Scattered light intensity cross correlation. I: Theory

It is shown that in a so-called cross-correlation set up, an experimental technique where two laser beams and two detectors are used, double scattering contributions to the measured intensity correlation function are negligibly small compared to the first order contribution. Furthermore, it is shown that the theoretical work of Phillies concerning this subject is incomplete, due to the use of an incomplete Siegert relation. In doing this, an important second order term is overlooked. The cross-correlation set up is best suited for experiments at 90° scattering angle. At other scattering angles experimental difficulties arise, due to the necessity, as is shown in this paper, to position the two detectors at different angles with respect to the laser beams.