FIGURE 9.

A comparison between bi-part and conventional prisms of the calculated extent of visual field as a function of eye scanning angle. For a patient with hemianopia, the extent of visual field is from primary gaze (eye scanning angle = 0 degrees; head direction) toward the blind side or prism base. At the foveal line of sight, the angle of incidence is equal to the eye scanning angle. The effective eye scanning range is limited by the angle of incidence, resulting in 50% transmittance. Within this range, the effective prism power increases as the patient scans farther toward the blind side. As a result, the increase in the extent of the visual field is larger than the scanning angle, representing prism minification (image compression). The increase in the extent of the visual field saturates when the eye scanning angle exceeds the critical angle. On eye scanning into the seeing side, the extent of field toward the blind hemifield is largely unchanged with the bi-part prism even when compared with the conventional 36Δ prism, and it is much more stable than with the conventional high-power prism. Compare with the thin dashed lines that represent a constant field extent at all scanning angles.