Digital holography for mechanical vibration measurements in rigidbody displacement: elimination of the latter by means of a variablefocal length adjustment

摘要

We present our investigation on the separation of mechanical vibrations from rigid body displacements. Pairs of digitalholograms acquired between two consecutive time intervals from this type of events produce phase maps that containboth the vibration and rigid body motion information, or even further fully decorrelated phase maps after computerprocessing. In order to compensate for body displacements, a conjugate object-image experimental arrangement fordigital holography is used to measure the mechanical vibrations in a rectangular flat plate. This is achieved by includingan extra lens with variable focal length adjustments in front of the typical lens-aperture combination used in the opticalhead of a digital holographic set up. Out of plane data is obtained from a framed metal plate subjected to a known modalvibration that is also allowed to move perpendicularly to its surface. We will demonstrate that due to the poweradjustment of the added lens the angular phase change in the digital hologram from the known object motion allows theseparation of the vibration mode at the image plane. The proposed lens addition into a new optical head arrangement indigital holography combined with an a priori knowledge of the rigid body displacement is able to accurately separate themechanical vibrations making it a promising method in experiments performed under noisy environments. This researchsuggests the inclusion of adaptive lenses to control the effective focal length when there is a need to separate twodistinctive motion types, i.e., vibration from rigid body motion.