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A moiré pattern is an interference pattern created when two grids are overlaid at an angle, or when they have slightly different mesh sizes.
 A moiré pattern, formed by two sets of parallel lines, one set inclined at an angle of 5° to the other The drawing on the right shows a moiré pattern. The lines could represent fibres in moiré silk, or lines drawn on paper or on a computer screen. The human visual system creates an imaginary pattern of roughly horizontal dark and light bands, the moiré pattern, that appears to be superimposed on the lines. More complex moiré patterns are created if the lines are curved or not exactly parallel.
The term originates from moire (or moiré in its French form), a type of textile, traditionally of silk but now also of cotton or synthetic fibre, with a rippled or 'watered' appearance.
Moiré patterns are often an undesired artifact of images produced by various digital imaging and computer graphics techniques, e.g. when scanning a halftone picture or ray tracing a checkered plane. This cause of moire is a special case of aliasing, due to undersampling a fine regular pattern.
In manufacturing industries, these patterns are used for studying microscopic strain in materials: by deforming a grid with respect to a reference grid and measuring the moiré pattern, the stress levels and patterns can be deduced. This technique is attractive because the scale of the moiré pattern is much larger than the deflection that causes it, making measurement easier.
Etymology
The history of the word moire is complicated. The earliest agreed origin is the Arabic-Persian mukhayyar, a cloth made from the wool of the Angora goat, from khayyana, 'he chose' (hence 'a choice, or excellent, cloth'). It has also been suggested that the Arabic word was formed from the Latin marmoreus, meaning 'like marble'. By 1570 the word had found its way into English as mohair. This was then adopted into French as mouaire, and by 1660 (in the writings of Samuel Pepys) it had been adopted back into English as moire or moyre. Meanwhile the French mouaire had mutated into a verb, moirer, meaning 'to produce a watered textile by weaving or pressing', which by 1823 had spawned the adjective moiré. Moire and moiré are now used interchangeably in English.
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