Fig. A.1.

The schematic illustration of the method of calculating the semi-analytical SIR. The square at the center of the figure represents the transducer element face. X and Y denote the local coordinate system for the transducer element of interest. S denotes the point photoacoustic source. P denotes the projection of S on the transducer face. The gray spherical shell represents the wavefront at time t. ${\{{\mathrm{C}}_i\}}_{i=1}^6$ represent the intersections of the wavefront and the edges of the transducer face. Θ₁ and Θ₂ denote the angle of the radius PC₁ and PC₂, respectively, measured from the positive X direction. Let Q be an arbitrary point on the intersection arcs. ∠QSP is denoted by Φ.