Speckle Metrology: Some Newer Techniques and Applications

Summary

A speckle pattern arises due to the self-interference of a multitude of waves with random amplitudes and phases which originate from the scattering centers on the surface, illuminated by a coherent wave. A fully developed speckle pattern results when the rms value of the surface roughness is greater than a wavelength. The contrast of the speckle pattern is unity. The speckle pattern undergoes both positional shift and irradiance changes when the generating surface is displaced or deformed. In order to relate the changes in the speckle pattern to those at the surface, imaging geometry is most often used. The measurement methods are divided into two groups, namely, speckle photography and speckle interferometry. Speckle photography measures the positional changes of the speckles while speckle interferometry measures the phase changes, and consequently the irradiance changes. It may be noted that both these changes occur simultaneously with one possibly predominating over the other.