In most of our experiments, the PA signals were measured at flat and reflective surfaces, and the probe beam of the interferometer was aligned perpendicularly to the surface. However, in real biological applications, we can think of the signal as suffering from the surface roughness of samples. To overcome the signal degradation and/or distortion caused by the sample surface roughness, several studies have been made.15,38,39 Additional materials such as water or oil were applied on the sample surface to increase its reflectance and/or reduce the roughness.15 A photorefractive crystal was also used to concentrate the beam scattered from the rough surface.38,39 In the proposed system, we will be able to improve its limitations by applying the surface smoothing materials as in the reported study15 or compensate the image distortion by measuring the roughness of the sample before the PA measurement. In addition, an important issue for biological imaging is the imaging speed. In our current configuration, unfortunately, the imaging speed was highly limited by the low-repetition rate of the pulsed laser; it was only 20 Hz. Therefore, when we got one sheet of a 2-D depth image under the experimental conditions of 1 cm scan length and 100 steps, the data acquisition time was as poor as 5 s. For the real-time image reconstruction, the imaging acquisition speed should be higher than 10 fps in general. By using the laser with a repetition rate of 2 kHz, we can acquire the same data within 0.05 s, which enables to get more realistic 3-D images in vivo. Therefore, it is definitely necessary to use a laser with a higher repetition rate.