A novel holographic particle-image velocimeter system has been developed for the study of threedimensional (3-D) fluid velocity fields. The recording system produces 3-D particle images with a resolution, a signal-to-noise ratio, an accuracy, and derived velocity fields that are comparable to high-quality two-dimensional photographic particle-image velocimetry (PIV). The high image resolution is accomplished through the use of low f-number optics, a fringe-stabilized processing chemistry, and a phase conjugate play-back geometry that compensates for aberrations in the imaging system. In addition, the system employs a reference multiplexed, off-axis geometry for the determination of velocity directions with the cross-correlation technique, and a stereo camera geometry for the determination of the three velocity components. The combination of the imaging and reconstruction subsystems makes the analysis of volumetric PIV domains feasible.

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.

Recent trends in optical metrology suggest that, in order for holographic measurement to become a widespread tool, it must be based on methods that do not require physical development of the hologram. While digital holography has been successfully demonstrated in recent years, unfortunately the limited information capacity of present electronic sensors, such as CCD arrays, is still many orders of magnitude away from directly competing with the high-resolution silver halide plates used in traditional holography. As a result, present digital holographic methods with current electronic sensors cannot record object sizes larger than several hundred microns at high resolution. In this paper, the authors report on the use of bacteriorhodopsin (BR) for digital holography to overcome these limitations. In particular, BR is a real-time recording medium with an information capacity (5000 line-pairs/mm) that even exceeds high resolution photographic film. As such, a centimetre-square area of BR film has the same information capacity of several hundred state-of-the-art CCD cameras. For digital holography, BR temporarily holds the hologram record so that its information content can be digitized for numeric reconstruction. In addition, this paper examines the use of BR for optical reconstruction without chemical development. When correctly managed, it is found that BR is highly effective, in terms of both quality and process time, for three-dimensional holographic measurements. Consequently, several key holographic applications, based on BR, are proposed in this paper.

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This paper demonstrates the use of bacteriorhodopsin (bR) as the holographic recording medium for a holographic particle image velocimetry (HPIV) system. Using an off-axis hologram in bR, double-exposed images of particles in a turbulent flow are recorded. A high numerical aperture configuration (NA = 0.75) ensures a maximal signal intensity of the holographic recordings. Using a CCD the real particle images, that were reconstructed in the original object space, were digitized. The reconstructed image has a theoretical depth of focus of 4.73 µm and a diffraction-limited resolution of 0.43 µm. Using a priori knowledge about the flow, the flow pattern is extracted from the double-exposed particle images. A liquid crystal shutter was employed during the reconstruction in order to minimize photo-induced erasure. Details of the experimental set-up, as well as the difficulties which were encountered during this investigation, are discussed in this paper. The paper also discusses various multiplexing methods and their suitability for use with bR to remove directional ambiguity in HPIV.