Development of a laboratory spectral backscattering instrument: design and simulation

A typical integrating sphere configuration for measuring backscatter includes a conventional cuvette with flat windows. This arrangement results in a significant amount of total internal reflection, preventing a large portion of the backscattered flux from entering the integrating sphere-detector system. Use of a semispherical cuvette overcomes this problem. Monte Carlo simulations of a semispherical cuvette window demonstrate that the detected signal varies monotonically with the attenuation, depending only on the probability of backscattering for a given single scattering albedo. That is, only the total backscattering probability matters, regardless of subtle differences in the scattering phase function in the backward direction. The system is calibrated by use of standard microspheres for which the size distribution and the refractive index are known; this makes it possible to compute the exact phase function based on Mie theory. We have performed Monte Carlo simulations for various measured volume scattering functions and for computed phase functions, using particle scattering codes. All the results indicate that the backscattering measurement errors are likely to be less than 10%.