Dichroic means "two color." Any film combination that has to perform to a specification at more than one wavelength zone is dichroic. The films may even have similar specifications, that is, high reflection at 450-650 nm and also high reflection at 1060 ± 10 nm. Generally, dichroic refers to high reflection for a certain distance and then low reflection for another distance. Examples: cold mirrors reflect 400-700 nm and pass 760-2000 nm; hot mirrors pass 400-700 nm and reflect 800-1200 nm. The definitions become complicated if the design is also used at a 45-deg angle.
High-quality coatings can be very difficult to manufacture, although designing is easier now with the use of computers and good software. However, errors in achieving the prescribed thickness of the layers can alter the spectral performance of the coating. A number of stacks and partial quarterwave layers are needed to smooth out ripple in the transmission zones.
By altering a few layers in the reflector, the performance of the coating can be profoundly improved. The stack (HL)9 L will become a yellow filter when the first four and last five layers are changed by the TFCalc program (Fig. 5.1). This is a long-pass filter. All of the wavelengths above the reflection zone are passed (transmitted).
Design: 0.4H 1.18L 0.82H 0.97L (H L)4 H 0.94L 1.18H 0.8L 85H 2.08L utilizing Ta2O5 (H) and quartz (L).
After selecting the position of the quarterwave stack by running the software and moving the wavelength, I instructed the computer program to find the best thickness for low reflection at 563-1300 nm (at 10 nm intervals) and high reflection at 440-510 nm. I then selected the layers that allow their thickness to be changed. It took approximately 5 sec of optimization to select these thicknesses. A short-pass filter has the opposite properties. The exact same starting design of the long-pass filter was used as the starting design for this filter. Targets of low reflection from 400-550 nm and high reflection from 590-690 nm were selected; the same layers were selected for modification. After 5 seconds of optimization (~125 steps), the design in Fig. 5.2 appeared.