern and, connected to it, a certain one Shade effect. The combination of several superimposed grids also results a pattern with a certain shade effect. Due to slight register shifts in Influences acting in the X and / or Y direction or the like now occur all possible partial pattern-like point coverings between the two, for example above cases described. However, if dots of different printing colors are next to each other, times are printed on top of each other on the substrate, the result is a very different color sensation. When printing, it is important to keep the register constant to a fraction of a point distance (167 µm with a 60-digit grid) technically impossible, if only because of the dimensional changes or dimensional differences of the Substrate.

In the conventional grid there is a repetition of an exact one Point coverage practically not at all because the orthogonal distances are irrational are (irrational grid), so that in connection with the electronic grid process The problem described does not exist.

The object of the invention is to provide a method for producing electronic To create raster extracts, preferably with digitally controlled laser beam exposure, that works in the given system, but the described instabilities as far as possible excludes.

This object is achieved according to the invention by a method for electronic Production of raster extracts using raster scanners solved, with with a scanning head analog image signals are supplied to a color computer for color and tonal value correction, then each signal into one Scale calculator, from there to create a raster program in a raster computer and a raster point memory is given, the analog signals being digitized beforehand then the raster signal is preferably used as a digital signal for modulation, in order to generate laser beams, preferably in a laser writing unit, which point to a photo paper or the like can be transferred. The new procedure stands out characterized in that in the combination structure of the superimposed grid the Repetition distance of the coverage cycle is increased.

In the known electronic raster method, the repetition distance is at every second or fourth point, i.e. exactly every second or fourth point comes to cover. The grids are rational. From it result even with the smallest register differences, very different patterns with the same or almost the same repetition distance. Two different patterns can be used here be so different that a certain number of points do not cover comes or that this certain number of points is congruent. It surrenders from this difference, however, a very different color perception for the eye. Surprisingly, it has been shown that this disadvantageous optical effect, which can occur, for example, with the slightest register differences, thus avoided can be that the repetition distance of the coverage cycle is increased, wherein preserve the possibilities of the electronic system with digitization of the measured values can stay.

The electronic system requires that a repetition of the coverage cycle not in the combination structure with constant screen ruling and screen angles can be completely avoided, since only with the digitally controlled laser beam exposure there is a finite, rational supply of values. In the method according to the invention The scanning head of the raster scanner delivers - as usual - analog image signals, which are fed to the color computer for color and tonal value correction.

After determining the desired scale, e.g. B. in the scale calculator, According to the invention, the raster computer must, in conjunction with the raster program, the laser writing unit control in such a way that to reduce repetitive integer point In short periods of two to four point intervals, these periods are significantly spaced be enlarged.

According to a particular embodiment of the invention, the repetition distance is at least seven grid points in the combination structure of two superimposed Grid. It is thus ensured that the angled grid points only after at least seven points a repeating point coverage with one point of the example not angled grid.

The method according to the invention is carried out particularly expediently, in that for the basic grid (e.g.

0 0 grid) a distance between the center points of the grid points or holes is chosen, which is a whole multiple. For example, are located at the 0 angulation the center points at a distance of 12 laser tracks. This results in a grid e.g.

with 60 L / cm. For the angled combination grid, however, a Selected point spacing that is a non-integer multiple of the laser tracks. This 0 is achieved z. B. at the 45 -angle by moving the center points of this Alternately or alternately after the eighth and then after the ninth laser track and this is repeated alternately.

This results in a distance between the center points of 8.5 laser tracks and a grid of 59.8 L / cm which is close to 60 L / cm.

Another useful embodiment of the invention 0 is given, if a screen angle of + 15.26 is selected. Repeat the midpoints then all 11 laser tracks in the X direction and all 3 laser tracks in the Y direction, which results in a grid with 63.1 lines per cm. For the - 15.26 ° angle the center repeats itself accordingly in the X-direction every 3 laser tracks and every 11 laser tracks in the Y direction. The grid also points in this case 63.1 lines per cm.

In the known electronic raster method described, results a coverage cycle after two to four points for all colors, all point distances and all raster programs, so that the disadvantages shown in almost all combination structures occur caused by z. B. register differences or the like arise. With the By means of the invention, this influence can be almost eliminated because of the combination structures do not have such great differences that the eye has a distinctly different one Perceives color sensation.

Claims (5)

Claims: 61 Process for the electronic production of grid extracts using raster scanners, with a scanning head analog image signals which are fed to a color computer for color and tonal value correction , then each signal in a scale computer, from there for creation given a raster program in a raster computer and a raster point memory the analog signals are digitized beforehand, then the raster signal preferably as a digital signal for modulation, preferably in a Laser writing unit to generate laser beams on a photo paper or the like be transmitted, d u r c h e -k e n n n z e i c h n e t that in the combination structure the overlapping grid increases the repetition distance of the coverage cycle will. 2. The method of claim 1, d a d u r c h g e -k e n n z e i c Note that the repetition distance should be at least seven grid points in the combination structure of two superimposed grids. 3. The method according to claim 1 and / or 2, d a d u r c h g e k e n n shows that for the basic grid there is a distance between the center points of the grid points or holes is used, which is a whole multiple, and for the angled Combination grid, a point spacing is selected that is a multiple that is not an integer of the laser tracks is. 4. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that for the 0 O angulation a center point distance of 12 laser tracks 0 and for the 45 ° angle, a center-to-center spacing alternating between 8 and 9 laser tracks is chosen. 5. The method according to one or more of claims 1 to 4, d a d u r c h g e k e n n z e i c h -n e t that a raster angle of the combination raster of +/- 15.26 ° is used.