Coudray explains how following time-honored recommendations for pairing halftone line coutns can lead to undesirable printed results.
Halftone orientation affects how many dots we can stack in a given area. When the dots are oriented at 90°, a vertical stack, the frequency is the stated value. At 65 lines/in., we would stack 65 vertical dots. As the angle is rotated, however, the shape and orientation of the dots also changes, reaching the maximum amount of change at 45°. In order to maintain the same frequency of 65 line/in. at this angle, the actual dot would have to be equivalent to a 74 line/in. dot at 90°. In other words, the dot's size would have to be reduced by 70.72% to fit at 65 lines/in.
In reality, when a 65-line/in. halftone is rotated 45°, the result is the equivalent of a 46 line/in. stack, which represents an increase in the diagonal of 141% over the original dot size. For printers, it's important to realize that the real, effective frequency is different for each angle in the halftone sequence. This frequency change is what causes the familiar rosette that forms when process-color halftones are correctly printed in relation to each other.
Now, let's move on to the mesh count itself. This is another item that is very elusive and constantly changing. Our first point of consideration is the accuracy of the thread count. You should understand from the start that all mesh is woven to metric standards, and the values we see are converted to English and rounded to the nearest five threads/ in. For example, our 305-threads/in. mesh is really 120 threads/cm. The conversion to English is 2.54 x 120 = 304.8, which means our approximation is pretty accurate. Some mesh counts aren't reported so accurately, such as our 390 threads/in. mesh, which is really 155 threads/cm and converts to 393.7, a figure that is then rounded down.
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