Figure A1.

Critical angle of incidence with 50% transmittance. Note negative sign in angle of incidence is toward the base side. (A) Transmittance and (B) effective prism power vary with angle of incidence in 57Δ prisms. Total internal reflection starts at −5.3° angle of incidence with 0% transmittance and maximizes the effective prism power. However, the transmittance is below 50% with angles of incidence higher than 4.7° toward the base side, which cannot be used for the field expansion due to the low visibility of the shifted view. Therefore, we set the critical angle of incidence for field expansion to −4.7°. At this angle of incidence, the effective prism power is 39° (≈81Δ). (C) Critical angle of incidence with 50% transmittance in various prism powers of OPS and EPS prisms. Dashed line indicates the maximum prism power of OPS prisms without TIR at 55° nasal field (Fig. 1B).