Use of a radial angular filter array to estimate the position of an optically attenuating object within a turbid medium

The Radial Angular Filter Array (RAFA) is a novel optical filter consisting of a radially-distributed series of micromachined channels, which converge upon a focal point several millimeters away from the edge of the device. It is designed to measure the angular distribution of light emitted from an object located at the focal point and is capable of selecting ballistic and quasi-ballistic photons at specific angles out of strong background noise due to scattering. We hypothesized that the device might be useful for examining optically absorbing features below the surface of a turbid medium via a depth mapping approach. In order to validate this concept, experiments were performed with a focused laser beam, a series of IntralipidTM solutions (0.1 wt% to 1.0 wt%), a 0.5 mm diameter graphite rod (absorber), and a RAFA optically coupled to a line camera. By scanning the position of the rod and comparing the light scattering profiles obtained by the RAFA at each scanning step, the location and the depth of the rod were successfully identified. Future work will be directed toward studying the performance of the technique with a collimated broadband illumination beam for spectroscopic applications.