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The moiré phenomenon, often referred to as moiré patterns or moiré fringes, is a visual effect that occurs when two superimposed, often regular, patterns are slightly out of alignment. This misalignment can be in terms of rotation, spacing, or scale. The resulting visual distortion creates a new, often large-scale, pattern that is not present in either of the original patterns individually.

Moiré effects are common in everyday life. For instance, looking through two fine mesh screens or window blinds at an angle can produce moiré patterns. In digital imaging, moiré can appear when a digital camera's sensor grid interacts with a finely patterned subject, such as fabric or a screen. This is known as an aliasing artifact. In printing, if the halftone screens used for different colors are not precisely angled relative to each other, moiré patterns can emerge in the printed image.

The underlying principle is related to wave interference. When waves of similar wavelengths are superimposed with a slight difference, they produce constructive and destructive interference at regular intervals, leading to the appearance of a larger, repeating pattern. The frequency of the moiré pattern is related to the difference in frequencies of the original patterns.

While often considered an unwanted artifact, the moiré phenomenon can also be harnessed for useful purposes. In metrology, moiré fringes are used for precise measurements of displacement, strain, and shape. In textile design, moiré is a deliberate decorative effect achieved by weaving or pressing fabrics to create a lustrous, wavy pattern. Understanding the moiré effect is crucial in fields ranging from optics and engineering to graphic design and digital image processing.