A TEM image simulation package is written which is used to investigate the strain field contrast of faceted voids and bubbles. The strain field is modelled either as spherically symmetric or using a Green's function technique and the results of the two methods are compared with each other and with real images. The system demonstrates that a gas pressure of between 0.5 and 0.75 GPa must be present in overpressurised bubbles for significant strain contrast to be observed. It is shown that there is not a linear relationship between the gas pressure and the extent of the strain field contrast and that defocussing the microscope can reduce, or even eliminate, the strain field contrast. Image reversal between voids and overpressurised bubbles is also demonstrated.