Printing halftones involves minimizing dot gain and maintaining a full tonal range. Achieving these two goals becomes more challenging as the halftone line count increases or the substrate becomes less smooth. In screen printing, higher line counts yield progressively higher dot-gain values, and, when combined with poor dot formation, result in tonal-range compression. A loss of tone at each end of the scale flattens the overall image contrast and produces a dark, muddy image.

Printing halftones involves minimizing dot gain and maintaining a full tonal range. Achieving these two goals becomes more challenging as the halftone line count increases or the substrate becomes less smooth. In screen printing, higher line counts yield progressively higher dot-gain values, and, when combined with poor dot formation, result in tonal-range compression. A loss of tone at each end of the scale flattens the overall image contrast and produces a dark, muddy image.

This article introduces a very useful technique that will help you increase tonal range and improve print contrast and the level of detail within an image. The method is called posterization, and while it may not work in every instance, it is reliable in most cases.

Posterization involves altering the number of grayscale steps in the halftone to minimize tonal compression caused by excessive dot gain. The approach is variable, meaning you can choose how many tonal steps will be represented in your image. You can also preview the effects of posterization before you output film to determine if the number of steps is suitable for your application.

How many levels of gray?

Postscript Levels 1 and 2 support 255 gray levels (162-1) between white and solid values. This means there are 255 steps across the scale. Each step is approximately 0.39%. Very light highlights are the only places such small shifts in tone density are important. The human eye can easily perceive these changes in very light grays, beiges, and flesh tones, and in very light gradations, such as shading of an egg or the transition of light sky blue to the horizon. This shortcoming has been remedied in Postscript Level 3, which supports 4095 tone levels. But only litho printers can bring this level of tonal resolution to their prints.

For screen printers, even 255 steps are overkill. Very few screen shops maintain enough control over their processes to reproduce the subtle differences this level of resolution affords. This lack of control overrides the tonal steps, resulting in contrast reduction.

Improving internal image contrast without sacrificing detail is a more immediate need for screen shops. Fortunately, this need can be fulfilled by judiciously reducing tone through image posterization.

How to use posterization

Posterization is nothing more than reducing continuous tone to a specific number of steps. Postscript Levels 1 and 2 posterize at a level of 255 steps. However, you have the power to re-duce the number of steps, thereby increasing the distance to the next tonal value that can be reproduced. For instance, if you posterize at 100 steps, each tonal step will be a 1% increment. A value of 50 will give 2% tonal steps.

To understand how posterization level fits in with the screen-printing process, you have to consider both your prepress and printing capabilities. In prepress, your imagesetter’s output resolution determines the maximum line count you can use. But the higher the line count, the more dot gain you will experience during printing and the fewer tonal steps you can produce.

First, consider the highest line count your imagesetter will support. Assuming a maximum tonal level of 255, you divide the output resolution by 16 to determine the maximum line-count value. For example, if your imagesetter is set for 1200 dpi, the maximum halftone line count would be 1200