Moiré Patterns

EluZions.com Illusions Moire :

Moiré fringes are an interference pattern that is formed when two similar grid-like patterns are superimposed. They create a pattern of their own that does not exist in either of the originals. The result is a series of fringe patterns that change shape when as the grids are moved relative to each other.

The word Moiré was first used by weavers and comes from the word mohair, a kind of cloth made from the fine hair of an Angora goat. Artists hate moiré patterns because they can sometimes ruin a printed photo or drawing. But they can also be great fun.

One of the more common occurrences of moiré patterns is found in computer monitors and television sets where we see an ordered wavy pattern is superimposed over the screen in a series of ripples. Moiré patterns also are a common problem in scanned images due to interference between the printed dot pattern and the reflection of the image.

This can happen, for example, when scanning large areas of what seems to be a single colour. Sometimes what we perceive is a block of colour, is actually a great many dots of ink. When printed, small random variations in value can occur due to the imperfections of print technology. The grid pattern of the scanning process combined with these almost-invisible random ink patterns can produce unexpected results. This is often in the form of "banding". Scanning an image slightly off "square" can increase the likelihood of this happening.

The basic pattern consists of lines radiating out from a common center. One copy of this pattern is fixed, and the other drifts about, creating a changing interference pattern. You can start and restart the applet by shift-clicking on the pattern. You can also click-and-drag to control the motion of the pattern yourself.