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Understanding the Behavior of Midtone Dot Gain

(November 2007) posted on Mon Nov 12, 2007

Coudray discusses the art of managing dot gain and the science of manipulating dots in the midtone region to improve image clarity and quality.

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By Mark A. Coudray

The shape of the halftone dot has a major bearing on how much dot gain you will experience in the midtones. If you’re not aware of the differences in the dot-shape options, you may be in for an unpleasant surprise. Programs like Adobe Photoshop often default to lithographic preferences and deliver halftone separations at the wrong angle and with the incorrect dot shape for screen printers. Even experienced prepress technicians fall victim to inappropriate default settings.

When referring to the shape of the dot, we concentrate our attention on the transition area of the midtones. This is where the dot area grows to the point where the corners of adjacent dots join. This is a huge problem for our printing process. When the corners of the joining dots connect, they immediately bleed together. We know this as dot gain, and the associated, simultaneous corner growth is referred to as midtone jump. The combination of the geometry of the joining corners and the high film thickness of screen-printing inks contributes to how much midtone jump you experience. The midtone jump can easily exceed 20%. Even the very best printers find it impossible to compensate for this kind of behavior.

A major point that I want to reinforce is the fact that the midtone value still has the maximum amount of dot gain. The midtone jump is in addition to the expected dot gain in the midtones, which is why the midtone jump is so incredibly damaging to an image. Where normal, expected dot gain might be 20% (50% becomes 70% value), the midtone jump may add up to 20% more gain. Therefore, 50% can suddenly yield a 90% tone value. This huge jump in tone value shows as dark banding and exaggeration of shadow transition. It can be totally disruptive and destructive to the tone values and reproduction of the image.

Round dots transition through the tone range as round dots. All four corners of the adjacent dots connect at a simultaneous tangent point in the midtone area. The angle formed at this tangent point is acute (less than 90°). Because it is so tight, it is very easy to fill with ink. The round dot is the absolute worst choice for our process. Figure 1 clearly illustrates the acute angle and the resulting filling that occurs.


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