Fig. 3.

PA spectrum of fresh pigment as a function of depth. A silicone tube was filled with purified pigment and cross-sectional US/PA images were acquired. The PA spectrum was analyzed within different regions of interest. “Top” and “bottom” indicate locations closest to, and further from, the transducer with the least and most light attenuation, respectively. (a) The PA spectrum of melanin was modified by depth due to light absorption of melanin, causing a significant change in the PA spectrum. (b) The experiment was repeated with the addition of a scattering layer (sclera), resulting in further modification of the measured PA spectrum of melanin.