Run-free color raster for multi-color printing

In engraving of printing forms particularly of gravure cylinders, it has previously been possible to produce run-free color raster configurations for only two colors. The present invention discloses rastering methods for run-free color production of four color impressions and polychromatic impressions.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The invention relates to a screening method for multicolor printing wherein 
raster elements are produced point or line-wise in chronological 
succession by engraving, exposure or electro-mechanically by means of high 
energy radiation or electro-optically wherein the types of raster utilized 
in both the line spacings in the various rasters of a set-up color 
separation have ratios of approximately small rational numbers and the 
raster point spacings within the lines of the set of rasters have ratios 
of approximately small rational numbers and wherein the number of raster 
points per surface area can be different in the rasters for the different 
individual colors. 
2. Description of the Prior Art 
In the art of electronic mechanical engraving of printing forms as, for 
example, in the engraving of gravure cylinders it has been described in 
detail in "Der Polygraph", No. 18/1965; "Deutscher Drucker", No. 
22-24/1974 and "Druck-Print", 12/68, pp. 931 through 938, the problem 
arises that in order to avoid Moire effects, an angling between the 
screens of the individual colors of a multi-color printing used in 
standard conventionally etched autotype forms cannot be accomplished. 
Primarily this is because the raster grid of an engraving for example of a 
gravure cylinder is formed by means of successively engraving all of the 
raster points such that the engraving head operating with a constant 
rastering frequency of its engraving tool engraves one axial extending 
line of the cylinder after another with raster cups of different depths 
but with constant center spacings from each other. Thus, the engraving 
head continuously moves slowly or step-by-step in the axial direction of 
the cylinder on the respective axial line spacing per cylinder revolution 
which is prescribed for the desired raster. 
Thus, the raster grid results from the interaction between the rotary 
motion of the drum, the forward feed of the engraving tool in the axial 
direction of the cylinder and the rastering frequency of the engraving 
tool. It is obvious that the geometry of the raster grid is based on the 
circumferential spacing and the forward feed direction as specified 
directions and that an angled position of the raster grid relative to the 
circumferential direction which is the subsequent direction of the run of 
paper during printing cannot occur. Since the number of cups on a given 
circumference as well as the length unit in the axial direction is 
extremely precise, Moire phenomena cannot occur even given non-angled 
rasters because Moire patterns result because raster points of two or more 
rasters are printed on top of one another or next to one another at 
periodically repeating intervals. 
In practice, however, a number of meters or yards of the paper which are to 
be printed may be present between two printing units of a printing machine 
and the paper can easily change its length by one-half of a raster point 
spacing due to stretching and shrinking due to moistening and dilatation 
between the printing units. As a result, the raster points of two or more 
colors may be accidentally printed on top of each other or be printed so 
close next to each other to obtain so-called color drift and these effects 
also cause Moire effects which are statistically much greater so that 
impressions of the same subject printed from various cylinder impressions 
have great color fluctuations which cannot in practice be accepted. 
It should also be noted that the standard angling of the rasters utilized 
in conventional practice does not cause the Moire to disappear. On the 
contrary, as a result of angling, the Moire is made so fine that its mesh 
is only slightly greater than the mesh of the raster itself. Thus, due to 
the finite power of resolution of the human eye at a reading distance, it 
will no longer be perceptible if a sufficiently fine screen is used. 
Since the existence of electro-mechanical engraving particularly of gravure 
form cylinders, it has become desirable to obtain raster geometries to 
produce forms which are equivalent in effect to conventional angling. 
A type of raster distortion in the engraving using a suitable selection of 
the raster production parameters has been disclosed in German LP 1,112,407 
(also in "Druck-Print" 12/68, pp. 931-938 also). The same effect as in 
"angling" is created by means of "crushing" or "elongating" the originally 
quadratic raster meshes in specific numerical ratios. However, it was not 
observed at the time of this work that there are only two practical 
combinations of raster geometries which meet the conditions desired which 
are that in the circumferential direction that three raster points of the 
second raster occur for two raster points of the first raster and 
vice-versa in the forward-feed direction that three raster points of the 
first raster occur for two raster points of the second raster. Compare 
FIG. 2b in German LP 1,112,407. In four-color printing, all of the 
combinations produce an unacceptable Moire effect. 
SUMMARY OF THE INVENTION 
For a long period of time, when printing four colors in combination 
operators printed the critical colors usually red and blue in one of the 
respective raster geometries. These were free of color drift and the Moire 
which occurred was sufficiently fine. The remaining colors black and 
yellow are not quite as sensitive relative to color drift and were 
therefore respectively assigned to one of the two raster geometries. The 
color drift with red and blue was acceptable but was always a critical 
point for the engraved printing forms. Since then, the object has occurred 
of finding combinations of the raster configurations which allow more than 
two colors to be printed free of color drift and with a sufficiently small 
Moire effect. Further, such configurations must also be adaptable and 
useable in the standard machines of reproduction technology which existed 
today whether they be engraving machines or scanners. The present 
invention has an object of disclosing a method of rastering which 
accomplishes these results. 
Other objects, features and advantages of the invention will be readily 
apparent from the following description of certain preferred embodiments 
thereof taken in conjunction with the accompanying drawings although 
variations and modifications may be effected without departing from the 
spirit and scope of the novel concepts of the disclosure and in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention comprises printing the raster configurations together which 
are to be varied not only by distorting them from the original quadratic 
shape, but also in the raster amplitude, in other words, in the actual 
number of raster elements per surface unit. The inventors have discovered 
that a great many more combinations can be used than the ratio of 2:3/3:2 
which is disclosed in German LP 1,112,407. It has been discovered that 
even noticeable deviations from a basic raster amplitude for the various 
colors do not result in a noticeable sacrifice in quality particularly 
when the deviation is in the direction toward finer screens. It has been 
discovered that using these preconditions that precise rational ratios in 
the "elongation" or "compression" of the quadratic original rasters need 
not be utilized. Slight deviations from the determined configuration do 
not result in color drift but rather only change the fine Moire effect to 
an extremely small degree and phase position, mesh shape and period. As 
examples of the invention, two practical configurations for a four-colored 
high-grade catalog gravure may be cited. 
In the first example, the originally employed distortions 2:3/3:2 are 
combined with a respectively coarser and finer quadratic screen. In FIG. 
1, the four configurations are illustrated wherein U indicates the 
direction on the impression cylinder in the circumferential direction and 
B indicates the forward feed direction on the engraving element in the 
axial direction of the cylinders. Corner points of the quadrangles are to 
be interpreted as the centers of associated raster points. The coarser 
quadratic raster whose raster points have the spacing of the raster points 
of the red screen in the U-direction and in the V-direction is 
advantageously assigned to the yellow color because this generally does 
not allow the raster structure in the printing to be perceived because of 
its greater brightness. Red and blue colors are printed in 2:3 or, 
respectively, 3:2 configuration. The black color has a quadratic raster 
which is twice as fine as the yellow raster which advantageously benefits 
the reproduction of fine print. Thus, in FIG. 1 the yellow configuration 
results in the center of the raster points being on a square which is the 
square illustrated and indicated as yellow. The red configuration has its 
circumferential points coincidence with the yellow but its axial direction 
points are in one-third from the yellow points toward the center of the 
arrangement. This configuration is indicated with "red" marking. The blue 
raster points have their axially extending dimensions coincidence with the 
yellow rasters and their circumferential points in one-third toward the 
center of the arrangement from the yellow points as illustrated. 
A separate embodiment is illustrated in FIG. 2 in which the yellow, red and 
blue rasters have the same relative positions as in the embodiment 
illustrated in FIG. 1. However, the black raster points do not lie in the 
corners of a square as in FIG. 1. Rather, the spacing of the black raster 
points in the axial direction "V" corresponds to the spacings of the 
points of the red screen and the spacings in the circumferential direction 
"U" is equal to one-half the spacing of the red screen in the U-direction. 
Other combinations are possible and the important point is that the 
elongation and compressions in the U or, respectively, V directions need 
not be precisely observed. Deviations in the magnitude by a few percent 
are not perceptible in the printed impression. This also applies to the 
raster amplitudes in the rasters of the individual colors. It is also 
possible to interchange the colors assigned to the individual rasters. It 
also is within the teaching of the invention to produce a set of color 
plates in which a departure from the previous method is made in only one 
raster and a raster according to the invention is employed only in this 
particularly separation. 
The method can be used not only in engraving of printing forms, but can 
also be used with the described raster geometries in the production of 
raster color separations by scanners for all printing methods even as a 
corresponding contact raster in the classic form production by means of 
etching in the graphic arts. 
The invention is being commercially utilized in the entire range of 
reproduction technology particularly in electronic reproduction 
technology. 
Although the invention has been described with respect to preferred 
embodiments, it is not to be so limited as changes and modifications can 
be made which are within the full intended scope of the invention as 
defined by the appended claims.