Moiré pattern

The moiré effect (of French moirer [ mwaʀe ], " moirieren; marbling " ) makes itself felt in the superposition of periodic fine screens with additional apparent coarse grid. These resulting patterns whose appearance is similar to the patterns of interference are a special case of aliasing effect by sub-sampling.

Declarations and occurrences

Causes are:

In multi-color process printing moiré effects are a known bug. The screen then do not have the same pitch, and / or the individual prints do not meet exactly one above the other.

When you print, in television, scanning and other imaging screening processes occur moire effects when the object itself is fine rasterized (clothing materials) or if the object is already a raster or bitmap.

Moiré effect in line grids

Planar grids are generally grid shaped, that is, 2 -dimensional. The grid lines is the general reduction of the grid in the one - dimensionality.

Examples

Image 2 ( 2 causes)

Picture 3

Picture 4

Picture 5

• Figure 1: Moire patterns occur when two raster lines twisted with the same pitch are stacked ( division 4 pixels, twisting 2 °, dividing the apparent grid about 115 pixels).
• Figure 2: Two superimposed lines grid show long-period brightness modulations when the little divisions vary ( division of the left screen 4 pixels, of the right 0.95 × 4 pixels, the apparent mean 76 pixels).
• Figure 3: Examples of moiré effects in screening of images: can ( here Portrait of Sarah Bernhardt ) arise. The original image is inserted as a reduced halftone image. The big picture top left is the initial screening ( semitone → height). If the raster image scaled to a new raster image created moire lines that overlay the image ( Raster → Raster ). The picture above right was reduced by 1%, the image including by 20%. In comparison, the raster image shows no disturbances at the bottom left, which resulted from a reduced to 20 % halftone image ( halftone → height).
• Figure 4: Electron micrograph of graphite. The resolution is too low to detect the vertical in the image extending basal levels ( one above the other in the object -line grid of about 0.3 nm pitch). But it looks dark horizontally running bands, which arise from a moiré superposition slightly tilted planes.
• Figure 5: Digital photograph of Castle Lötzen. Here, the periodic structures of the image converter overlap with those of the brick pattern, a relatively common problem.

Applications

The effect is used as a design element, for example, the case of fabrics and papers ( see moire ).

Due to the superposition of appropriate structures (eg line patterns of different period) on transparent supports the shift of the carrier can be calculated to each other by the moire effect at constant resolution with higher accuracy. This method is used in photolithography for aligning the mask and wafer.

Also the vernier for accurate length determination with a caliper works on this principle. Two line patterns of different period abut. In the measurement, the best alignment lines - pair is evaluated.

A playful application is shown on the left moire clock. Minute display: A perforated black hour disc rotates over a black dial with white bars. The rotating moire pattern simulating the missing minute hand. Second indication: the hour disc of the clock a minute disc rotates. The analog resulting moiré pattern simulating the missing seconds pointer.

Due to a higher-level structure in at least one of the two line patterns are also changing characters can be generated. One example is the so-called moiré beacon, in which the initially uniform image merges with change of perspective in an arrow-like display. The arrow indicates that the ship has deviated from the direct course towards the fire, and has to be corrected on that side.

Another example of a higher-order structure in one of the two line patterns is a digital sundial ( right). The sunlight changes throughout the day his direction. The two- line pattern translucent light generates a digital time display that changes every five minutes.