The following screen angle sets are all valid and are in common use. However, if certain jobs have images where two of the process colors predominate and where those two colors are less than 30 degrees apart, then that screen set should be avoided and a different one used instead. Most printers will have a standard screen angle set that is used for all their jobs. To avoid this problem, some vendors utilize shifted angles of 7.5° to introduce "noise" around the edges of the dots in order to break up and eliminate the visibility of single channel moiré. At angles other than 0° and 45° a type of moiré patterning within one screen "single channel moiré" may occur. These standard screen angles are based on analog photomechanical screens and do not work best with electronic screens. Right: Yellow at a higher frequency to help reduce visible moiré. Left: Yellow at the same frequency as Cyan. To help reduce the visibility of the Y/C moiré, most screening systems run the Yellow at a slightly higher frequency (lpi) – typically 108% of the frequency of the C, M, and K printers. The visibility of the moiré can be exacerbated if the Yellow becomes contaminated by ink traveling into it from previous press units. They also form the least objectionable moiré – the rosette pattern ( more on rosettes here).īecause the Yellow printer is only 15° from the Cyan printer it produces moiré. These angles represent a best all around compromise for most pictures and represent the standard, most commonly used screen angles. The cyan and magenta are then placed between these two. Then the most visible color, black, is placed at 45°. First the least visible color, yellow, is placed at the most visible angle 0° (90°). However, in four color process printing, four different screens must be superimposed and all four screens must be angled within the 90° limitation.Ī set of standard screen angles has been established that is based on a combination of theory and experience. The best angle between two screens that is least likely to cause moiré, and is most forgiving to small degrees of error, is 45°.
Below are two overlaid halftone grids angled at 5 degrees and 10 degrees apart with the resulting moiré pattern: 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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