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The speckle effect is observed when radio waves are scattered from rough surfaces such as ground or sea, and can also be found in ultrasonic imaging.
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speckle and effect
The speckle effect is a result of the interference of many waves of the same frequency, having different phases and amplitudes, which add together to give a resultant wave whose amplitude, and therefore intensity, varies randomly.
When lasers were first invented, the speckle effect was considered to be a severe drawback in using lasers to illuminate objects, particularly in holographic imaging because of the grainy image produced.
It was later realized that speckle patterns could carry information about the object's surface deformations, and this effect is exploited in holographic interferometry and electronic speckle pattern interferometry.
The speckle effect is also used in stellar speckle astronomy, speckle imaging and in eye testing using speckle.
For many years telescope performance was limited by this effect, until the introduction of speckle interferometry and adaptive optics provided paths to remove this limitation.
speckle and is
The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon.
The speckle pattern which is seen when using a laser pointer is another diffraction phenomenon.
An example of deleterious mechanically induced modulation is speckle noise created in a multimode fiber by an imperfect splice or imperfectly mated connectors.
This is a subjective speckle pattern.
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.
A less familiar example of speckle is the highly magnified image of a star through imperfect optics or through the atmosphere ( see speckle imaging ).
A speckle pattern can also be seen when sunlight is scattered by a fingernail.
When we observe an illuminated surface, we detect the average energy of the light at the surface ; thus the brightness of a given point on a surface which has been illuminated by a set of random scatterers with a single frequency, is constant over time, but varies randomly from point to point, i. e. it is a speckle pattern.
If light of low coherence ( i. e. made up of many wavelengths ) is used, a speckle pattern will not normally be observed, because the speckle patterns produced by individual wavelengths have different dimensions and will normally average one another out.
When an image is formed of a rough surface which is illuminated by a coherent light ( e. g. a laser beam ), a speckle pattern is observed in the image plane ; this is called a “ subjective speckle pattern ” – see image above.
It is called " subjective " because the detailed structure of the speckle pattern depends on the viewing system parameters ; for instance, if the size of the lens aperture changes, the size of the speckles change.
speckle and observed
However, speckle patterns can be observed in polychromatic light in some conditions.
The change in speckle size with lens aperture can be observed by looking at a laser spot on a wall directly, and then through a very small hole.
Although it was observed to be binary by speckle interferometry in 1985, subsequent observations show no sign of binarity, and the detection appears to have been spurious.
speckle and when
< div style =" float: right ; width: 160px ; margin: 0 0 0 10px ; clear: right ;">< gallery > Image: Eps_aql_movie_not_2000. gif | Slow-motion speckle imaging movie, showing what you see through a telescope when you look at a star at high magnification ( negative images ).
speckle and waves
An analogy with water waves may help to understand the speckle phenomenon.
This setup is used to avoid speckle noise form being generated from interference of the two waves within the scattering medium, which would occur if they were both propagated through the medium.
speckle and are
A photograph was then taken of the speckle pattern formed on the wall ( strictly speaking, this also has a second subjective speckle pattern but its dimensions are much smaller than the objective pattern so it is not seen in the image )
In one technique called shift-and-add ( also called image stacking ), the short exposure images are lined up by the brightest speckle and averaged together to give a single output image.
speckle and scattered
When laser light which has been scattered off a rough surface falls on another surface, it forms an “ objective speckle pattern ”.
If a photographic plate or another 2-D optical sensor is located within the scattered light field without a lens, a speckle pattern is obtained whose characteristics depend on the geometry of the system and the wavelength of the laser.
The speckle pattern in the figure was obtained by pointing a laser beam at the surface of a mobile phone so that the scattered light fell onto an adjacent wall.
speckle and from
A laser shining into the mixture produces a speckle pattern that results from the motion of the particles.
Laser speckle on a digital camera image from a green laser pointer.
In the output of a multi-mode optical fiber, a speckle pattern results from a superposition of mode field patterns.
The light at a given point in the speckle pattern is made up of contributions from the whole of the scattering surface.
The fact that many of the speckle imaging methods have multiple names results largely from amateur astronomers re-inventing existing speckle imaging techniques and giving them new names.
By shining a laser ( whose smooth wavefront is an excellent simulation of the light from a distant star ) on a surface, the resulting speckle pattern can be processed to give detailed images of flaws in the material.
In 1970 the French astronomer Antoine Labeyrie showed that information could be obtained about the high-resolution structure of the object from the speckle patterns using Fourier analysis ( speckle interferometry ).
In the 1980s methods were developed which allowed images to be reconstructed interferometrically from these speckle patterns.
One more recent type of speckle interferometry called speckle masking involves calculation of the bispectrum or closure phases from each of the short exposures.
Image: Zeta bootis short exposure. png | Typical short-exposure image of this binary star from the same dataset, but without using any speckle processing.
In particular, the speckle noise in an HPIV recording often prevents traditional image-based correlation methods from being used.
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes.
It has since been augmented by a large number of measurements, from Hipparcos and Tycho, speckle interferometry, and other sources.
speckle and such
Other methods can achieve resolving power exceeding the limit imposed by atmospheric seeing, such as speckle imaging, aperture synthesis, lucky imaging, and space telescopes, such as the Hubble Space Telescope.
While it may provide less anatomical detail than techniques such as CT or MRI, it has several advantages which make it ideal in numerous situations, in particular that it studies the function of moving structures in real-time, emits no ionizing radiation, and contains speckle that can be used in elastography.
# It causes the images of point sources ( such as stars ), which in the absence of atmospheric turbulence would be steady Airy patterns produced by diffraction, to break up into speckle patterns, which change very rapidly with time ( the resulting speckled images can be processed using speckle imaging )
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