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Maintaining Dot Predictability

(August 2002) posted on Fri Aug 30, 2002

Coudray discusses the concept of linearization and explains how to apply it in digital prepress.

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By Mark A. Coudray

In the past, I have devoted considerable time to discussing the causes and control of dot gain on press. My previous treatments of this subject focused mostly on the physical printing issues surrounding mesh selection, screen tension, press setup, ink, printing parameters, and substrate variances. But this month, I will focus on the fundamentals of linearizing digital prepress in order to achieve dot predictability on the final substrate.


Dot gain occurs in all printing processes, and each process and specific application is characterized with a unique dot-gain profile. In screen printing, to compensate for the inevitable gain on press, prepress functions such as film production and screenmaking must be adjusted and controlled.


The amount of prepress compensation you'll need to apply for a particular gain profile depends on how accurately you can maintain image characteristics throughout each stage of the printing process and repeat the procedure to achieve the same results. Without this control, it is not possible to make meaningful prepress adjustments that can be measured on press.


Linearization is another way of saying that the production process follows a linear, predictable flow from start to finish. It involves identifying variances throughout the process and correcting the variances to bring the image back into a linear progression. It means you are confident that the digital information you wish to reproduce is being faithfully and accurately moved along at each step of the prepress journey. In the end, you'll still experience dot gain or loss at each step, but the gain or loss will be consistent and predictable. And you'll be able to make corrections to the digital file to compensate for the dot gain or loss you expect to occur in screenmaking and on press.


Linearization is fundamental to achieving good halftone color reproduction. But optimizing your system for the best and most consistent results is somewhat of a mysterious process, especially since any correction you make influences another variable, which in turn affects another variable, and so on.


Two principles serve as the foundation for linearization. The first is that you can never assume the information on a film positive or screen is accurate. The second is that you must have some way to measure the process.



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