  We study the effects of high optical depth and density on the performance of
a light-atom quantum interface. An in-situ imaging method, a dual-port
polarization contrast technique, is presented. This technique is able to
compensate for image distortions due to refraction. We propose our imaging
method as a tool to characterize atomic ensembles for high capacity spatial
multimode quantum memories. Ultracold dense inhomogeneous Rubidium samples are
imaged and we find a resonant optical depth as high as 680 on the D1 line. The
measurements are compared with light-atom interaction models based on
Maxwell-Bloch equations. We find that an independent atom assumption is
insufficient to explain our data and present corrections due to resonant
dipole-dipole interactions.
