Fig. 1. Optical path in dual band imaging system. (a) Imaging channel FP1; (b) imaging channel FP2; (c) dual band freeform prism

Fig. 2. Optical path deviation caused by small perturbations on optical surfaces. (a) Refractive surface; (b) reflective surface

Fig. 3. Center and edge rays of normalized field of view

Fig. 4. Margin rays and nodes of all-reflective system and infrared dual band freeform prism. (a) All-reflective system; (b) infrared dual band freeform prism

Fig. 5. Design result of infrared dual band freeform prism

Fig. 6. MTF curves of optical system at -20 ℃, 25 ℃, and 70 ℃. (a)(b) -20 ℃; (c)(d) 25 ℃; (e)(f) 70 ℃

Fig. 7. Field curvature and distortion of optical system under different bands. (a) Middle wave infrared band; (b) long wave infrared region

Fig. 8. Diffraction enclosed energy of optical system under different bands. (a) Middle wave infrared region; (b) long wave infrared region

Fig. 9. MTF curve obtained by Monte Carlo analysis under different bands. (a) Middle wave infrared region; (b) long wave infrared region

Fig. 10. Optical constant curves of high and low refractive index materials. (a) Ge coating material; (b) ZnS coating material

Fig. 11. Coating structure of dichroic film

Fig. 12. Coating design spectra of dichroic film. (a) Reflectance spectral curve; (b) Transmittance spectral curve

Fig. 13. Manufacture and test of freeform prism. (a) Optical layout of long wave infrared freeform prism; (b) sample of long wave infrared freeform prism; (c) designed MTF curve; (d) measured MTF curve; (e) captured image