Spatial localization of absorbing bodies by interfering diffusive photon-density waves

The use of the destructive interference of diffusive photon-density waves for the localization of an absorbing (and scattering) body in a scattering medium was studied. The objectives of this approach in the reflectance mode were as follows: first, to reduce sensitivity to absorption features occurring in superficial layers while sensitivity to bodies lying deeper is maintained; second, to establish a confined depth region of maximum sensitivity in which the distance of an absorbing body could be determined through phase measurement. Intensity and phase data were acquired with a modified frequency-domain spectrometer at modulation frequencies up to 600 MHz as an absorbing body was moved in three dimensions. The experimental results are compared with simulations based on a numerical solution of a time-dependent photon-diffusion equation.