Bandpass and edge filters are examined in this chapter. Filters are designed to be used at a 45-deg angle in air. Reflective bandpass filters are discussed first; they are designed to absorb the radiation that is not reflected as a bandpass. Two types of designs are shown. For low wavelengths a long-pass dielectric filter deposited on a dark mirror performs well. The filters described in Chapter 6 are used as the starting designs for these dichroic reflective color filters.
Bandshape cannot be easily improved beyond the performance shown for the following designs. Multicavity filters are not allowed. The filters are designed to function only in the visible spectrum area (for the most part), and further blocking is possible with more layers and less contrast. To achieve low polarization, two sets of targets, one for each polarization mode, are provided for the spectral zone affected (Fig. 11.1).
Design: 20nmI 0.52qwQ 1.57qwT 1.83qwQ 19nmI 1.97qwT 0.26qwQ 1.32qwT 3.18qwQ 1.24qwT 0.59qwQ 0.38qwT 1.9qwQ 2.66qwT 1.92qwQ 0.27qwT 0.47qwQ 1.23qwT 2nmI 0.72qwT 1.18qwQ referenced to 490 nm.
The bandwidth is about 60 nm and displays little polarization in both the reflective and absorptive regions. The filter's properties are uncontrolled above 800 nm. The purpose of the filters is color control. The wavelength may be altered for this configuration of layers to produce different colors; the layer thicknesses are then slightly adjusted to keep low reflectance in the blocking region of interest. A green filter with 21 layers shows similar shape and blocking characteristics (Fig. 11.2).