Optical design of the freeform reflective imaging system with wide rectangular FOV and low F-number

In recent years, freeform optical surfaces have been used increasingly in a variety of off-axis reflective imaging systems with high performance levels. However, it is a difficult and challenging task to design an off-axis reflective imaging system that has a low F-number and a wide rectangular field-of-view (FOV) simultaneously. In this paper, the field extension construction (FEC) design method is proposed to design a freeform imaging system with both a low F-number and a wide rectangular FOV. When using an existing normal system with a small rectangular FOV as the input, an initial configuration that simultaneously has both a low F-number and a wide rectangular FOV can be obtained after several system construction attempts. A freeform reflective infrared camera is then successfully designed after optimization and has a focal length of 9.3 mm, an F-number of 1.39, and a wide rectangular FOV of 40 degrees x 30 degrees. The image quality is close to the diffraction-limited level in the farinfrared band and the maximum relative distortion is 5.5%. In addition, the system prototype is presented after fabrication and its good practical imaging performance is demonstrated.