CMOS-ESPI-system with in-line digital phase stabilization using unresolved speckles

摘要

In combination with phase shifting techniques electronic speckle pattern interferometry (ESPI) is a versatile tool in the field of deformation measurements. However, in applications outside the laboratory, it suffers from the influence of external disturbances, especially mechanical vibrations and temperature fluctuations. These effects result in global phase fluctuations that are constant over the field of measurement, but vary in tune. Phase fluctuations of this kind can be compensated by an active phase stabilization system. In previous papers we introduced a DSP-controlled digital phase stabilization system on the basis of a synthetic heterodyne technique which needs no additional optical components in the ESPI set-up and stabilizes the phase at one point of the field of measurement. In this paper we will report on further improvements of the system. The functionality of further components has been integrated in the DSP, making the handling of the system and the variation of parameters of the control system even simpler. Furthermore, a high speed CMOS-camera with high full well capacity is used in the set-up instead of a CCD-camera and the system is operated with unresolved speckles. This CMOS-camera makes not only the tracking of fast deformation processes and the observation of objects with strongly varying brightness possible, but it can simultaneously generate the input signal for the control system. Finally, the control signal can be analyzed in order to get further information about object movements, especially rigid body motions and the sign of an object deformation itself.