  The survival of time-reversal symmetry in the presence of strong multiple
scattering lies at the heart of some of the most robust interference effects of
light in complex media. Here, the use of time-reversed light paths for imaging
in highly scattering environments is investigated. A common-path Sagnac
interferometer is constructed which is able to detect objects behind a layer of
strongly scattering material through up to 14 mean free paths total attenuation
length. A spatial offset between the two light paths is used to suppress
non-specific scattering contributions, limiting the signal to the volume of
overlap. Scaling of the specific signal intensity indicates a transition from
ballistic to quasi-ballistic contributions as the scattering thickness is
increased. The characteristic frequency dependence for the coherent modulation
signal provides a path length dependent signature, while the spatial overlap
requirement allows for short-range 3D imaging. The technique of common-path,
bistatic interferometry offers a conceptually novel approach which could open
new applications in diverse areas such as medical imaging, machine vision,
sensors, and lidar.
