High-resolution time-average electronic holography for vibration measurement

Abstract

Time-average electronic holography (also known as TV holography) is a widespread technique used in real-time, non-contact, whole-field vibration measurement. The quantitative interpretation of time-averaged fringe patterns is difficult because of speckle noise and of the decreasing visibility of higher-order Bessel fringes. Speckle noise may only be reduced during image acquisition, by multiple-frame averaging. The objective of this paper is to present two new methods allowing to obtain time-averaged fringe patterns presenting a high resolution and a high contrast between dark and bright fringes, as well as the associated quantitative interpretation. They are based on the elimination of the random phase component and on the reduction of the random intensity component of the time-averaged fringe pattern. The fringe patterns obtained by this method may be efficiently filtered in order to provide the precise locations of the Bessel-type fringes associated with the object vibration. The methods rely on the use of the simplest standard equipment, without any need for modulators or stroboscopic devices. Quantitative fringe interpretation is straightforward and based on the method of fringe skeletonizing and indexing. Fringe contrast is so high that skeletonizing may be done by simple thresholding.