A schlieren method for ultra-low-angle light scattering measurements

We describe a self-calibrating optical technique that allows to perform absolute measurements of scattering cross-sections for the light scattered at extremely small angles. Very good performances are obtained by using a very simple optical layout similar to that used for the schlieren method, a technique traditionally used for mapping local refraction index changes. The scattered intensity distribution is recovered by a statistical analysis of the random interference of the light scattered in a half-plane of the scattering wave vectors and the main transmitted beam. High-quality data can be obtained by proper statistical accumulation of scattered intensity frames, and the static stray light contributions can be eliminated rigorously. The potentialities of the method are tested in a scattering experiment from non-equilibrium fluctuations during a free-diffusion experiment. Contributions of light scattered from length scales as long as Lambda = 1 mm can be accurately determined.