Moire Patterns are patterns of wavy lines, bars, geometric patterns, or dots that appear when two partially transparent patterns are superimposed. The Moire patterns are caused by images that overlap to different extents in different places, thus causing some areas of what were intended to be uniformly shaded areas to have more area visible than other areas and appear darker. They were once used by weavers to compare fabrics and the name itself is a corruption of the word 'mohair'.

The following website has good illustrations of Moire patterns:

Moire patterns can occur when computer input or output is resolved into individual dots. They can be interesting and attractive, but not when they appear on your monitor or printer in areas that should have uniform color and intensity. They appear on computer monitors because the raster image pattern painted by the electron tube in a CRT is superimposed on the color discriminating pattern from the shadow mask or tension mask. They can appear in scanned images devices if the source material is composed of discrete dots as with half tone printing because of the interaction with the scanning pixel pattern.

In principle, Moire patterning will occur whenever the superimposed patterns are not identical. They may not be noticeable if one pattern has much finer resolution than the other. In practice, the worst problems seem to occur when the pattern frequencies are similar, but are not integer multiples.

Moire patterns should not be confused with interference patterns caused by undesired signals being picked up by the monitor cables. The two may be similar in appearance, but interference patterns can be altered by better shielding or reorienting the signal cable whereas Moire patterning can not.

It is possible for Moire patterning to be worse with some colors than others. Use of high resolutions and sharply focused electron/scan beams can exacerbate Moire patterning. Deliberate defocusing of monitors is sometimes used to reduce Moire patterning. Similar problems with imaging can sometimes be reduced by rotating the material being imaged or by the use a "Gaussian Blurring" (a sort of deliberate defocusing) or "descreening" filters during processing. Experimentation with scan rates can help with imaging. Turning up brightness and or contrast on monitors can reduce the severity of Moire patterning by defocusing the electron beam a bit. Use of larger numbers of colors tends to reduce Moire affects caused by interaction of dots of identical color used for dithering.

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