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.