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Development of New Processes for the Refurbishment of Infrared Search and Track (IRST) Germanium (Ge) Domes


OBJECTIVE: Develop a process to refurbish hemispherical Ge domes, which will restore infrared (IR) transmission and meet the operational performance specifications of the IRST system. DESCRIPTION: After extensive use in adverse environments such as sandstorms, rain, and fog, IR domes develop pits and surface anomalies, which lead to a reduction in the average transmission to below acceptable limit. New domes are quite costly since they are constructed of single-crystal germanium (Ge) with very sophisticated inner and outer surface coatings. A process is needed that is capable of restoring the ge domes to a state or condition that will pass all imaging and environmental specifications set for the original dome design. The refurbishment process must take into consideration the tight tolerances on inner and outer radii to maintain focal length and eliminate any image distortion. A refurbishment process that results in a change in dome thickness, and therefore focal length, should be detailed. A process to rebuild the ge dome to its original thickness should be included in the contractor's report. The IRST system operates in the nominal 7.5-13 um region. Ge is a wide-band gap semiconductor with a refractive index of n=4.00 (10 um) and is naturally 54 percent transparent to IR light in the wavelength range of 2-14 um. No IR energy is lost in the material but a reduction of 56 percent transmission occurs due to reflections at the front and back surfaces of a highly polished ge window. Antireflective (AR) coatings deposited onto these surfaces can help to virtually eliminate surface reflections and increase IR transmission to greater than 95 percent over specific wavelength ranges. The contractor must include recommendations for the alternative AR coating materials and rain and sand erosion protection will both maximize IR transmission in the designated IR wavelength band and provide durability to withstand the operational environment. PHASE I: Develop a process for refurbishing Ge Domes. Research existing processes and provide a conceptual analysis of the proposed process. Provide pertinent calculations, which demonstrate the knowledge required to design, develop and produce a process to re-polish and recoat existing IRST Ge domes that have not passed the transmission test. Provide an estimate of the cost benefit of refurbishment compared to dome replacement. PHASE II: Produce and verify a process that will refurbish the Ge domes to an operationally ready state. Refine a process to rebuild the domes to their original thickness while maintaining the required performance specifications of the IRST system. A test case should be generated to validate the rebuild/refurbish process developed by the contractor. PHASE III: Restore IRST domes to the original specifications required by the IRST system. The restoration shall, at a minimum, make ready for issue (RFI) all domes that have previously been removed from the inventory. Domes that cannot be restored should be evaluated for further reconditioning or recommended for condemnation. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The electric power utility industry utilizes IR sensors in the same bandwidth as the IRST sensor to perform evaluation on above ground power transmission lines for insulation leaks. These power line insulator leaks result in power transmission loss costing the electric utility companies millions of dollars annually. To identify failures in power line insulation, public utilities use specially configured aircraft that fly over the nation's power lines using an IR sensor to detect the leaks. The IR systems used in the utility company's aircraft generally have the same type of IR window used in an IRST system. While less harsh (lower speed), the utility company aircraft's IR system is subject to much of the same environment. However, the utility companies operate their aircraft IR systems more than the Navy uses the IRST. Further, the flights are made at lower altitudes where airborne particulate matter has a greater opaquing effect on the IR window. As a result, failure of the IR window occurs as often if not more often than in the Navy. Development of a refurbishment process can be directly applied to this particular commercial application. The same applies to the oil and gas industry in the identification of leaks in their pipelines.
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