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The speckle pattern which is seen when using a laser pointer is another diffraction phenomenon.
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speckle and pattern
A laser shining into the mixture produces a speckle pattern that results from the motion of the particles.
The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon.
This is a subjective speckle pattern.
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.
A speckle pattern can also be seen when sunlight is scattered by a fingernail.
In the output of a multi-mode optical fiber, a speckle pattern results from a superposition of mode field patterns.
If the relative modal group velocities change with time, the speckle pattern will also change with time.
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.
If the position of the imaging system is altered, the pattern will gradually change and will eventually be unrelated to the original speckle pattern.
A photograph of an objective speckle pattern.
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.
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 )
speckle and which
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.
The " size " of the speckles is a function of the wavelength of the light, the size of the laser beam which illuminates the first surface, and the distance between this surface and the surface where the speckle pattern is formed.
Rotating diffusers — which destroys the spatial coherence of the laser light — can also be used to reduce the speckle.
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 )
In the 1980s methods were developed which allowed images to be reconstructed interferometrically from these speckle patterns.
Unfortunately, the phase differences between adjacent image picture elements (" pixels ") also produce random interference effects called " coherence speckle ", which is a sort of graininess with dimensions on the order of the resolution, causing the concept of resolution to take on a subtly different meaning.
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.
Another issue is the presence of substantial speckle noise which lowers the overall signal-to-noise ratio of particle images.
speckle and is
An example of deleterious mechanically induced modulation is speckle noise created in a multimode fiber by an imperfect splice or imperfectly mated connectors.
A less familiar example of speckle is the highly magnified image of a star through imperfect optics or through the atmosphere ( see speckle imaging ).
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.
speckle and seen
Spatial coherence of laser beams also manifests itself as speckle patterns and diffraction fringes seen at the edges of shadow.
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 using
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.
The speckle effect is also used in stellar speckle astronomy, speckle imaging and in eye testing using speckle.
In 1976, component A was itself discovered to be a binary star, using speckle interferometry and the 2. 1-meter telescope at the Kitt Peak National Observatory.
A search for a faint companion using the Hubble Space Telescope Wide Field Planetary Camera revealed nothing, nor did a search with near infrared speckle interferometry.
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 ).
All of these were obtained using speckle imaging and have higher resolution than can be obtained with e. g. the Hubble Space Telescope:
Image: Zeta bootis short exposure. png | Typical short-exposure image of this binary star from the same dataset, but without using any speckle processing.
This star has been examined for an orbiting companion using speckle interferometry in the near infrared part of the spectrum.
speckle and laser
Laser speckle on a digital camera image from a green laser pointer.
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.
A more detailed discussion on laser speckle reduction can be found in
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.
Optical engineering metrology uses optical methods to measure micro-vibrations with instruments like the laser speckle interferometer or to measure the properties of the various masses with instruments measuring refraction.
However, high speckle noise along with thermal instability in the image remains a major challenge, primarily due to the pumped green laser.
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