Hot conditions force infrared imaging system designers to have thoughtful control of stray light. One must simultaneously optimize both the emission and reflection properties of an imaging system to effectively reject unwanted infrared radiation in a hot, compact environment. In aviation systems, infrared sensors may be placed behind severely angled windows to remain both aerodynamic and compact. We present a reflective baffle design that effectively rejects stray radiation and masks emission from hot structures, allowing an infrared imager to see outside of its harsh environment.
Additive manufacturing (AM) can change the way we design opto-mechanics. One can realize complex structures with AM that are difficult or impossible to achieve with conventional fabrication techniques. In a recent project, the US Army attempted to demonstrate athermalization of an infrared lens using AM design and manufacturing techniques. The goal was to demonstrate these new techniques in a small-scale prototype lens for a missile application.
U.S. Army Combat Capabilities Development Command, Aviation and Missile Center teamed with Materials Sciences LLC (MSC) to apply a previously-reported topology-optimization technique [1] to develop an additively manufactured multi-material, opto-mechanical structure. The composite structure was engineered to maintain the location of the detector focal plane at the focus of a lens as it changes with temperature. MSC initially fabricated several prototype lens housings of additively manufactured titanium and cast urethane which failed at the material interfaces during temperature cycling over the intended operational range. MSC is currently conducting fabrication trials with an injection molded thermoplastic polymer to replace the cast urethane. These unfinished prototype feedstocks are being evaluated for structural integrity under temperature cycling. Initial results are promising; but additional fabrication trials are required to produce fully dense feedstocks for finish machining.
Composite structures allow tailoring of mechanics to react as designed to temperature and vibration. But it is nearly impossible to model all aspects of a composite structure. Prototyping and testing can quickly reveal limitations of materials and fabrication methods that can inform future optimization efforts. This effort documents these iterations and learned lessons, and presents evidence that this AM-composite technique is useful for designing robust opto-mechanics and that the current manufacturing method can be matured to deliver operational components.
Additive manufacturing (AM) is a rapidly advancing area in which new mechanical design techniques can be used to greatly benefit optical sensor designs. Emerging technologies now allow us to utilize many different materials to print three-dimensional structures in complex, accurate shapes. This enables designers to build volumes with optimal material distributions, allowing them to tailor a structure’s response to temperature or vibration. Introduction of these methods provides an excellent opportunity for a designer of optical sensors to make significant for harsh and varied environments. The Army is currently funding research of a Robust Seeker Optomechanics project to design novel mechanical structures promising to athermalize and vibration-isolate a missile seeker’s imaging components. The structures would eliminate the need for moving parts or expensive components such as gimbals or complex lenses. Methods for topology optimization allow a computer algorithm to fill the volume around a missile seeker lens and camera focal plane with a uniquely shaped composite material. These uniquely constructed spaces can both move or maintain the position of the imaging components when the missile experiences large temperature variations and vibrations thus improving performance. Army Aviation and Missile Research and Development Center (AMRDEC) lens designers teamed with Materials Sciences Corporation (MSC) to develop topology optimization algorithms based on frequency tailoring of structures specific to another project underway. The initial phase of this effort is to demonstrate mechanical athermalization of a simple long-wave infrared camera in a small, 2.75-inch-diameter missile seeker configuration. MSC created a series of volume optimizations and algorithm refinements to study the solution space for a 45mm, F/1.4, 14-degree lens and a FLIR Quark2 640 sensor between -35°C and +50°C. The example problem provides a relevant, achievable athermalization goal as well as an opportunity to include vibration damping in later phases. The results of initial volume optimization reveal that one can achieve a buildable athermalized design by separating the detector mount structure from the lens retention structures. The study also showed that mechanical and optical design need to be coupled into a common optimization solution to achieve the best results. MSC measured the mechanical properties of many additivelyproduced metals and plastics to use in the optimization. The team produced a lens in a prototype housing, and will measure it for thermal stability soon after this paper is submitted.
Foveated imaging can deliver two different resolutions on a single focal plane, which might inexpensively allow more
capability for military systems. The following design study results provide starting examples, lessons learned, and
helpful setup equations and pointers to aid the lens designer in any foveated lens design effort.
Our goal is to put robust sensor in a small package with no moving parts, but still be able to perform some of the
functions of a sensor in a moving gimbal. All of the elegant solutions are out (for various reasons). This study is an
attempt to see if lens designs can solve this problem and realize some gains in performance versus cost for airborne
sensors. We determined a series of design concepts to simultaneously deliver wide field of view and high foveal
resolution without scanning or gimbals.
Separate sensors for each field of view are easy and relatively inexpensive, but lead to bulky detectors and electronics.
Folding and beam-combining of separate optical channels reduces sensor footprint, but induces image inversions and
reduced transmission. Entirely common optics provide good resolution, but cannot provide a significant magnification
increase in the foveal region. Offsetting the foveal region from the wide field center may not be physically realizable, but
may be required for some applications.
The design study revealed good general guidance for foveated optics designs with a cold stop. Key lessons learned
involve managing distortion, telecentric imagers, matching image inversions and numerical apertures between channels,
reimaging lenses, and creating clean resolution zone splits near internal focal planes.
Computational imaging techniques can be used to extend the depth of field of imaging sensors such that the sensors become less expensive to build and athermalize with no loss to performance. Optical phase can be manipulated to create an image that is optimized for a detection and tracking algorithm as well as reconstructed digitally to form an image suitable for viewing. A typical low-cost sensor which is used for target detection and tracking may run an algorithm which requires different features and resolution from its imagery than would a system optimized for a human. This offers a unique opportunity to optimize both optics and image processing for a system which can maximize mission performance as well as minimize production cost. Simple computational techniques have not yet been successful in passive, low-signal environments due to noise issues. This study examines the use of a simple computational technique in an algorithmic application in which optimal reconstruction may occur with lower noise. This paper will describe the model, simulation, and prototype which resulted from a detailed and novel system design and modeling process. The goal of this effort is to accurately model the anticipated performance and to prove actual cost savings of a tracking sensor which employs computational imaging techniques.
Optical design is one of the major limiting factors for making any sensor and display feasible in a head-mounted application. This is especially the case for multispectral (dual-color) sensing systems because they may employ two sensors instead of one. Various approaches can be made to incorporate a multispectral imaging system in a head mounted application. One of these is to use an entrance aperture that is common to two separate sensors. This type of design is attractive in that it provides co-aligned sensors in a relatively compact package. However, multispectral, head-mounted optics pose unique challenges that must be overcome by both the designer and fabricator of such a system. This paper will examine these challenges and pose conceivable optical solutions.
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