Athermalization And Near Focus Analysis Of The Gun Scope Optical System With An Uncooled LWIR Thermal Imaging Sensor

Gun scopes equipped with uncooled thermal imaging sensors operating in the LWIR (Long-Wave Infrared) range of 8–12 µm are advanced optical devices essential for military and law enforcement applications. These forces operate in extreme environments, from freezing mountain ranges to scorching deserts, requiring equipment that performs reliably under all conditions. This paper presents an athermal optical design for a Gun scope optimized for extreme temperatures ranging from -10°C to +60°C. A near-focus analysis is conducted at 10 m for a temperature of 20°C, as well as across the full temperature range. The optical system is built around an uncooled (LWIR) microbolometer sensor with a 640 × 480-pixel focal plane array (FPA) and a 17 µm pixel pitch [1]. Designed using SYNOPSYS™ [2] optical design software, the system features an F-number of 1.5, a 105 mm focal length, and a 7.42° full field of view. It consists of three lenses, including one aspheric element, with a total weight of 210 grams and an optical length of 130 mm, ensuring a compact and lightweight design [1]. Active athermalization is achieved through the controlled movement of a single lens along the optical axis. Performance analysis reveals that the polychromatic Modulation Transfer Function (MTF) at the Nyquist frequency, with a value of 0.446 at on-axis and 0.44 at 0.75 field, decreases to less than 5% at 60°C and to less than 2% at -10°C when utilizing an aluminum housing [3]. For near-focus conditions, the on-axis polychromatic MTF drop at the Nyquist frequency remains below 5% across the entire temperature range. Additional image quality metrics, including Optical Path Difference (OPD) and Strehl ratio, as well as image analysis using extended sources, are also presented. The active athermalization approach is chosen for its cost-effectiveness and simplicity. The lens movement is implemented through a manual thread-based linear focusing mechanism, ensuring a lightweight and efficient solution.