Compared with the traditional visible light imaging technology, infrared spectral thermal imaging technology utilizes the thermal radiation emitted by objects to obtain images of target objects, with unique advantages in target detection and tracking. Particularly in the long-wave infrared (LWIR) bands, it demonstrates superior transmittance, increased propagation distance, and enhanced detection performance. Consequently, infrared cameras possess application significance in the high-end commerce, monitoring, and other fields. However, conventional thermal imaging cameras are limited by a fixed focal length, which enables observation only within a specific field of view or area and hampers search and observation capabilities. To this end, the infrared zoom thermal imaging camera is developed. By continuously adjusting the field of view and magnification relationship via a continuous zoom optical system, seamless range size adjustment is achieved, with stability and image clarity maintained. However, the majority of existing LWIR zoom systems incorporate diffractive surfaces, which results in complex design requirements, elevated processing and assembly demands, and increased system costs. Furthermore, the design of certain LWIR zoom systems overlooks the influence of ambient temperature on image quality, subsequently compromising practicality. Thus, it is imperative to devise a low-cost, compact, and uncooled long-wave infrared continuous zoom optical system that preserves excellent image quality across a wide range of temperature variations and exhibits strong practicality. We aim to make the design outcomes contribute to advancements in military weapon targeting, handheld thermal imaging cameras, unmanned vehicles, and related fields.
To meet the requirements of the specific application environment, we have determined the appropriate initial structure for the design. The mechanical positive group compensation method is chosen as the compensation technique for the system. Additionally, the introduction of sulfur glass helps control chromatic aberration and minimize thermal defocus within the system. Meanwhile, the temperature compensation group employs the smallest aperture lens in the system to address temperature variations and maintain image quality. We incorporate the electro-mechanical active non-thermalization method, allowing the temperature compensation mirror group to be adjusted and ensuring excellent imaging quality across a wide temperature range. Additionally, we utilize Zemax OpticStudio software to optimize the design to help control the system size and improve overall image quality. By adopting this iterative process, we design a non-thermalized continuous zoom optical system for LWIR. The designed system takes into account the practicality of implementation, cost-effectiveness, and compactness while delivering excellent image quality and addressing thermal variations. This design has significant potential for applications in handheld thermal imaging cameras, unmanned vehicles, and other related areas.
After implementing the Zemax OpticStudio software for optimization, a continuous zoom optical system for LWIR consisting of seven lenses is designed. The materials chosen for the lenses are ZnSe and ZnS for the first and second lenses, IRG24 for the fourth lens, and Ge for the third and fifth to seventh lenses (Table 2). In this optical system, five even-ordered aspherical surfaces are employed, and their feasibility for machining is analyzed (Table 3 and Fig. 6), with the remaining surfaces being standard spherical surfaces. The evaluation of the system's imaging quality produces the following results. The modulation transfer function (MTF) exceeds 0.32 at all focal lengths, which is close to the diffraction limit (Fig. 3). The aberration values are also found to be less than 1.8% at the short focus and 0.4% at the intermediate and long focuses (Fig. 4). Furthermore, the energy of the field-of-view envelope is more than 82% at the short focus, 70% at the intermediate focus, and 77% at the long focus for a pixel size of 25 μm×25 μm. At short focal length, it exceeds 70% and is greater than 77% at the long focal length (Fig. 5). The out-of-focus amount of the image plane of the optical system at different temperatures is within the depth of focus of the system (Table 4). Additionally, tolerance analysis demonstrates that the system is easily machinable and has a high degree of realizability (Tables 5 and 6). Meanwhile, the cam curve of the lens displays a smooth trend without any inflection point (Fig. 7).
For the LWIR 320 pixel×320 pixel infrared detector, a continuous zoomable non-thermalized uncooled LWIR optical system is designed by mechanical positive group compensation and electromechanical active compensation. The system achieves MTF values close to the diffraction limit at all focal lengths, indicating excellent image sharpness. It has a compact structure, minimal aberrations, long working distance, and high overall imaging quality. The design strategy focuses on cost reduction by incorporating only aspherical surfaces while maintaining system performance. This approach helps minimize the system's size and weight, simplifies its complexity, and ensures smooth motion curves for both the zoom and compensation groups. The cam mechanism chosen for this design is relatively straightforward to process. Given these features and advantages, the system holds application significance in various fields such as searching, tracking, and detecting, and can be effectively utilized in scenarios where high-quality infrared imaging is crucial.
