Speckle (interference)

Speckle patterns are used in a wide range of metrology techniques, as they generally allow high sensitivity and simple setups.

[1][2][3][4] Speckle is not external noise; rather, it is an inherent fluctuation in diffuse reflections, because the scatterers are not identical for each cell, and the coherent illumination wave is highly sensitive to small variations in phase changes.

The vast majority of surfaces, synthetic or natural, are extremely rough on the scale of the wavelength.

Speckle results from these patterns of constructive and destructive interference shown as bright and dark dots in the image.

[6] Speckle in conventional radar increases the mean grey level of a local area.

Although scientists have investigated this phenomenon since the time of Newton,[11] speckles have come into prominence since the invention of the laser.

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.

However, two points in the image which are illuminated by areas in the object which are separated by the diameter of the Airy disk, have light intensities which are unrelated.

We can observe the change in speckle size with lens aperture by looking at a laser spot on a wall directly, and then through a very small hole.

Also, the speckle pattern itself will change when moving the position of the eye while keeping the laser pointer steady.

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.

By contrast, a very interesting feature of near field speckles is that their statistical properties are closely related to the form and structure of the scattering object: objects that scatter at high angles generate small near field speckles, and vice versa.

Under Rayleigh–Gans condition, in particular, speckle dimension mirrors the average dimension of the scattering objects, while, in general, the statistical properties of near field speckles generated by a sample depend on the light scattering distribution.

[19] Speckle patterns have been used in a variety of applications in microscopy,[20][21] imaging,[22][23] and optical manipulation.

In the case of near field speckles, the statistical properties depend on the light scattering distribution of a given sample.

[28] 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).

This can be used in a wavemeter configuration, with a resolution around 1 attometre,[29] (equivalent to 1 part in 1012 of the wavelength, equivalent to measuring the length of a football field at the resolution of a single atom[30]) and can also stabilise the wavelength of lasers[31] or measure polarization.

[32] The disordered pattern produced by speckle has been used in quantum simulations with cold atoms.

The randomly-distributed regions of bright and dark light act as an analog of disorder in solid-state systems, and are used to investigate localization phenomena.

[36] The Mitsubishi Laser TV appears to use such a screen which requires special care according to their product manual.

multi-look processing), averaging out the speckle by taking several "looks" at a target in a single radar sweep.

The main reason for the use of multiscale processing is the fact that many natural signals, when decomposed into wavelet bases are significantly simplified and can be modeled by known distributions.

[40] The first multiscale speckle reduction methods were based on the thresholding of detail subband coefficients.

The angular momentum density is given by:[45] Typically vortices appear in speckle pattern in pairs.

[47] Apart from formal decomposition in Fourier series the speckle pattern may be composed for plane waves emitted by tilted regions of the phase plate.

3D numerical emulation demonstrates the intertwining of vortices which leads to formation of ropes in optical speckle.

Laser speckle on a digital camera image from a green laser pointer. This is a subjective speckle pattern. (Note that the color differences in the image are introduced by limitations of the camera system.)
A photograph of an objective speckle pattern. This is the light field formed when a laser beam was scattered from a plastic surface onto a wall.
A green laser pointer. Reduction of the speckle was necessary to photograph the laser's Gaussian profile , accomplished by removing all lenses and projecting it onto an opaque liquid (milk) being the only surface flat and smooth enough.