Fig. 1. Different off-axis structures. (a) 4-type structure off-axis two-reflective system; (b) Z-type structure two-reflective system; (c)-(j) Eight structural types of off-axis three-reflective system

Fig. 2. Schematic of the principle of partial differential equation method

Fig. 3. Schematic of the principle of SMS

Fig. 4. Schematic of the principle of CI-2D^[27]. (a) Initial structure; (b) Sampling feature rays; (c) Construction of surface and iteration; (d) Starting-point of freeform surface imaging optical system

Fig. 5. Schematic of expending FOV. (a) Expanding the FOV in the X; (b) Expanding the FOV in the Y

Fig. 6. Flow chart of alternate optimization method based on freeform surface

Fig. 7. Optical path of ellipsoidal mirror system. (a) Single ellipsoid mirror; (b) Overlapping double ellipsoidal mirrors; (c) A head-mounted display system^[37]

Fig. 8. Design of single reflector^[38]

Fig. 9. Design examples of fusion of parabolid. (a) Czerny-Turner imaging spectrometer^[40]; (b) f-θ telecentric scanning system^[41]

Fig. 10. Design steps of fusion of parabolid^[42]. (a) Split FOV/aperture; (b) Solve parameter of sub-mirror of off-axis paraboloid; (c) Surface fusion