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Absolute measurement of absorption and scattering coefficients spectra of a multiply scattering medium

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On the basis of the diffusion theory model, frequency-domain spectroscopy allows for a quantitative determination of the absorption (μa) and scattering (μs') coefficient spectra of a homogeneous multiply scattering medium. We performed measurements using an intensity modulated light emitting diode (LED) as the light source. The LED's spectral distribution permits the study of a spectral region extending for about 80 nm. Data sets (phase shift and average intensity) at two different source-detector distances are acquired: The absorption and scattering coefficient spectra of the medium are then calculated from analytical expressions for μ and μ 's Methylene blue (peak absorption wavelength 656 nm) is used as a test absorbing material. The methylene blue is dissolved in an aqueous Liposyn solution which serves as the multiply scattering medium. The relative amounts of absorber and scatterer are chosen such that the values of μ 's and μs' match typical values in tissues. The results obtained for μa(λ) with this LED based technique are in quantitative agreement with those obtained with a standard spectrophotometer in a non-scattering regime.

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