According to the interior stray light analysis of infrared refractive zoom optical system. An optical system of mid-wavelength infrared waveband for cooled system is designed. The F-number of the system is 4 and full field of view 2ω=11.96°-2.84°. The system chooses a double imaging configuration and the efficiency of the cold stop is 100%. With the help of stray light analysis software of LightTools,a optical system with mechanical components is built. The critical objects are then judged. The heat radiation received on image plane of the detector is computed and the the minimum radiance of the target is computed according to the signal to noise. In this paper,the narcissus effects of dif-ferent focal lengths in optical system are analyzed. A method of inverse path tracing rays was proposed for analyzing the magnitude of narcissus of each surface in the optical system. These results demonstrate that the optical system had better restrain effect of narcissus.%针对折射式红外变焦光学系统内部的杂散辐射，设计了焦距240~60mm、系统F/#为4、视场11.96°~2.84°、工作波段为3.7~4.8μm的制冷型中波红外变焦光学系统。该系统为二次成像结构，并且有100%的冷光阑效率。利用杂散光分析软件Light Tools，建立光学机械结构模型并判断出关键表面，分析得到探测器像面接受的热辐射，根据信噪比推断出目标的最小辐亮度。采用光路倒置的方式分析光路中各光学表面产生的冷反射大小，分析了光学系统在不同焦距时的冷反射效应，结果表明该光学系统具有较好冷反射抑制效果。
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