Stacatto - Connecting the Dots
Printers continually strive to increase reproduction fidelity and presswork consistency, while lowering production costs. For over 125 years, the mechanism to reproduce imagery in presswork has been halftone screening.
With this process, the tones of the original art are converted into small dots arranged on a fixed grid that, seen at a distance, create the illusion of a continuous tone reproduction.
The dots may be variably sized and positioned at fixed spacings. This technique is commonly referred to as "AM" screening, because the amplitude of the dot is modulated. Another screening method is where the dots are fixed in size but placed at variable spacing. Since the frequency of the dot is modulated, this technique is referred to as "FM" screening.
Just as there are different AM screen designs reflected in the final dot shape (elliptical, round, square, etc.), patterns used for the distribution of microdots in FM screens vary from vendor to vendor.
In addition, there are two classes of "hybrid" screens. In the first, FM spaced dots grow in size (length or area), forming larger "clusters" or "features." These FM screens are commonly referred to as "Hybrid FM" or "Second Order FM" screens.
The other is "Hybrid AM" screening, where the AM structure forms the main part of the tone scale, but at some point (typically 1% to 10% and 90% to 99%) it switches to an FM-like pattern. Hybrid AM screens are most often seen in flexographic applications, because of the inherent plate imaging limitations of that process.
AM and hybrid AM screening is usually invisible to the casual observer, unless a reproduction problem causes them to become visible or generate unwanted artifacts. For example:
1. Loss of photographic visual impression caused by the fixed pattern of dots that form the image.
2. Subject moiré: the undesirable pattern caused by regular spaced AM dots interfering with regularly spaced patterns in the original art (e.g., a pinstriped shirt).