Fig. 4.

(a) Photograph of the cross-section of the 6 cm-thick phantom at the central $Y$ plane ($Y=3\mathrm{cm}$) showing three embedded absorption objects. The needle is used as an ultrasonic fiducial marker for the alignment of the $Y$ and $Z$ positions between the embedded objects and the ultrasound focus. (b) UOT image at the central $Y$ plane. The color bar corresponds to the modulated-light power normalized to the peak of each A-line. (c) Modulated-light power distribution along the horizontal dashed line in (b). The blue squares are the measured data, and the red curve the FFT smoothed results. (d) Modulated-light power distribution along the vertical dashed line in (b).