SCREEN PRINTING

Presenting a helpful technique for maximizing tone range, print contrast, and image detail in your prints.
By Mark A. Coudray
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.
 
Figure 1
Testing Tones Creating a screen with a range ofposterized gradient patterns is a great way to determine what numberof tonal steps gives you the best prints. This example shows thedifference between a 36- and 255-step grayscale pattern.
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 ÷ 16 = 75 lines/in. But if the imagesetter is only capable of 600 dpi, the picture changes. In this case, 600 ÷ 16 = 37.5 lines/in., meaning you can´t achieve 255 tonal steps without sacrificing resolution.
If you want to determine the number of tonal steps you can reproduce on film using your current line count, divide the imagesetter resolution by the halftone line count and square the result. So if you have a 600-dpi imagesetter and halftones at 60 lines/in., your film positives would represent 100 tonal steps (600 ÷ 60 = 10, 102 = 100).
Now it´s time to shift focus to how many tonal steps you can actually reproduce on press. The best way to determine this is to conduct a simple test. Begin by creating a test image in Photoshop that consists of a 1 x 14-in. grayscale document. Next, set the foreground color to white and the background color to black. Using the gradient tool, make a blend that starts at 0% and ends at 100%. This will be the gradient that you test print.
Make 12 copies of this image. Next, apply a different number of steps (posterization levels) to each copy. I suggest using squares of whole numbers for each copy (e.g., 25, 36, 49, 64, 81, 100, 132, 144, 169, 196, 225, plus the 255 maximum). Adjust these steps by selecting >Image>Adjust>Posterize and entering one of the values for each copy. You will have 12 different grayscale images. You will also notice that the fewer steps you choose, the more banding that appears in the image. This is what you are after.
The next step is to combine all of the grayscale images into one document and output the file at the line count and halftone angle you normally use. If you know what your dot gain is, apply the correction curve to the document be-fore you output. Compare the finished film to what you see on screen. If your imagesetter is compressing the tone range, you will see banding in the higher level gradients.
In the final step, prepare screens as you normally would and print the image. Again, compare the print to the image on your computer monitor. You will notice that the tone range looks more complete when you posterize at lower levels, and you´ll be able to pick out the individual, discrete tonal steps. The goal is to move up on the tonal steps until the grayscale appears almost continuous. In most cases, this will be in the 64-, 81-, or 100-step range. Anything higher than that is internal resolution that you are not capable of reproducing.
Decreasing dot gain, altering the tone range of the original, or decreasing the halftone line count are the only ways to capture the lost steps. Each approach has its merits--you may end up using all of them. You simply have to understand the limitations and find the compromise that produces the best result with the least visual damage to the image. And you can possibly find a level of posterization that actually improves the way the image looks. That is the goal with this technique. The only real downside is the possibility of banding in very soft, very smooth, long gradients.
Once you have run this test, you will be able to determine which gradient gives you the best reproduction. Then it is simply a matter of posterizing your image to this level while it is in RGB mode. In doing so, you are selectively reducing the amount of information you will be reproducing, while simultaneously increasing the internal contrast of the image. Both of these changes will make the image more forgiving on press and easier to print.