Friday, January 20, 2012

AM screening - 150 lpi Elliptical Halftone Dot

[click "Play" to view animation - may take a moment to buffer]

Note that the dots are not exactly the same at each percentage. In part, this is to avoid single channel moiré. Also note that in the very light tones (as well as in the darkest tones) as you go through the tones, sometimes there are dots one pixel is size with gaps between them which are then fill with more one pixel sized dots. Then slowly some of the single pixel dots become two pixel dots mixed in with the single pixel ones. This is because a RIP will only image full individual pixels to form a halftone dot. So, in the case of a 2,540 dpi device, each pixel is 10 microns in size (10.6 microns for a 2,400 dpi device). Therefore, if, for example, the the dot diameter needs to be 15 microns in size, since the RIP cannot image a half a pixel to achieve 15 microns, instead the RIP will alternate between 1 pixel dots (10 micron) and 2 pixel dots (20 micron) which results in an effective 15 micron dot average for that tone value.

The elliptical dot shape attempts to avoid the "optical bump" at 50% seen with traditional Euclidean (round/square/round) dots by splitting the point at which dots first touch to 40% and 60% rather than solely at 50%. While it's commonly used, it is not an optimal dot shape in a CtP environment, due to the fact that individual dot shapes are different at each screen angle and that they are directional in nature. When used at low frequencies (

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