[permalink] [id link]
The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon.
from
Wikipedia
Some Related Sentences
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 seen when using a laser pointer is another 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 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 laser
Laser speckle on a digital camera image from a green laser pointer.
A more detailed discussion on laser speckle reduction can be found in
Spatial coherence of laser beams also manifests itself as speckle patterns and diffraction fringes seen at the edges of shadow.
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.
speckle and light
The light at a given point in the speckle pattern is made up of contributions from the whole of the scattering surface.
UCA is used to reduce the speckle effects of dithering, especially with very light colors.
In the aperture masking technique, the bispectral analysis ( speckle masking ) method is typically applied to data taken through masked apertures, where most of the aperture is blocked off and light can only pass through a series of small holes ( subapertures ).
Although the signal-to-noise of speckle masking observations at high light level can be improved with aperture masks, the faintest limiting magnitude cannot be significantly improved for photon-noise limited detectors ( see Buscher & Haniff ( 1993 )).
Dark spots or blotches may speckle the back half of the body, and some individuals have alternating dark and light scales near the posterior, giving the impression of lateral bars.
The main challenge is to generate a flat field illumination for short time exposures and wavelengths, to avoid speckle ( in monochromatic light conditions ) and statistical fluctuations of the light stream that become most obvious at short integration times.
0.142 seconds.