INFLUENCE OF A SUPERFICIAL LAYER IN THE QUANTITATIVE SPECTROSCOPIC STUDY OF STRONGLY SCATTERING MEDIA

We have experimentally investigated the meaning of the effective optic al absorption [mu(a)((eff))] and the reduced scattering [mu(s)'((eff)) ] coefficients measured on the surfaces of two-layered turbid media, u sing the diffusion equation for homogeneous, semi-infinite media. We p erformed frequency-domain spectroscopy in a reflectance geometry, usin g source-detector distances in the range 1.5-4.5 cm. We measured 100 s amples, 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 similar to 0.4 cm thick, whereas they are representative of the superficial laye r if it is more than similar to 1.3 cm thick. (C) 1998 Optical Society of America OCIS codes: 170.5280, 170.6510, 170.7050, 300.0300, 300.63 40.