An extremely small-angle scatterometer for measuring the phase function of turbid media

Summary form only given. As light travels through turbid media such as ocean water, clouds or tissue, it is attenuated by absorption and scattering. In this type of media, scattering is due mainly to particles that are much larger than the wavelength of light. The angular scattering probability (known as the scattering phase function) is often extremely forward-peaked and difficult to measure. Measurement of the scattering phase function is needed as an input to theoretical models that predict optical system performance. A lack of good measured phase functions for these types of media has led to disagreements between measurements and theoretical predictions. We describe an extremely small-angle scatterometer (ESAS) instrument which is capable of measuring optical phase functions with high angular resolution and dynamic range at very small angles. The instrument is currently configured for table-top liquid samples. One key feature of this instrument is the use of the Fourier transform to map the angular changes in the transmitted light (caused by scattering in the liquid cell) into spatial changes at the focal plane of a long focal length camera lens.