Fig. 7.

$⟨z_{max}|t⟩$ at different time of flight $t=0.50,0.75,1.00,1.25,1.5,2.0,3.0,4.0\text{ns}$ (color code from brown to blue) as a function of the thickness of the upper layer $s_1=5,10,15,20,25\text{mm}$ for a two-layer medium with constant total thickness ( $s_{\mathit{\text{tot}}}=90\text{mm}$ , $n_1=n_2=1.4$ ). Panels (a)-(d) have same scattering ( ${\mu }_{s1}^{\mathrm{\prime }}={\mu }_{s2}^{\mathrm{\prime }}=1.0\text{m}{\text{m}}^{-1}$ ) and different absorption: (a) ${\mu }_{a1}=0.005\text{m}{\text{m}}^{-1}$ , ${\mu }_{a2}=0.015\text{m}{\text{m}}^{-1}$ ; (b) ${\mu }_{a1}=0.025\text{m}{\text{m}}^{-1}$ , ${\mu }_{a2}=0.015\text{m}{\text{m}}^{-1}$ ; (c) ${\mu }_{a1}=0.015\text{m}{\text{m}}^{-1}$ , ${\mu }_{a2}=0.005\text{m}{\text{m}}^{-1}$ ; (d) ${\mu }_{a1}=0.015\text{m}{\text{m}}^{-1}$ , ${\mu }_{a2}=0.025\text{m}{\text{m}}^{-1}$ . Panels (e)-(h) have same absorption ( ${\mu }_{a1}={\mu }_{a2}=0.015\text{m}{\text{m}}^{-1}$ ) and different scattering: (e) ${\mu }_{s1}^{\mathrm{\prime }}=0.5\text{m}{\text{m}}^{-1}$ , ${\mu }_{s2}^{\mathrm{\prime }}=1.0\text{m}{\text{m}}^{-1}$ ); (f) ${\mu }_{s1}^{\mathrm{\prime }}=1.5\text{m}{\text{m}}^{-1}$ , ${\mu }_{s2}^{\mathrm{\prime }}=1.0\text{m}{\text{m}}^{-1}$ ); (g) ${\mu }_{s1}^{\mathrm{\prime }}=1.0\text{m}{\text{m}}^{-1}$ , ${\mu }_{s2}^{\mathrm{\prime }}=0.5\text{m}{\text{m}}^{-1}$ ); (h) ${\mu }_{s1}^{\mathrm{\prime }}=1.0\text{m}{\text{m}}^{-1}$ , ${\mu }_{s2}^{\mathrm{\prime }}=1.5\text{m}{\text{m}}^{-1}$ ). In all cases we set $\rho =30\text{mm}$ , $n_{e1}=n_{e2}=1.0$ . The dashed line is the unity line for which $⟨z_{max}|t⟩=s_1$ .