Effect of compensation for spherical aberration on the transverse displacement of laser speckle patterns

Abstract

When a rough surface illuminated by coherent light is displaced perpendicularly to the optical axis of an imaging optical system the speckle pattern in the conjugate plane is transversally displaced too. This displacement has two components. The first one is proportional to the object displacement, and the second one depends on wave-front aberrations and, consequently, is strongly related to the optical system that is used. Usually, well-corrected photographic objectives are used for the measurement of transverse displacements by double-exposure laser speckle photography. Since in well-corrected objectives aberrations tend to compensate one another, it seems that the complementary displacement of the speckle pattern, caused by aberrations, is near zero and does not affect the accuracy of the measurement. Here it is analytically shown that the compensation of spherical aberrations does not guarantee a negligible complementary displacement. From the results obtained it follows that well-corrected objectives for laser speckle photography can be regarded as a particular class of photographic objectives, since they not only yield high-quality images but also minimize complementary displacement.