Coudray discusses the art of managing dot gain and the science of manipulating dots in the midtone region to improve image clarity and quality.
Diamond and elliptical dots were designed to further support even tone transition. They seek to address the geometry of the midtone join and shift the values at which the corners connect. In these two designs, the halftone dot starts off round, transitions to diamond (Figure 3) or ellipse, and then transitions back to round, before becoming a solid value. Changing the shape of the dot allows us to connect two ends of the dot at a lower tonal value—usually in the 40% range. The dot continues to grow until the opposing axis corners join in the 60% region. The corners can join at almost any percentage value when we control the aspect ratio of the dot.
Besides the huge value of connecting at different tonal values, the geometry of the joining corners is now obtuse, meaning the angle formed by the connecting points is greater than 90°. This greater open angle is less prone to filling as the ink spreads. The result is a reduction of the midtone jump to near zero.
The chance of thread eclipsing increases as the dot flattens to a diamond or ellipse. These two shapes are particularly susceptible to moiré. The flatter dot also creates a more visually objectionable delineation of the regular halftone pattern. This effect can be particularly distressing in areas of large, uniform value or in areas of slow tonal change. Examples include a sky gradient in the first case and a flesh tone in the second. Carefully selecting the halftone-angle set helps minimize these visual disturbances.
Halftone-dot design is a highly technical undertaking that requires extensive knowledge of mathematics. The ability to engineer a specific dot shape or transition sequence involves equally extensive knowledge of the imaging process and the method by which the halftone is printed. It’s entirely possible that the ideal halftone dot (or dots) for screen printing has yet to be invented. Until that happens, we’re pretty much limited to working with diamond or elliptical dots in an attempt to minimize tonal jump, an issue that has been with us since we first used halftones in our printing process.
Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Specialty Graphic Imaging Association Int'l (SGIA) and as chairman of the Academy of Screen Printing Technology. Coudray has authored more than 250 papers and articles over the last 20 years, and he received the SGIA's Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine. He can be reached via e-mail at firstname.lastname@example.org.
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