For the measurements, the detection head was scanned using two translation stages and the OCT and PAI data were simultaneously acquired. After each step of the translation stages, the measurements were paused for a short time period to reduce mechanical vibrations, before the next measurements were started. This lead to increased scanning times, and was one of the reasons why we simultaneously acquired OCT and PAI data, i.e., to speed up the measurement. The long measurement times lead to drying of the samples. Consequently, their shape changed slightly during the measurement. By measuring OCT and PAI simultaneously, perfect coregistration of both modalities could be achieved for 3-D imaging of the agarose phantom and for 2-D imaging of the chicken skin phantom. However, even with simultaneous scanning, the measurement times and the resulting changes in the chicken skin phantom were too high to allow for a 3-D measurement. Simultaneous measurement, however, comes at a cost. To maximize light collection for ncPAI, the focus was placed onto the specimen’s surface. As the same focusing lens was used for OCT and ncPAI, it was not possible to choose the focus for OCT independently. Usually, in case of highly reflecting surfaces, the focus of OCT is either set significantly beyond the surface inside the sample or the sample surface is tilted. Thereby, local reflections of high intensity at the surface are avoided in favor of reflections at inner structures. Tilting was not an option because it would have significantly reduced light collection for ncPAI and, in the presented setup, the chromatic focal shift of the achromatic objective lens fixed the focus for OCT above the sample surface, at the cost of a degradation of the lateral resolution and low signal intensity. The latter has been compensated by averaging over 200 A-scans. In the presented setup, the spot diameter at the sample surface of the interrogation beam was about , which was also the lateral resolution of OCT on the sample surface. Deeper inside the structure, the lateral resolution degraded. For example, in a depth of 0.5 mm, the beam diameter broadened to about . This resulted in lateral blurring of the structures, as can be seen, e.g., in Fig. 5. The nominal diameter of the structure of could be reproduced in the axial direction, while in lateral direction a size of was obtained by OCT. In the PA image, a diameter of can be reproduced in the lateral and axial directions. No serious broadening was expected for PAI, as the system parameters were chosen to allow for a resolution of about : the spot diameter of permits acquisition of acoustic waves of frequencies up to 50 MHz. This frequency is matched to the bandwidth of the photodetector of 45 MHz; in the measurement, the step size was chosen to be , fitting the aforementioned parameters.