FIGURE 6.

The rays’ paths through a bi-part double Fresnel prism. The two Fresnel prism segments with apical angle α are inclined relative to each other at angle &thetas;. The total deflection of light δ is the sum of the deflection powers of the two segments (δ = δ₁ + δ₂). As with a conventional prism, the effective prism power of the bi-part prism is increased by a negative angle of incidence and is limited by TIR (blue ray).11 This design offers some flexibility through a trade-off between a wider eye scanning range and a higher nominal power (see Fig. 9). When the angle of incidence increases, the effective prism power of the bi-part prism increases (from red rays to blue rays on the left). If the eye scanning angle in the first prism (closer to the eye) is just under the critical angle of incidence (blue rays), the angle of incidence in the second prism should be higher than the critical angle of incidence to prevent TIR (blue rays).11 This is achieved by increasing angle &thetas;.