Speckle pattern
As a speckle pattern, Lichtgranulation or short the granular speckle interference phenomena are referred to, which can be at a sufficiently coherent illumination optically rough object surfaces observed ( bumps in the order of the wavelength ).
The term speckle, which is available both for a single light spot as well as for the entire interference pattern is derived from speckle (English for speckles, spots, stains). Depending on the imaging system used will also distinguished in the predominantly English-language literature between subjective and objective speckle: one forms the speckle without the aid of a lens or other optical devices directly on a screen from, we speak of objective speckle. In contrast, it is the subjective speckle at the image of the interference pattern with the aid of a lens or more complex optical systems - this also includes the human eye.
Formation
Plausible is the speckle formation when the unevenness of the illuminated surface can be considered as scattering centers which give rise to spherical waves of different phase, which interfere in the far field. This creates a spatial structure with randomly distributed intensity minima and maxima. As three-dimensional interference pattern, there are longitudinal and transverse speckle, which depend on the respective longitudinal and transverse coherence. Transversal speckle have a greater distance greater significance because, the individual spherical wave components simplify more and more as plane waves. The speckle contrast can therefore be used as a measure of consistency.
Appearance
When viewed from speckle to a coherently illuminated surface with the naked eye you take a moving pattern true. This is caused by the different interference on the retina of the eye movement. The direction of movement is dependent on the position of the focal plane relative to the retina - not short-sighted and far-sighted eyes akkommodierte persons will perceive different directions of movement. In addition to the brilliance speckle is perceived as a typical laser property.
As you can see only the time- integrated interference with a relatively slow for the period of oscillation of the light detector, the speckle size decreases with larger bandwidth, even if there are still speckle within a narrow spectral range. The effect of transverse coherence in the speckle can be suppressed in this manner. In spectral decomposition of white ( longitudinally incoherent ) ultra-short pulse laser light therefore appear across the spectrum speckle, although these are not perceived in the white light, whereas the highly incoherent white light of an incandescent lamp always appears with no visible phase contrast.
Speckle outside the visible spectrum
The emergence of speckles is not limited to optical wavelength, but also occurs in other regions of the electromagnetic spectrum, and wherein the interference of sound waves. It can even be in particle streams that have just as wave properties, proven. In radar, the phenomenon is also known under the name of glitter.
Application
The surface has no systematic structures, so the speckle pattern is completely random. Is there a globally repetitive fine structure, the local diffraction or scattering effects add up constructively, which is exploited in the X-ray crystallography to determine structures in the wavelength.
Macroscopic changes (eg scratches, surface structure ) have an impact on the overall picture and are larger than the individual speckle in the image. This circumstance is also makes use of the measurement technique where laser speckle ( ESPI ) are used for analysis of vibration, mechanical stresses and for the detection of submicroscopic surface irregularities in the electronic speckle pattern interferometry. In addition to other classical application areas such as the Stellar speckle interferometry and the Speckleholographie also " exotic" applications such as digital fingerprinting of documents have been shown in recent years.
Suppression
Not only in the field of holography, but in almost all applications that are based on the use of coherent waves, including ultrasonography and optical coherence tomography, which Signal-/Bildqualität is greatly reduced by the presence of speckle. Usually speckle is removed by changing the phase conditions: modulation of the reference surface through prolonged integration time or summation of mutually phase- modulated multiple shots. However, numerical methods of the multiplicative noise suppression are applied.