Figure 2.

Channel prisms. (A) Channel prism lens available from Chadwick Optical. There are three sector prisms per carrier lens, with the apices outside the borders of the patient's functional field when at primary gaze. In this case, the lateral prisms are 12Δ, base-in and base-out, on the nasal and temporal side, respectively, and the lower prisms are 8Δ, base-down, matching the configuration and prism powers that were used by InWave. Note the break in the visibility of the spectacles' temples due to the apical scotomas. (B) Photo through a molded InWave channel prism trial lens. A polar grid with 5° radius increments is seen through the lens. The channel width is 6 mm (17°), the smallest offered by InWave. Note that the central circle in the grid is 10° in diameter. Approximately 6° laterally are lost to each of the left and right prisms' apical scotomas to achieve a corresponding lateral field substitution (extension, not expansion), and 4° lost to the lower scotoma, hiding sections of the original grid from view. (C) 6 mm channel lens fabricated using 40Δ 3M Press-On™ Fresnel prisms. High-power prisms have correspondingly large apical scotomas. (The spectacles are just a few millimeters from the paper, yet the apical scotoma changes each fox into an ox.)