The results of this study clarify the influence of probe geometry on spectroscopic measurements obtained from the surface of a turbid biological tissue. We show that the transition between the measurement of the predominantly backward-propagating and the predominantly forward-propagating photon fluxes is marked by the separation between the source probe and the detector probes at which the dependence of the fluence on small changes in scattering coefficient vanishes. This is the probe separation at which a variable scattering background has the least influence on the measurement of optical absorption in turbid materials. Estimates of the optimum probe spacing for typical values of absorption and scattering coefficients of soft tissue in the near-infrared spectral region (800–2500 nm) are derived from an analytical solution of the diffusion equation. The estimates were verified by Monte Carlo simulations and experiments on particle suspensions with optical properties similar to those of skin tissue.
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