We have experimentally investigated the meaning of the effective optical absorption [μa (eff)] and the reduced scattering [μs′(eff)] coefficients measured on the surfaces of two-layered turbid media, using the diffusion equation for homogeneous, semi-infinite media. We performed frequency-domain spectroscopy in a reflectance geometry, using source–detector distances in the range 1.5–4.5 cm. We measured 100 samples, each made of one layer (thickness in the range 0.08–1.6 cm) on top of one semi-infinite block. The optical properties of the samples were similar to those of soft tissues in the near infrared. We found that the measured effective optical coefficients are representative of the underlying block if the superficial layer is less than ∼0.4 cm thick, whereas they are representative of the superficial layer if it is more than ∼1.3 cm thick.
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