Abstract
Periodic clustered-dot screens are widely used for electrophotographic printers due to their print stability. However,
moir´e is a ubiquitous problem that arises in color printing due to the beating together of the clustered-dot,
periodic halftone patterns that are used to represent different colorants. This beating or interference phenomenon
introduces spurious low frequency (large period) patterns in the printed output that are very objectionable to
the viewer. The traditional solution in the graphic arts and printing industry is to rotate identical square screens
to angles that are maximally separated from each other. For example, the classic three-color screen set rotates
three identical square screens to the angles 15°, 45°, and 75°, respectively. However, the effectiveness of this
approach is limited when printing with more than four colorants, i.e. N-color printing, where N >4. Moreover,
accurately achieving the angles that have maximum angular separation requires a very high resolution plate
writer, as is used in commercial offset printing.
In this paper, we propose a systematic way to design color screen sets for periodic, clustered-dot screens
that offers more explicit control of the moir´e properties of the resulting screens when used in color printing. We
find a general concept for moir´e-free screen design that is called lattice-based screen design. The basic concept
behind our approach is the creation of the screen set on a 2-dimensional lattice in the frequency domain and
then picking each fundamental frequency vector of the individual colorant planes in the created spectral lattice
according to the desired properties. The halftone geometry of a screen set is the set of angles and frequencies
in units of lines per inch (LPI) of each screen plane. The lattice-based screen design offers more flexibility in
designing N-color screen sets with different halftone geometries, and all of them are guaranteed to be all-orders
moir´e-free. For example, by creating a square lattice in the frequency domain, square N-color moir´e-free screen
sets that consist of N rotated square screens can be achieved. The proposed approach maintains the advantage
of square clustered-dot screen design and is based on low addressability of digital printing. We also propose
several symmetry measures, and use them to compare the proposed 4-color square screen set and the screen
sets based on a previous moir´e-free N-color non-orthogonal approach. The proposed screen set is shown to have
better symmetry properties.
