Abstract
The Green's function for the diffusion equation is widely used to describe photon transport in turbid media. We have performed aseries of spectroscopy experiments on a number of uniform turbid media with different optical properties (absorption coefficient in the range 0.03-0.14 cm(-1), reduced scattering coefficient in the range 5-22 cm(-1)). Our experiments have been conducted in the frequency domain, where the measured parameters are the dc intensity (I(dc)), ac amplitude (I(ac)), and phase (?) of the light intensity wave. In an infinite medium, the Green's function predicts a linear dependence of ln(rI(dc)) and ? on the source-detector separation r. Our measurements show that the intercepts of these straight lines predicted by the Green's function do not agree with the experimental results. To reproduce the experimental results, we have introduced an effective photon source whose spatial extent and source strength depend on the optical properties of the medium. This effective source term has no effect on the slopes of the straight lines predicted by the Green'sfunction at large values of r.
Highlights
The properties of light propagation in scattering media are important in many areas of physics and engineering such as remote sensing of the atmosphere,[1,2] studies of interstellar dust,[3] industrial production monitoring,[4] and medical diagnostics.[5]
We have investigated the properties of the effective sourcespatial extension and strengththat are consistent with the experimental results
In a previous publication,[19] we proposed a multidistance measurement protocol for the absolute determination of a and sЈ in optically dense media. This protocol consists of acquiring data at a numberat least twoof different source– detector separations, from which one can determine the slopes of the straight lines lnrIdc, lnrIac, and ⌽ as a function of r
Summary
The properties of light propagation in scattering media are important in many areas of physics and engineering such as remote sensing of the atmosphere,[1,2] studies of interstellar dust,[3] industrial production monitoring,[4] and medical diagnostics.[5]. In the strongly scattering regime, the Boltzmann transport equation reduces to the diffusion equation, which describes heat conduction in solids, diffusion of neutrons in condensed media, and gas diffusion. NearinfraredNIRlight is strongly scattered inside most biological tissues, so the diffusion equation has been widely employed in studies focused on NIR imaging and spectroscopy of living tissue. These applications to medical diagnostics have raised much interest in recent years because of a number of promising preliminary results and because of their significant potential.[6] Optical measurements in biological tis-
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