Fig. 5.

$L_1$ , $L_2$ , $L_3$ , $L_4$ and $L_T$ in a four-layered medium vs. $\rho$ for different thicknesses of layer 2. (a) to (e): ${\mu }_{a,1}=0.0067$ ${\mathrm{mm}}^{-1}<{\mu }_{a,3}=0.0125$ ${\mathrm{mm}}^{-1}$ . (f) to (j): ${\mu }_{a,1}=0.0125$ ${\mathrm{mm}}^{-1}>{\mu }_{a,3}=0.0067$ ${\mathrm{mm}}^{-1}$ . The rest of the parameters are: ${\mu }_{a,2}={\mu }_{a,4}=0.016$ ${\mathrm{mm}}^{-1}$ , ${\mu }_s^{\mathrm{\prime }}=1$ ${\mathrm{mm}}^{-1}$ (for all layers), $l_1=5$ mm, $l_3=2$ mm.