Moiré pattern

Etymology

The term originates from moire (moiré in its French adjectival form), a type of textile, traditionally of silk but now also of cotton or synthetic fiber, with a rippled or "watered" appearance.

The history of the word moiré is complicated. The earliest agreed origin is the Arabic mukhayyar (مُخَيَّر in Arabic, which means chosen), a cloth made from the wool of the Angora goat, from khayyara (خيّر in Arabic), "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 (pronounced "mwar") and moiré (pronounced "mwar-ay") are now used somewhat interchangeably in English, though moire is more often used for the cloth and moiré for the pattern.

"Watered textile" refers to laying part of the textile on top of another part, and pressing the two layers when wet. The similarity of the spacing of individual threads (warp and weft), which is, however, not perfect spacing, creates characteristic patterns when the layers are pressed together; when dry, the patterns remain.

In the liquid crystal display industry, moiré is often referred to by the Japanese word mura, which is roughly translates to "unevenness; irregularity; lack of uniformity; nonuniformity; inequality."

Pattern formation

Moiré patterns are often an artifact of images produced by various digital imaging and computer graphics techniques, for example when scanning a halftone picture or ray tracing a checkered plane (the latter being a special case of aliasing, due to undersampling a fine regular pattern). This can be overcome in texture mapping through the use of mipmapping and anisotropic filtering.

The drawing on the upper right shows a moiré pattern. The lines could represent fibers in moiré silk, or lines drawn on paper or on a computer screen. The nonlinear interaction of the optical patterns of lines creates a real and visible pattern of roughly parallel dark and light bands, the moiré pattern, superimposed on the lines.

More complex line moiré patterns are created if the lines are curved or not exactly parallel. Moiré patterns revealing complex shapes, or sequences of symbols embedded in one of the layers (in form of periodically repeated compressed shapes) are created with shape moiré, otherwise called band moiré patterns. One of the most important properties of shape moiré is its ability to magnify tiny shapes along either one or both axes, that is, stretching. A common 2D example of moiré magnification occurs when viewing a chain-link fence through a second chain-link fence of identical design. The fine structure of the design is visible even at great distances.

The moiré effect also occurs between overlapping transparent objects. For example, an invisible phase mask is made of a transparent polymer with a wavy thickness profile. As light shines through two overlaid masks of similar phase patterns, a broad moiré pattern occurs on a screen some distance away. This phase moiré effect and the classical moiré effect from opaque lines are two ends of a continuous spectrum in optics, which is called the universal moiré effect. The phase moiré effect is the basis for a type of broadband interferometer in x-ray and particle wave applications. It also provides a way to reveal hidden patterns in invisible layers.

Geometrical approach

Let us consider two patterns made of parallel and equidistant lines, e.g., vertical lines. The step of the first pattern is p, the step of the second is p + δp, with 0 < δ < 1.

If the lines of the patterns are superimposed at the left of the figure, the shift between the lines increase when going to the right. After a given number of lines, the patterns are opposed: the lines of the second pattern are between the lines of the first pattern. If we look from a far distance, we have the feeling of pale zones when the lines are superimposed (there is white between the lines), and of dark zones when the lines are "opposed".

The middle of the first dark zone is when the shift is equal to p/2. The nth line of the second pattern is shifted by n δp compared to the nth line of the first network. The middle of the first dark zone thus corresponds to

that is

The distance d between the middle of a pale zone and a dark zone is

the distance between the middle of two dark zones, which is also the distance between two pale zones, is

From this formula, we can see that:

The principle of the moiré is similar to the Vernier scale.

Mathematical function approach

The essence of the moiré effect is the (mainly visual) perception of a distinctly different third pattern which is caused by inexact superimposition of two similar patterns. The mathematical representation of these patterns is not trivially obtained and can be somewhat arbitrary. In this section we shall give a mathematical example of two parallel patterns whose superimposition forms a moiré pattern, and show one way (of many possible ways) these patterns and the moiré effect can be rendered mathematically.

The visibility of these patterns is dependent on the medium or substrate in which they appear, and these may be opaque (as for example on paper) or transparent (as for example in plastic film). For purposes of discussion we shall assume the two primary patterns are each printed in greyscale ink on a white sheet, where the opacity (e.g., shade of grey) of the "printed" part is given by a value between 0 (white) and 1 (black) inclusive, with 1/2 representing neutral grey. Any value less than 0 or greater than 1 using this grey scale is essentially "unprintable".

We shall also choose to represent the opacity of the pattern resulting from printing one pattern atop the other at a given point on the paper as the average (i.e. the arithmetic mean) of each pattern's opacity at that position, which is half their sum, and, as calculated, does not exceed 1. (This choice is not unique. Any other method to combine the functions that satisfies keeping the resultant function value within the bounds [0,1] will also serve; arithmetic averaging has the virtue of simplicity—with hopefully minimal damage to one's concepts of the printmaking process.)

We now consider the "printing" superimposition of two almost similar, sinusoidally varying, grey-scale patterns to show how they produce a moiré effect in first printing one pattern on the paper, and then printing the other pattern over the first, keeping their coordinate axes in register. We represent the grey intensity in each pattern by a positive opacity function of distance along a fixed direction (say, the x-coordinate) in the paper plane, in the form

where the presence of 1 keeps the function positive definite, and the division by 2 prevents function values greater than 1.

The quantity k represents the periodic variation (i.e., spatial frequency) of the pattern's grey intensity, measured as the number of intensity cycles per unit distance. Since the sine function is cyclic over argument changes of 2π, the distance increment Δx per intensity cycle (the wavelength) obtains when k Δx = 2π, or Δx = 2π/k.

Consider now two such patterns, where one has a slightly different periodic variation from the other:

such that k₁ ≈ k₂.

The average of these two functions, representing the superimposed printed image, evaluates as follows:

where it is easily shown that

and

This function average, f₃, clearly lies in the range [0,1]. Since the periodic variation A is the average of and therefore close to k₁ and k₂, the moiré effect is distinctively demonstrated by the sinusoidal envelope "beat" function cos(Bx), whose periodic variation is half the difference of the periodic variations k₁ and k₂ (and evidently much lower in frequency).

Other one-dimensional moiré effects include the classic beat frequency tone which is heard when two pure notes of almost identical pitch are sounded simultaneously. This is an acoustic version of the moiré effect in the one dimension of time: the original two notes are still present—but the listener's perception is of two pitches that are the average of and half the difference of the frequencies of the two notes. Aliasing in sampling of time-varying signals also belongs to this moiré paradigm.

Rotated patterns

Let us consider two patterns with the same step p, but the second pattern is rotated by an angle α. Seen from afar, we can also see darker and paler lines: the pale lines correspond to the lines of nodes, that is, lines passing through the intersections of the two patterns.

If we consider a cell of the lattice formed, we can see that it is a rhombus with the four sides equal to d = p/sin α; (we have a right triangle whose hypotenuse is d and the side opposite to the angle α is p).

The pale lines correspond to the small diagonal of the rhombus. As the diagonals are the bisectors of the neighbouring sides, we can see that the pale line makes an angle equal to α/2 with the perpendicular of each pattern's line.

Additionally, the spacing between two pale lines is D, half of the long diagonal. The long diagonal 2D is the hypotenuse of a right triangle and the sides of the right angle are d(1 + cos α) and p. The Pythagorean theorem gives:

that is:

thus

When α is very small (α < π/6) the following small-angle approximations can be made:

thus

We can see that the smaller α is, the farther apart the pale lines; when both patterns are parallel (α = 0), the spacing between the pale lines is infinite (there is no pale line).

There are thus two ways to determine α: by the orientation of the pale lines and by their spacing

If we choose to measure the angle, the final error is proportional to the measurement error. If we choose to measure the spacing, the final error is proportional to the inverse of the spacing. Thus, for the small angles, it is best to measure the spacing.

Printing full-color images

In graphic arts and prepress, the usual technology for printing full-color images involves the superimposition of halftone screens. These are regular rectangular dot patterns—often four of them, printed in cyan, yellow, magenta, and black. Some kind of moiré pattern is inevitable, but in favorable circumstances the pattern is "tight"; that is, the spatial frequency of the moiré is so high that it is not noticeable. In the graphic arts, the term moiré means an excessively visible moiré pattern. Part of the prepress art consists of selecting screen angles and halftone frequencies which minimize moiré. The visibility of moiré is not entirely predictable. The same set of screens may produce good results with some images, but visible moiré with others.

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.

Television screens and photographs

Moiré patterns are commonly seen on television screens when a person is wearing a shirt or jacket of a particular weave or pattern, such as a houndstooth jacket. This is due to interlaced scanning in televisions and non-film cameras, referred to as interline twitter. As the person moves about, the Moiré pattern is quite noticeable. Because of this, newscasters and other professionals who appear on TV regularly are instructed to avoid clothing which could cause the effect.

Photographs of a TV screen taken with a digital camera often exhibit moiré patterns. Since both the TV screen and the digital camera use a scanning technique to produce or to capture pictures with horizontal scan lines, the conflicting sets of lines cause the moiré patterns. To avoid the effect, the digital camera can be aimed at an angle of 30 degrees to the TV screen.

Marine navigation

The Moiré effect is used in shoreside beacons to mark underwater hazards (usually pipelines or cables). The Moiré effect creates arrows that 'point' towards an imaginary line marking the hazard; as navigators pass over the hazard, the arrows on the beacon appear to become vertical bands before 'changing' back to arrows pointing in the reverse direction. An example can be found in the UK on the East shore of Southampton water, opposite Fawley oil refinery (50°51′21.63″N 1°19′44.77″W). Similar Moiré effect beacons can be used to guide mariners to the centre point of an oncoming bridge; when the vessel is aligned with the centreline, vertical lines are visible.

Strain measurement

The moiré effect can be used in strain measurement: the operator just has to draw a pattern on the object, and superimpose the reference pattern to the deformed pattern on the deformed object.

A similar effect can be obtained by the superposition of an holographic image of the object to the object itself: the hologram is the reference step, and the difference with the object are the deformations, which appear as pale and dark lines.

See also: theory of elasticity, strain tensor and holographic interferometry.

Image processing

Some image scanner driver programs provide an optional filter, called a "descreen" filter, to remove Moiré-pattern artifacts which would otherwise be produced when scanning printed halftone images to produce digital images.

Banknotes

Many banknotes exploit the tendency of digital scanners to produce moiré patterns by including fine circular or wavy designs that are likely to exhibit a moiré pattern when scanned and printed.

Microscopy

In super-resolution microscopy, the Moiré pattern can be used to obtain images with a resolution higher than the diffraction limit, using a technique known as structured illumination microscopy.

In scanning tunneling microscopy, Moiré fringes appears if surface atomic layers have a different crystal structure than the bulk crystal. This can for example be due to surface reconstruction of the crystal, or when a thin layer of a second crystal, e.g. graphene is on the surface.

In transmission electron microscopy, Moiré fringes can be seen when two overlapping crystals are imaged, e.g. thin films. The Moiré indicates relative orientation and lattice mismatch of the two crystals.