  A low-cost adjustable illumination scheme for hand-held photoacoustic imaging
probe is presented, manufactured and tested in this paper. Compared with
traditional photoacoustic probe design, it has the following advantages: (1)
Different excitation modes can be selected as needed. By tuning control
parameters, it can achieve bright-field, dark-field, and hybrid field light
illumination schemes. (2) The spot-adjustable unit (SAU) specifically designed
for beam expansion, together with a water tank for transmitting ultrasonic
waves, enable the device to break through the constraints of the transfer
medium and is more widely used. The beam-expansion experiment is conducted to
verify the function of SAU. After that, we built a PAT system based on our
newly designed apparatus. Phantom and in vivo experimental results show
different performance in different illumination schemes
