We consider a wave propagating through a disordered slab of thickness L (and infinite in transverse directions). We calculate fluctuations in transmission for an arbitrary width W of the incident beam. Our results demonstrate that one should distinguish between two regimes: L≪W (plane wave) and L≫W (narrow beam). Whereas the average transmission is the same in both regimes, fluctuations in transmission are quite different and have a different dependence on L and W. We also consider long-range correlations in the intensity of the wave.

• Received 9 November 1988

DOI:https://doi.org/10.1103/PhysRevB.39.6986