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 Λ = 1mm can be accurately determined. Low angle static light scattering is a powerful tool to investigate the long wavelength fluctuations that appear in a great variety of important phenomena, like phase transitions and nucleation processes [1, 2], aggregation of colloidal systems [3, 4], and non equilibrium systems [5]. As the typical length scale Λ increases, the angular range of interest δΘ = λ/Λ becomes smaller, and this creates unsormountable difficulties stemming from the cramped solid angle of collection for the scattered light, tighter angular definition for the scattering angle, ever increasing difficulty with stray light that is preferentially scattered at smaller angles, and finally main beam diffraction spilling that also piles up at the low angle scattering end. In this letter we describe a new technique that allows to overcome most of the above mentioned difficulties and can be pushed to arbitrary small angles. This technique relies on a detection scheme totally different from the traditional angle-resolved scattering method [6], and based on an optical setup similar to schlieren imaging technique, traditionally used to map refraction index modulations [7]. The technique belongs to a family of scattering measurement techniques recently introduced, Near Field Scattering (NFS) [8, 9, 10]. These techniques allow to measure scattered intensity distributions from the statistical analysis of the intensity modulations due to the random interference of the scattered waves in the near field. In Heterodyne NFS (HNFS) [10], the scattered light is mixed with the transmitted beam that acts as a reference beam. The