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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

Measurement begins by working backwards from the final print. This helps you get a mental picture of what is happening at each step. The following is a good generalization of a typical screen-printing image profile:


1. Final print: Dot loss may appear in highlight and quarter-tone areas; gain in mid tones, three-quarter tones, and shadows.


2. Screenmaking: Dot loss usually occurs in highlights, with less dot loss across the remaining tonal range. Loss is due to thread eclipsing and light undercutting during exposure.


3. Film output: Dots can exhibit loss or gain depending on a number of factors. Often, dots may appear consistent across most of the tonal range, but highlights drop and shadows plug.


4. Monitor: The image displayed on screen may appear as if it has experienced dot loss or gain. Factors that influence this virtual gain include gamma, white point, black point, and color temperature of the monitor, as well as ambient light levels in the design area.


5. Digital file: You may face loss or gain that is built into the original digital file, depending on how it was acquired or adjusted to compensate for its appearance on the computer monitor.


Compensating for dot gain or loss


In prepress, you must make sure that what you are doing is stable and predictable. If it is not, the next step will either cancel, or add, to the error. More importantly, you will have no confidence in the accuracy of the tonal reproduction at any later step in the process. It is difficult enough to get it right at the screenmaking and printing stages; if the other steps in the process are unpredictable, your chances of success are less than at a craps table in Vegas.


For the sake of this discussion, I will not spend any time discussing dot defects in the original digital file. Instead, I'll start with the monitor. First, make sure that no direct light falls on the monitor face. Working in subdued daylight is the best. Work areas with high levels of ambient light will cause the monitor to represent images inaccurately. Next, verify that the monitor's representation of white is pure, with an RGB value of 255, 255, 255, and that pure black gives a value of 0, 0, 0.



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