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We describe an experimental breadboard developed for the investigation of nonequilibrimn fluctuations induced by macroscopic temperature and concentration gradients under microgravity conditions. Under these conditions the amplitude of the fluctuations diverges strongly for long wavelengths. The setup was developed at the University of Milan and at the University of California at Santa Barbara within the gradient-driven fluctuations experiment (GRADFLEX) project of the European Space Agency, in collaboration with the National Aeronautics and Space Administration. The apparatus uses a quantitative shadowgraph technique for characterization of the static power spectrum of the fluctuations S(q) and the measurement of their dynamics. We present preliminary experimental results for S(q) obtained in the presence of gravity for gradient-driven fluctuations for two cases, those induced in a liquid mixture with a concentration gradient produced by the Sorer effect and those induced in a single-component fluid by a temperature gradient. OCIS codes: 120.5820, 070.2580, 110.6760, 040.2840, 100.2000.

This special issue of Applied Optics contains research papers on photon correlation and scattering, many of which were presented at the OSA Topical Meeting that was held 16-18 August 2004. OCIS codes: 290.0290, 040.0040, 120.0120, 300.0300, 120.3180.

The utility of the one-beam cross-correlation dynamic light-scattering system for sizing small particles in suspension was previously limited by its small-intensity signal-to-baseline ratio for strongly turbid suspensions. We describe three improvements in the optical system and sample cell that raise the ratio to a value comparable with that of other cross-correlation dynamic light-scattering systems. These improvements are (i) using a square cross-sectional sample cell to minimize the attenuation of the incident beam and singly scattered light, (ii) placing a 200-[[micro]meter]-wide slit between the sample cell and the detector fibers to mask off the region of weak single scattering and strong multiple scattering from the detectors' field of view, and (iii) aligning the center of the detectors' field of view with the region of strongest single scattering. We analyze a number of suspensions of polystyrene latex spheres with a diameter between 65 and 562 nm in water using this improved one-beam instrument and find that the measured radius is determined in a 2-min data collection time to better than [+ or -]10% for volume fractions of the suspended polystyrene latex spheres up to a few percent.

We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented. Key words: Particle sizing, dynamic light scattering, cross correlation, multiple-scattering suppression, static light scattering.

A general-purpose, multifunction light-scattering instrument has been developed at the NASA Lewis Research Center for Space Shuttle and Space Station colloid crystallization and other microgravity experiments. For a single sample, the instrument can measure two-dimensional Bragg scattering from 0.5 [degrees] to 60 [degrees], dynamic and static light scattering from 10 [degrees] to 170 [degrees], the shear modulus of samples before and after crystallization, and digital color images of the sample. A carousel positions any one of eight 3-ml samples into the test position for separate experiments. Program challenges and flight results from the STS-83 Space Shuttle mission are discussed. Key words: Light scattering, hard sphere, Bragg scattering, dynamic light scattering, quasi-elastic light scattering, crystal growth, microgravity, colloidal crystal.

Obtaining true and beautiful data from any photon correlation experiment demands serious attention to optimizing both the measuring system and experimental conditions. The laser must have sufficient power, be stable under all likely conditions, and usually be restricted to a single transverse mode. The beam-delivery optics must be carefully designed, built, and verified. The scattering medium must contain a proper concentration of suitably sized scatterers with appropriate characteristics. Surfaces surrounding the point of measurement must not introduce optical noise. Flare reduction in the receiver optics may be improved with ghost analysis, spatial and spectral filtering, and careful choice of stops, baffles, and surface coatings. The photon detector must have adequate speed and sensitivity with suitably low internal correlations and noise. The choice of correlator is crucial. Sometimes the equipment must be small to reach inaccessible places. Performance may be compromised by thermal, mechanical, or electrical instabilities caused by exposure to environmental excesses. Errors may even be introduced by preprocessing hardware and software before proper information is extracted. With so many conditions and potential problems, how does one obtain beautiful data, leading to correct results and enlightening information? That is the focus of our work. Key words: Photon correlation, dynamic light scattering, laser velocimetry.

Bounding the errors of measurements derived from correlation functions of light scattered from some physical systems is typically complicated by the ill conditioning of the data inversion. Parameter values are estimated from fitting well-chosen models to measurements taken for long enough to look acceptable, or at least to yield convergence to some reasonable result. We show some simple numerical simulations that indicate the possibility of substantial and unanticipated errors even in comparatively simple experiments. We further show quantitative evidence for the effectiveness of a number of ad hoc aspects of the art of performing good light-scattering experiments and recovering useful measurements from them. Separating data-inversion properties from experimental inconsistencies may lead to a better understanding and better bounding of some errors, giving new ways to improve overall experimental accuracy. OCIS code: 290.0290.

This feature issue of Applied Optics contains 31 research papers on photon correlation and scattering, many of which were presented at an OSA Topical Meeting that was held 21-23 August 2000 in Whistler, British Columbia, Canada. These papers focus on research in dynamic light scattering, surface light scattering, photon correlation, and laser velocimetry and their applications to physical, chemical, and biological processes.