Bandpass and edge filters are examined in this chapter and are designed for use at a 45-deg angle. These are slab filters; therefore, the second surfaces of transmissive filters require a nonpolarizing reflection-reducing coating (Fig. 12.1). A design with nine layers using three materials provides excellent performance. The bulk of the design uses TiO₂ and SiO₂ with one layer of MgF₂. The coating retains hardness by covering the MgF₂ layer with a 20-nm-thick SiO₂ layer. Fewer layers are effective for a narrower spectral range.

Design: 0.22qwT 0.43qwQ 1.05qwT 0.11qwQ 0.74qwT 2.2qwQ 1.93qwT 0.82qwM 0.22qwQ all layers in qw units at 520 nm.

About 10% of the S-polarized light is reflected without a coating.