User:Dbrogioli/Near Field Speckles

TO BE ADDED TO THE PAGE speckle pattern

Near Field Speckles
Objective speckles are usually obtained in the far field (also called Fraunhofer region, that is the zone where Fraunhofer diffraction happens). This means that they are generated "far" from the object that emits or scatters light. Speckles can be observed also close to the scattering object, in the near field (also called Fresnel region, that is, the region where Fresnel diffraction happens). This kind of speckles are called Near Field Speckles. See near and far field for a more rigorous definition of "near" and "far".

The statistical properties of far field speckle pattern (i.e., the speckle form and dimension) depend on the form and dimension of the region hit by laser light. By contrast, a very interesting feature of near field speckles is that theier 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.

Actually, the condition under which the near field speckles appear has been described as more strict than the usual Fresnel condition.

Applications
In the case of near field speckles, the statistical properties depend on the light scattering distribution of a given sample. This allows to use the near field speckles analysis as a way to detect the scattering distribution; this is the so-called Near Field Scattering technique.