Abstract
I discuss dynamic properties of multispectral speckle in the context of digital holographic interferometry and image correlation. I outline the correlation of speckles in free space, in an imaging system, and, in the case of interferometric detection, caused by reflection off an inclined diffuse surface. It is shown that interferometric phase gradients and speckle movements are closely related where in fact the phase gradients are the generator of speckle movements in a defocused plane. The theory is exemplified by three typical situations encountered in image-plane digital holographic interferometry.
Highlights
In a way, speckles are the ultimate manifestation of coherence in a wave field, as they show up only in coherent fields
We may call the diffusor a secondary source and call the actual source a primary source. It is this property of producing a complete set of spatial frequencies that is utilized in optical imaging metrology
It is seen that interferometric phase differences are caused by changes in wave number in relation to some relative propagation length, as well as surface movements in relation to the sensitivity vector of the setup and movements of the detection point in relation to the detection vector, respectively
Summary
Speckles are the ultimate manifestation of coherence in a wave field, as they show up (become visible) only in coherent fields. We may call the diffusor a secondary source and call the actual source a primary source It is this property of producing a complete set of spatial frequencies that is utilized in optical imaging metrology. If we change the wavelength of the coherent beam, deform the object, or somehow change the microstructural distribution of scatterers, the phase, position, and microstructure of the speckle pattern change. It is this phase change, speckle movement, or decorrelation that is utilized in metrology. The results are put into perspective in the final section of the article
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