设计了工作于3~5 μm和8~12 μm双波段、F数为1.2的大相对孔径的红外消热差光学系统。该系统全视场角为22°,有效焦距为50 mm，系统总长78 mm。系统采用锗和AMTIR I(Ge、As、Se混合材料)两种材料,为三片镜结构。通过引入双层谐衍射元件，大大提高了衍射效率，减小了色差，并使系统重量减轻。系统在-40 ℃~60 ℃的温度范围内性能稳定,适用于像元尺寸为25 μm，像元数为640 pixel×480 pixel的凝视式双波段焦平面阵列探测器。设计结果表明：当探测器的尼奎斯特频率为20 lp/mm时，温度取不同值时的调制传递函数(MTF)值均大于0.5，成像质量良好，实现了消热差设计。可在作用距离为2 km时，分辨3 m范围的目标，满足**侦察的需求。

An infrared athermal optical system with F number of 1.2 for dual-band of 3~5 μm and 8~12 μm is designed. In this system, the view of vision angle is 22°, total length is 78 mm. Germanium and AMTIR I (mix of Ge, As and Se) and double-layer harmonic diffraction element are used to simplify the three-lens structure, decrease weight, and improve the quality of the imaging. Across temperature range of -40 ℃~60 ℃, the system has a stable performance and applies to gaze-type dual-band focal plane array detector with pixel size of 25 μm, and pixel number of 640 pixel×480 pixel. The design results show that, the modulation transfer function (MTF) is greater than 0.5 at different temperatures when the Nyquist frequency of detector is 20 lp/mm. The imaging quality is good, and athermal design is realized. It can distinguish a target of 3 m when the distance is 2 km. So this system can meet the needs of military reconnaissance.