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Moiré Patterns and How They're Caused

A moiré (pronounced "more-ray") pattern is sometimes seen in printed materials. Moiré patterns come about when two halftone screen patterns come into conflict — something that both designers and printers want to avoid. There's a few common ways for this to happen.

A common method of causing a moiré pattern is by scanning an image that has been previously printed with a halftone screen. A typical example would be a printed photograph in a book or magazine. (One can tell if an image contains a screen by looking very closely with a loop or a magnifying glass.) If a screened image is scanned and then printed, there's a good chance a moiré pattern will appear on the image. The original screen in the image conflicts with the screen that the printer or imagesetter is using to render the image and creates a nasty optical pattern. A designer can sometimes get around this issue by blurring a scanned image with an image-editing program. Image clarity can be severely sacrificed with this method but it's often better than a moiré pattern.

Moirés can also be created through incorrect screen angle settings in color printing. Usually, page-layout programs, which are used to import photos into page layouts, set default angles which will print correctly. Occasionally, a printer will have their own preference of screen angles and a moiré can occur.

The standard process printing screen angles are Cyan = 105 degrees, Magenta = 75 degrees, Yellow = 90 degrees and Black = 45 degrees. The important thing to remember is that each color must have a distinctive screen angle of its own. These angles are unique and will help avoid an unpleasant moiré pattern.

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