Speckle pattern

specklelaser specklespeckle noiselaser speckle imagelaser speckle imagesLaser Speckle Velocimetrynear field specklesspecklessubjective speckle pattern
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.wikipedia
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Wave interference

interferencedestructive interferenceconstructive interference
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.
Prime examples of light interference are the famous double-slit experiment, laser speckle, anti-reflective coatings and interferometers.

Laser

laserslaser beamlaser light
This phenomenon has been investigated by scientists since the time of Newton, but speckles have come into prominence since the invention of the laser and have now found a variety of applications.
Diffuse reflection of a laser beam from a matte surface produces a speckle pattern with interesting properties.

Diffraction

diffraction patterndiffractdiffracted
When a surface is illuminated by a light wave, according to diffraction theory, each point on an illuminated surface acts as a source of secondary spherical waves.
The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon.

Holography

hologramholographicholograms
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 resulting pattern is the sum of all these 'zone plates', which combine to produce a random (speckle) pattern as in the photograph above.

Eye testing using speckle

The speckle effect is also used in speckle imaging and in eye testing using speckle.
When a surface is illuminated by a laser beam and is viewed by an observer, a speckle pattern is formed on the retina.

Dynamic speckle

When the speckle pattern changes in time, due to changes in the illuminated surface, the phenomenon is known as dynamic speckle, and it can be used to measure activity, by means of, for example, an optical flow sensor (optical computer mouse).
In physics, dynamic speckle is a result of the temporal evolution of a speckle pattern where variations in the scattering elements responsible for the formation of the interference pattern in the static situation produce the changes that are seen in the speckle pattern, where its grains change their intensity (grey level) as well as their shape along time.

Electronic speckle pattern interferometry

Electronic Speckle Pattern InterferometerESPI
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 component under investigation must have an optically rough surface so that when it is illuminated by an expanded laser beam, the image formed is a subjective speckle pattern.

Airy disk

Airy diffraction patternAiry Patterndiffraction
The size of this area is determined by the diffraction-limited resolution of the lens which is given by the Airy disk whose diameter is 2.4λu/D, where λ is the wavelength of the light, u is the distance between the object and the lens, and D is the diameter of the lens aperture.
Speckle pattern

Speckle noise

speckleImage Despeckling
Speckle noise
The speckle can also represent some useful information, particularly when it is linked to the laser speckle and to the dynamic speckle phenomenon, where the changes of the speckle pattern, in time, can be a measurement of the surface's activity.

Lidar

laser altimeterlaser radarlight detection and ranging
Speckle is the chief limitation of coherent lidar and coherent imaging in optical heterodyne detection.
It can be used for imaging Doppler velocimetry, ultra-fast frame rate (MHz) imaging, as well as for speckle reduction in coherent lidar.

Optical heterodyne detection

synthetic array heterodyne detectionheterodyneoptical heterodynes
Speckle is the chief limitation of coherent lidar and coherent imaging in optical heterodyne detection.
In laser scattering this is known as speckle.

Diffusing-wave spectroscopy

DWS
Diffusing-wave spectroscopy
This technique either uses a camera to detect many speckle grains (see speckle pattern) or a ground glass to create a large number of speckle realizations (Echo-DWS ).

Intensity (physics)

intensityintensitieslight intensity
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.

Wavefront

wave frontwavefrontssurfaces of constant phase
A speckle pattern is an intensity pattern produced by the mutual interference of a set of wavefronts.

Isaac Newton

NewtonSir Isaac NewtonNewtonian
This phenomenon has been investigated by scientists since the time of Newton, but speckles have come into prominence since the invention of the laser and have now found a variety of applications.

Digital image correlation and tracking

digital image correlationcorrelationDIC
The term speckle pattern is also commonly used in the experimental mechanics community to describe the pattern of physical speckles on a surface which is useful for measuring displacement fields via digital image correlation.

Diffuse reflection

diffusemattediffuse interreflection
Speckle patterns typically occur in diffuse reflections of monochromatic light such as laser light.

Light scattering by particles

scatteringLight scatteringscattered
Such reflections may occur on materials such as paper, white paint, rough surfaces, or in media with a large number of scattering particles in space, such as airborne dust or in cloudy liquids.

Random walk

random walkssimple random walkrandom walker
If each wave is modelled by a vector, then it can be seen that if a number of vectors with random angles are added together, the length of the resulting vector can be anything from zero to the sum of the individual vector lengths—a 2-dimensional random walk, sometimes known as a drunkard's walk.

Wavelength

wavelengthsperiodsubwavelength
If the surface is rough enough to create path-length differences exceeding one wavelength, giving rise to phase changes greater than 2π, the amplitude, and hence the intensity, of the resultant light varies randomly.

Fraunhofer diffraction

Fraunhoferdiffraction patternelsewhere
is the zone where Fraunhofer diffraction happens).

Fresnel diffraction

FresnelFresnel coefficientsFresnel diffraction equation
scattering object, in the near field (also called Fresnel region, that is, the region where Fresnel diffraction happens).

Near and far field

far fieldnear fieldnear-field
This kind of speckles are called near field speckles. See near and far field for a more rigorous definition of "near" and "far".

Holographic interferometry

Holographic interferometer
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.

Speckle imaging

speckle interferometryspeckle interferometricimaging
The speckle effect is also used in speckle imaging and in eye testing using speckle.