Polar nephelometer based on a rotational confocal imaging setup

Rapid measurement of the angular distribution of light scattered by particles, the scattering phase function, is achieved by using a new type of polar nephelometer, a device for measuring the angular scattered-light intensity distribution, with a high angular precision and across many orders of magnitude of intensity. The design offers high-speed measurements and avoids many of the problems often associated with traditional goniometers when they are used for measurements of light scattering from small particles or biological cells in suspension. Our system relies on confocal imaging of the test space with off-axis parabolas, using a rotating mirror to scan the angular field of view at the second focus of a pair of conjugated parabolic mirrors, with the test space located at the first focus. The angular resolution of the system is limited mainly by the data-acquisition sampling frequency. In this proof-of-principle demonstration the system performs multiple scans of a 55 deg(square) field of view in a very short time (<1 s). To significantly increase the signal-to-noise ratio, we averaged the successively acquired scans during this time. Polystyrene spheres dispersed in water at low concentrations were used to test the system. The scattering patterns obtained were found to be in good agreement with Mie theory calculations.