Optical profiles with 180 μm resolution of objects hidden in scattering media

We measure the time-of-flight distributions of near-infrared photons emerging from thick scattering media within a collection angle of 0.6 mrad about the incident light direction by means of a time-correlated single-photon counting apparatus endowed with <35 ps resolution. These measurements, which are performed with a picosecond laser beam, allow us to isolate the weakly scattered (ballisticsnake) photons from the multiply scattered ones. By scanning the incidence position across a target we find variations in the fraction of detected unscattered photons that are significative of local changes in the optical parameters of the target. In particular, if either opaque, light diffusing, or transparent objects are embedded in the scattering medium, their profiles can be reconstructed and their nature can be assessed. Opaque objects embedded in realistic tissue phantoms are detected with <180 μm spatial resolution. © 2007 American Institute of Physics