Handling unfavourable polarization states in a polarization-based shearing speckle interferometer

Abstract

A polarization-based shearing speckle interferometer requires the incoming scattered light to have proper polarization characteristics: the light should be equally distributed over two orthogonal polarization states in order to obtain two interfering beams with equal intensities and the highest possible modulation. Many surfaces scatter linearly polarized light randomly, providing equal intensities for the interfering beams on average. However, when a diffusely reflecting metallic surface is illuminated by a linearly polarized laser beam, the polarization direction of the illuminating beam is retained to a large extent in the scattered light. As a result light entering the interferometer will not be randomly polarized, which may lead to low modulation due to unbalanced object and reference beams. A solution that handles the problem effectively is proposed. It consists of a quarter-wave plate positioned in front of the interferometer, oriented at 45 • . It is shown that unfavourable predominant polarization states encountered when testing unprepared metallic surfaces can be converted into favourable ones, thereby obtaining well balanced object and reference beams, irrespective of the polarization direction of the incoming light.

A new Method for Processing Time Averaged Vibration Patterns: Linear Regression

“…However, the actual measurement is taken according to Equation (13). To correct for the phase difference, we write cos φ and sin φ as…”

“…The object was excited by a loudspeaker on the backside, at a frequency of 147 Hz. The speckle interferometer used was a modified subsystem of an existing shearography system . The subsystem, a polarisation‐sensitive shearing interferometer in Michelson configuration, is shown in Figure .…”

Shearography technology and applications: a review

“…Phase maps obtained using this method will have reduced spatial resolution compared to temporal phase stepping since each image occupies only a fraction of the available area on the camera's sensor, although this may not be too much of a problem if a large format sensor is used. Another system that employs spatial phase shifting involves a polarization-based shearing interferometer which divides an incoming image into two and images them side-by-side on a camera with a π relative phase shift [70]. Analysis showed that maintaining sub-pixel image registration was essential to maintain accurate measurements [71].…”

“…Analysis showed that maintaining sub-pixel image registration was essential to maintain accurate measurements [71]. A more recent paper [70] discusses an improvement of the system when dealing with unfavourable polarization states. …”