Fig. 4.

$L_1$ , $L_2$ , $L_3$ and $L_T$ in a three-layered medium vs. $\rho$ for different thicknesses of layer 2. (a) to (d): ${\mu }_{a,1}=$ 0.0067 ${\mathrm{mm}}^{-1}<{\mu }_{a,3}=$ 0.0125 ${\mathrm{mm}}^{-1}$ ; (e) to (h): ${\mu }_{a,1}=$ 0.0125 ${\mathrm{mm}}^{-1}>{\mu }_{a,3}=$ 0.0067 ${\mathrm{mm}}^{-1}$ . The rest of the parameters are: ${\mu }_{a,2}=$ 0.016 ${\mathrm{mm}}^{-1}$ , ${\mu }_s^{\mathrm{\prime }}=1$ ${\mathrm{mm}}^{-1}$ (for all layers), $l_1=$ 7 mm.