In the experiments, the transducer was first moved along the Y direction to position the US focal point 2 mm deep in the scattering layer (the same Y plane where the inclusions were embedded). Both the light and the ultrasound were kept stationary, while the phantom was scanned along the X direction with a step size of 0.127 mm. At each position, a TRUE signal was obtained as discussed in Fig. 2. A dc signal and a time-reversed direct current (TRDC) signal were also recorded at PD2 and PD1, respectively, when the AOM tuning and US modulation were turned off. The result of the scan is shown in Fig. 3, where the normalized signal intensities are plotted as a function of X. The dc and TRDC images have spatial resolutions of 3.3 and 2.8 mm, respectively, based on the FWHM of their Gaussian fits,3 and thus lack the ability to resolve the three objects due to the light diffusion. For TRUE, however, the embedded objects are evident against the background. Their fitted widths, based on the Gaussian fit, measure 1.0, 1.1, and 1.0 mm, respectively, agreeing well with the actual widths of the objects. Spacings between the adjacent objects are also consistent with the actual positions. In addition, Obj 2 produced a lower TRUE signal intensity than the other two targets due to its higher absorption coefficient, suggesting less light was focused back to the US focus at Obj 2's position. Finally, the spatial resolution of the TRUE image, computed from the FWHM of the Gaussian fit, was 0.63 mm, which is approximately 1/ of the US focal width and consistent with the square law.3 All of these findings lead us to conclude, although indirectly, that the reflection-mode TRUE focusing system was able to focus light back to the US focal zone within a turbid medium.