Fig. 5.

Target fluorescence intensities as a function of the target depth, $z_T$, and the threshold calculated by Eq. (17) with ${\xi }_{\alpha }=3$ (the $3\sigma$ limit). Solid curves are the target fluorescence intensities, and the different colors indicate the different SD distances, $\rho$. The dashed line segments show the thresholds. The black dashed line indicates the threshold when the background emission is ignored, i.e., $F_B^{\mathrm{obs}}=0$. The small circles indicate the intersection points where Eq. (17) is satisfied for each SD distance, i.e., the depth detection limit $z_T^{\lim }$.