As well as attenuation due to absorption and spreading, the loss at the reflectors should be considered. The image reconstruction algorithm was based on the assumption that the reflectors had a pressure reflection coefficient of 1, i.e., they are 100% reflective. In practice, waves normally incident to reflectors will experience a reflection coefficient . Silicon and tungsten carbide have high acoustic impedances and reflection coefficients of 0.92 (velocity of and density of 23) and 0.97 (velocity of and density of 19), respectively, but are optically opaque which complicates the illumination of the sample. A water-air boundary has a pressure reflection coefficient of , and the reconstruction algorithm could be adapted to this case, but in implementing such a boundary it would be necessary to use a membrane to contain the water, and the water-membrane-air boundary only exhibits this ideal behavior over a narrow range of frequencies.24 As PA is inherently broadband, this is a drawback. In these experiments, borosilicate glass was used: it has quite a large acoustic impedance () and it is optically transparent, which allows the sample to be illuminated from all sides. At incident angles above 25.6 deg relative to the normal of the reflecting boundary, the waves will undergo total internal reflection, matching the assumption of the perfect reflecting boundary used in the reconstruction. However, these waves will undergo a phase change on reflection, so their contributions to the final reconstructed image will be blurred to some extent. For the majority of reflected waves with incident angles greater than 25.6 deg relative to the normal to the boundary, the phase shift in the reflected wave is . As this phase shift remains relatively constant for a range of reflection angles, it may be possible to compensate for it in later arrivals by introducing a virtual boundary offset in the positions of the walls used in the reconstruction by a distance equivalent to the phase shift.25 This is not implemented on the results presented here, but is left as future work.