Experimental study of the diffraction of photon density waves by an absorbing edge in highly scattering media

Near-infrared imaging of bodily tissues is of prime importance in medicine. The authors present an investigation aimed at understanding the physical principles behind the optical processes occurring in these highly scattering media. Frequency-domain methods are used to study the diffraction of transmitted intensity-modulated light waves on an absorbing edge which could be carried in position relative to the illumination and detection fiber optics. The medium was highly scattering and had a variable, adjustable absorption. The authors report the experimental conditions for which the highest spatial resolution is obtained and discuss the influence of the various photon paths. The addition of ink is shown to increase the sharpness of the edge because it causes deletion of the longer photon paths. The same effect can be obtained by increasing the modulation frequency used in the frequency-domain experiments, since higher modulation frequencies are more attenuated than lower modulation frequencies.<>