Simultaneous measurements of the spatial covariance of intensity and the angular spectrum of plane waves have been made for a diverging He–Ne laser beam propagating over a 1-km horizontal path. Most of the data are taken during strong scintillation. The measured angular spectrum is used to estimate the wave-structure function D(s). The propagation equation for the intensity covariance depends on the medium only through the longitudinal derivative of D(s). Thus, for a homogeneous medium, D(s) is sufficient to calculate the intensity covariance, although such a calculation has not been done in general. The estimated D(s) is used to calculate the intensity covariance by using weak scintillation theory and also by using an asymptotic theory for strong scintillation. The measured intensity covariances are then compared with these calculations. In both cases there is qualitative agreement, but the quantitative comparison is poor. It is shown that the inner scale of the turbulence is an important factor in weak scintillation, and it is argued that this inner scale is a dominant factor in strong scintillation.
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