The most serious moiré patterns occur at very small angles between screens.
When two (or more) screens are printed on top of each another, a visually objectionable pattern known as moiré may occur. For that reason, black and white halftones are usually printed with 45° angled screens – particularly with coarser screens. The screen pattern is very noticeable when positioned at 0° and is least visible when rotated 45° as illustrated below. For example, a screen rotated 15° is at the same angle as 105°, 195°, and 285°.Ī black and white halftone image consists of a single screen.
The screen can only be rotated up to 90° before it repeats itself. The rotation angle away from the vertical axis is known as the screen angle. When the screen is aligned parallel to the paper edges, the screen angle is said to be 0° or 90°. The vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. With the exception of FM (stochastic screening), all screens consist of dots arranged in a regular pattern or matrix.
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