We have performed frequency-domain optical measurements on the surface of two- and three-layered turbid media (in vitro) and on infant heads (in vivo). For the in vitro experiment we measured a number of combinations of one layer (thickness in the range 0.1-1.6 cm) on top of one semi-infinite block, with or without a clear layer (thickness of 0.1 cm) sandwiched in between. For the in vivo experiment, we did measurements on 20 preterm infants at the Meyer Children's Hospital in Florence, Italy. In the two-layered samples, we measured the effective absorption and reduced scattering coefficients with a multi-distance method based on data taken at source-detector distances in the range 1.5-4.5 cm. We found that the effective optical coefficients reproduce those of the underlying block if the superficial layer is less than about 0.4 cm thick, whereas they are representative of the superficial layer if it is more than about 1.3 cm thick. Larger source detector separations reduce the effect of the superficial layer on the recovery of the effective optical coefficients. Measurements in the presence of the clear layer gave results that did not reproduce, even qualitatively, the in vivo measurements on infant heads.

© 1998 Optical Society of America

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