Measurement of the local optical properties of turbid media by differential path-length spectroscopy.

We report on the development of an optical-fiber-based diagnostic tool with which to determine the local optical properties of a turbid medium. By using a single fiber in contact with the medium to deliver and detect white light, we have optimized the probability of detection of photons scattered from small depths. The contribution of scattered light from greater depths to the signal is measured and subtracted with an additional fiber, i.e., a collection fiber, to yield a differential backscatter signal. Phantoms demonstrate that, when photons have large mean free paths compared with the fiber diameter, single scattering dominates the differential backscatter signal. When photons have small mean free paths compared with the fiber diameter, the apparent path length of the photons that contribute to the differential backscatter signal becomes approximately equal to 4/5 of the fiber diameter. This effect is nearly independent of the optical properties of the sample under investigation.