Heterodyne displacement measuring interferometry offers high resolution, high accuracy displacement measurement for non-contact applications. However, the accuracy is limited by nonlinearities, or periodic errors, which depend on misalignments in the optical setup. These errors are non-cumulative in nature and repeat with each unit wavelength change in the optical path. Apart from periodic errors, there are also other sources of error which are not periodic in nature, such as cosine error, Abbe error, variations in refractive index, thermal deformations, beam shear, non-planar wave-fronts, and wavelength instability of the laser source.
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