Phantom experiments were carried out to test the system performance. A set of well-controlled tissue-like liquid phantoms were prepared by mixing water, intralipid (Fresenius Kabi, Uppsala, Sweden; ), and India ink (Pelican Fount, Hannover, Germany; 1:100 stock solution prepared in our laboratory) or bovine blood (purchased from a local supermarket). The optical properties were chosen to have a good correspondence to real biological tissue. PpIX stock solution was prepared by dissolving 0.056 g PpIX powder (Fisher Scientific, Pittsburg, Pennsylvania) in 20 ml dimethyl sulfoxide (DMSO) (Merck, Darmstadt, Germany) and 80 ml distilled water, resulting in a PpIX concentration of 1 mM. When diluting the PpIX solution in the phantom, 2% volume of Tween (Scharlau Chemie S.A., Barcelona, Spain) was added to prevent PpIX from aggregation. The ink phantom contained 3.75% volume of intralipid, 0.25% volume of ink, and 100 nM PpIX. In total 270 blood phantoms were prepared containing different concentrations of intralipid (3%, 4%, 5%, 6%, 7%, and 8%, v:v), bovine blood (0%, 1%, 2%, 3%, and 4%, v:v), and PpIX (0, 100, 200, 300, 400, 500, 600, 750, and 1000 nM), respectively. Each phantom was placed in a cylindrical glass container. The phantoms were stirred for 4 h prior to the measurements using a magnetic stirrer (to ensure that the temperature of each APD was stable) and kept stirred during the measurements.