Image Quality Metric Analysis- A Core Design Perspective- For 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 offer significant advantages due to their affordability, reliability, and compact design. These systems provide clear imaging even in complete darkness and can penetrate through smoke, fog, dust, and adverse weather conditions such as rain and snow [1-4]. Gun scopes play a crucial role in military and law enforcement applications, including enhanced target acquisition [5], surveillance and reconnaissance [6], and search and rescue operations [6]. The Gun scope optical system is designed around an uncooled LWIR thermal imaging sensor, a microbolometer, featuring a focal plane array (FPA) with a resolution of 640×480 pixels and a 17 µm pixel pitch. The optical system is designed using SYNOPSYSTM optical design software. The system has an Fnumber of 1.5, a focal length of 105 mm, and a full field of view of 7.42°. The optical design consists of three lenses, including one aspheric surface, with a total weight of 210 grams and an optical length of 130 mm. This configuration ensures a simple, compact, and lightweight system [7]. Performance analysis is conducted using both geometrical and diffraction-based image quality metrics, including polychromatic modulation transfer function (MTF), spot diagrams, encircled energy, and distortion [7]. These metrics directly impact the image quality produced by the optical system [8]. The design criteria include polychromatic MTF to be greater than 0.42 at Nyquist frequency and diffraction limited over the full field of view, the RMS spot size to be less than the detector pixel pitch 17µm, encircled energy to be more than 80%, and distortion to be less than 2%. The optical system design accomplished these design requirements successfully [7]. This paper focuses on key image quality metrics relevant to the core of the design process but not typically specified in the end-product specifications. These include optical path difference (OPD), transverse ray curves, astigmatic field curves, Strehl ratio, and image analysis using extended sources. Understanding these parameters aids in optimizing optical performance during the design phase.