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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

Printers select round dots for two reasons. The first is that this dot is more tolerant to moiré formation. The uniformity of the dot helps screen printers to minimize the effect of thread eclipsing. The second reason has to do more with the printing process—not necessarily the screen-printing process. On high-speed web and sheeted offset, the dot distorts as the paper passes between the blanket and impression roller. The round dot tends toward an ellipse. As we shall soon see, this helps to minimize the midtone jump. In the case of offset litho, the distortion is controlled by shape compensation. A round dot was developed for that purpose. Screen printing sees no benefit from this compensation.

The square dot was developed to address the problems of the round dot. In this example, the halftone dot starts out round. As tone darkens toward the midtones, the halftone shape transitions from a round dot to a square with round corners, finally becoming a perfect square at the 50% tone (Figure 2). The four corners of the square now connect simultaneously at an improved 90° angle. The possibility still exists for the corners to fill and midtone jump to occur. Square dots will spontaneously increase tonal value by 10-12% under ideal conditions. The jump is greater when printing conditions are poor. Past the 50% value, the dot shape reverses itself, eventually becoming a negative round dot before finally becoming a solid tone.

Square dots are noted for their ability to carry extremely sharp detail in the midtone region. The downside of the square dot is that it tends to be less tolerant of moiré. The mesh threads easily eclipse the narrow corners of the dot. Because all four corners connect simultaneously, any disruption of the connecting pattern is readily apparent to the human eye, giving the final printed image a grainy or rough look.


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