Object conjugate reconstruction (OCR): a step forward in holographic metrology

Abstract

This paper reports the development of an entirely new technique in holographic metrology which we have called ‘object conjugate reconstruction’ (OCR). It unifies the techniques of holographic interferometry and holographic velocimetry, being equally applicable to both solid and fluid mechanics respectively. Three–dimensional vector displacement of seeding particles in a fluid or scattering elements on a solid surface are provided from a double–pulsed holographic recording. In solid mechanics, OCR supersedes fringe counting in traditional holographic interferometry and avoids problems associated with fringe ‘localization’. For the first time, surface displacement can be measured on a pointwise basis for an arbitrary motion consisting of solid body translation and/or tilt and/or deformation. Importantly, with complex correlation processing, OCR is immune to image aberrations which have formerly precluded the use of holographic imaging methods for three–dimensional measurements. In addition, inherent in the technique is a simple means of removing directional ambiguity in the measured displacement vector. We present the defining theory for the technique and demonstrate its accuracy by application to rigid–body translation and deformation. Submicrometre displacement vector resolution is achieved commensurate with an interferometric system. OCR provides an important step forward in holographic metrology, achieving its full potential in experimental fluid and solid mechanics.