In digital holographic particle image velocimetry, the particle image depth-of-focus and the inaccuracy of the measured particle position along the optical axis are relatively large in comparison to the characteristic transverse dimension of the reconstructed particle images. This is the result of a low optical numerical aperture (NA), which is limited by the relatively large pixel size of the CCD camera. Additionally, the anisotropic light scattering behaviour of the seeding particles further reduces the effective numerical aperture of the optical system and substantially increases the particle image depth-of-focus. Introducing an appropriate Fourier filter can significantly suppress this additional reduction of the NA. Experimental results illustrate that an improved Fourier filter reduces the particle image depth-of-focus. For the system described in this paper, this improvement is nearly a factor of 5. Using the improved Fourier filter comes with an acceptable reduction of the hologram intensity, so an extended exposure time is needed to maintain the exposure level.

A theoretical analysis describing the dependence of the signal-tonoise ratio (SNR) on the number of pixels and the number of particles is presented for in-line digital particle holography. The validity of the theory is verified by means of numerical simulation. Based on the theory we present a practical performance benchmark for digital holographic systems. Using this benchmark we improve the performance of an experimental holographic system by a factor three. We demonstrate that the ability to quantitatively analyze the system performance allows for a more systematic way of designing, optimizing, and comparing digital holographic systems.

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.

Bacteriorhodopsin (bR) is a reversible photochromic protein that can be used as a holographic medium. The dichroic absorption of the bR molecule is polarization dependent, thereby allowing for the recording of polarization holograms. The properties of polarization holograms can be used to multiplex two independent images in a single bR film. A new technique and associated polarization-multiplexing scheme are demonstrated that allow for simultaneous readout of two orthogonally polarized images while achieving a high normalized diffraction efficiency for each of the individual images.

In a typical digital holographic PIV recording set-up, the reference beam and the object beam propagate towards the recording device along parallel axes. Consequently, in a reconstructed volume, the real image of the recorded particle field and the speckle pattern that originates from the virtual image of the recorded particle field overlap. If the recorded particle field experiences a longitudinal displacement between two recordings and if the two reconstructed complex-amplitude fields are analysed with a 3D correlation analysis, two separate peaks appear in the resulting correlation-coefficient volume. The two peaks are located at opposite longitudinal positions. One peak is related to the displacement of the real image and the other peak is related to the displacement of the speckle pattern that originates from the virtual image. Because both peaks have a comparable height, a sign ambiguity appears in the longitudinal component of the measured particle field displacement. Additionally, the measured longitudinal particle field displacement suffers from a bias error. The sign ambiguity and the bias error can be suppressed by applying a threshold operation to the reconstructed amplitude. The sign ambiguity, characterized by , is suppressed by more than a factor of 60. The dimensionless bias error is reduced by a factor of 5.