Infrared imaging system has been widely applied in infrared night vision, infrared detection, and infrared guidance. However, the detection accuracy of conventional infrared imaging systems in a single wave band is easily affected by regional terrain, ambient temperature, and target camouflage. If an infrared imaging system can obtain the target information in two infrared wave bands at the same time, the interference information can be easily suppressed during complex environment detection applications. The conventional refraction, reflection or reflector dual-band optical systems are usually designed with two channels or a channel including a spectroscope. The former design configuration contains two independent imaging systems, and the latter design configuration contains a reflective system or a broadband achromatic system. To solve the complex configuration of dual-band infrared imaging system, this paper proposes a dual-band freeform prism in the middle and long wave infrared region.

The freeform prism is a highly integrated optical system consisting of multiple asymmetric surfaces. The direct optimization design strategy can easily obtain invalid optical path structures. Therefore, a progressive optimization design strategy that gradually complicates the optical path can be adopted. Firstly, the on-axis reflective system can be designed according to the optical parameters. Then, the off-axis reflective system can be designed based on the above rotationally symmetric system. Finally, the media air in the optical path can be substituted by refractive materials. Additionally, the achromatic and thermal aberration corrections of the infrared freeform prism should also be considered. After the freeform prism fabrication, surface adjustment cannot be conducted, so it is necessary to decrease the influence of the fabrication tolerance sensitivity during the optimization design. When the deviation between the incident angle and the refraction angle of the refraction surface is smaller, the optical path change in this region is less sensitive to small disturbance. As for the reflective surface, the optical path change due to small disturbance depends mainly on the incident angle of the light on the reflector. Based on a low sensitivity optimization method of limiting the tracing ray angle in the design, a dual-band (3.7-4.8 μm and 8.0-12.0 μm) infrared optical system with an F number of 1, a focal length of 20 mm, and a field of view of 21.8°×16.4° has been designed.

The modulation transfer functions of the dual-band freeform prism are above 0.79 and 0.67 at a spatial frequency of 20 lp/mm for the middle wave infrared (MWIR) and long wave infrared (LWIR), respectively (Fig. 6). The maximum field curvature is less than 0.05 mm, and the maximum distortion is less than 3.5 % (Fig. 7). The distortion of the optical system is only symmetrical about the YOZ plane of the coordinate system, and the most deformation is at the edge of the field of view. The energy concentration of the middle wave infrared channel is greater than 91%, and that of the long wave infrared channel is greater than 78% (Fig. 8). The final optimized color separation film layer structure has been designed by OptiLayer (Fig. 11). When the incidence angles are 31°, 45° and 52°, the average reflectance of the film at 3.7-4.8 μm is greater than 90%, and the average transmittance at 8.0-12.0 μm is greater than 93% (Fig. 12). The diamond machined long wave infrared freeform surface prism and the uncooled infrared detector have been assembled. The LensCheck optical transfer function instrument is adopted to measure the transfer function of the freeform surface prism. After testing, the average transfer function over each field of view is better than 0.2 at 20 lp/mm (Fig. 13), and the captured image at the center and edge field of view are both clear.

A dual-band freeform prism in the middle and long wave infrared region has been designed based on low sensitivity optimization method of limiting the tracing ray angle in this paper. When the system is at a spatial frequency of 20 lp/mm, the modulation transfer functions of the designed system are above 0.79 and 0.67 for MWIR and LWIR, respectively. The maximum field curvature is less than 0.05 mm, and the maximum distortion is less than 3.5%. The imaging quality of the optical system is good in the temperature range of -20-70 ℃. According to tolerance analysis, the designed freeform prism can be fabricated by single point diamond turning technique, and a long wave freeform prism with working wavelength range of 8.0-12.0 μm has been fabricated and measured. The average modulation transfer function over each field of view is better than 0.2 at 20 lp/mm. At present, the developed dual-band design configuration in this paper has the advantages of high performance and compact structure.