13th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 26-29 June, 2006 Optimizing a Holographic PIV system using a Bacteriorhodopsin (BR) film. Thomas Ooms 1 , Joseph Braat 2 and Jerry Westerweel 3 1: Faculty of Mech. Engineering, Delft University of Technology, Delft, The Netherlands, t.a.ooms@wbmt.tudelft.nl 2: Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands 3: Faculty of Mechanical Engineering, Delft University of Technology, Delft, The Netherlands Abstract Since Bacteriorhodopsin (BR) was presented in 2000 as a high-resolution optical memory, it has developed quickly as a recording medium for Holographic Particle Image Velocimetry (HPIV). This work continues to investigate the possibilities of BR by studying the amount of tracer particles that can be holographically recorded on a BR-film and reconstructed with a sufficient signal-to-noise ratio (SNR) for an effective correlation analysis. The number of recorded tracer particles is a result of the illuminated cross- section area of the flow volume and the particle-integrated-area-density. These terms are optimized by using strongly scattering tracer particles, adding a high-pass optical Fourier filter to the recording set-up, optimizing the energy ratio between the object beam and the reference beam and optimizing of the intensity of the reconstruction beam. As a result, more than 100.000 particles, distributed over a transverse area of ~800 mm 2 , are successfully recorded on a BR-film and reconstructed with sufficient SNR to yield correct velocity vectors with a 3D-PIV-correlation analysis. A part of the flow near a vortex ring in water is recorded and analyzed to illustrate the system's ability to perform realistic flow measurements. 1. Introduction Since the 1990's, impressive 3-dimensional 3-component (3D3C) Holographic Particle Image Velocimetry (HPIV) flow measurements have been performed using silver-halide based films as a recording medium (Barnhart et al 1994, Herrmann & Hinsch 2004, Meng & Hussain 1995, Pu & Meng 2000, Sheng et al 2003). Despite positive results, the inconvenience and time-consuming nature of chemical processing has stimulated further research. Work on digital HPIV has lead to the realization of some flow measurements (Malkiel et al 2003), however, the severely limited information capacity of present electronic sensors (i.e. CCD chips) currently prevents this method from being a realistic tool for turbulent flow measurements (Barnhart et al 2002). The presentation of bacteriorhodopsin (BR) as a high-resolution optical memory (Hampp 2000) and the suggestion that BR could be successfully applied as a recording medium for HPIV (Barnhart et al 2004), presents a solution to the disadvantages of the earlier-described methods. Bacteriorhodopsin does not require any chemical processing, has a resolution of about 5000 line-pairs per millimeter (Hampp 2000) and is sensitive to the polarization-state of the recorded light which allows polarization-multiplexing (Koek et al 2004). These properties make BR a very suitable recording medium for HPIV measurements. In 2004, Chan presented the use of BR as a recording medium in an HPIV system (Chan et al 2004). Displacements of a moving particle field were recorded without directional ambiguity by polarization multiplexing techniques (Koek et al 2004). However, real flow measurements were not performed as the particles (glass, diameter 100 μm) were unsuitable as tracer particles in water. Also, the transverse dimension of the recorded particle field was limited by a configuration where the unscattered object light was filtered out by a beam-stop on the film. The current work builds on these results with the aim to increase the measurement volume and increase the amount of recorded tracer particles while maintaining an acceptable signal-to-noise ratio - 1 -