The problem is illustrated in Fig. 1. The upper half shows a calculated diagram of the image through a prism placed base left (as fitted for left HH) in front of a perimetry grid, assuming (as in 1) uniform deflection through the prism at all angles of incidence, the constant deflection angle (CDA) assumption, and not considering internal reflections. The lower half is an actual photograph of the grid through a rigid Fresnel prism. The view through the prism at the apex side is slightly magnified, whereas the view to the left is severely compressed (minified). Given the high angle of incidence at the prism apex, the prism has less than its rated (nominal) power and thus less “prism jump” at its apex (and a smaller apical scotoma) than in the upper diagram. At incidence angles greater than about 5 deg to the left of the grid center, total internal reflection (TIR) occurs. In that region, none of the “intended” view is seen, and dim reflections from unintended portions of the scene become visible. Note, however, that when the eye is at PG, the strongest reflections fall in the blind hemifield and are thus only consequential when the patient is gazing to the left. In this paper, we call the image portion seen through the prism as field expansion the intended image, and any other image portions seen through the prism are considered spurious images.