The angles of AM screens
26 Flexo & Gravure Global 2-2018
The angles of AM screens
In halftone printing, the choice between moiré and colour drift is a decision between two evils
Prof Dr Martin Dreher
The angles of amplitude modulated screens (AM screens) is quite a current printing problem. Even so it hardly attracts the interest of experts and the solutions available are anything but satisfactory. However potential future developments could make this issue a major focus of concern.
The dilemma of halftone multi-colour printing
Most graphic printing methods use halftone printing. Gravure printing is an exception, but this process also runs in a semi-autotypical way today (variable depth gravure cells) and therefore is also the object of the following technical conditions.
Halftone printing processes only consist of two inking conditions: Inking of the image elements or non-inking (on/off; yes/no; binary) of the background. For the generation of halftones for the reproduction of printed images screening technologies are used. Comparable to the electronic image which is captured in pixels, the printing areas are split into small sub-areas, each provided with a degree of ink coverage according to the respective tonal value. This means: the lower the tonal value, the thinner the ink coverage.
This level is determined by the positioning of individual, even-sized printing dots supposedly randomly distributed throughout the printing surface. This process is called frequency modulated screening (FM). However, the degree of ink coverage can also be achieved by using single, variable sized dots which is called amplitude modulated screening (AM). The variable dots are called halftone dots.
Frequency modulated screening offers many advantages in terms of the current problems. In practice, however, they have some serious disadvantages which have, as yet, prevented a wide scale technological breakthrough. Rather they are mainly used for special effects and require a higher degree of process control and care. The print quality achievable with this process is threatened by the appearance of a visually perceived graininess which results in mottling.
For these and other reasons, the amplitude modulated screening dominates the current print market and also offers the required smoothness within halftone areas and are therefore preferred by the client. AM screening means the arrangement of the screen dots in a regular and symmetric pattern. In multi-colour printing this involves the need to rotate the screens relative to each other, which is referred to as the screen angle. Otherwise, the overlap of the screen dots would cause interferences known as colour drift and/or moiré.
In practice, this can lead to severe colour changes of the consecutively printed results. This is caused by the fact, that when the dots from a single colour meet on the substrate the corresponding dots only partially overlap depending on the image tone. If this degree of overlap changes by even a small amount due to unavoidable register fluctuations, changes in
“AM screening dominates the current print market and offers the required smooth-ness in half-tone areas”
Figure 1: Current screen angles in four-colour halftone printing
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