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Meller Optics has developed a technique for shortening turnaround times on sapphire windows for sensor gimbals in UAVs and other unmanned systems (writes Rory Jackson). The sapphire crystal is grown by a supplier before being delivered to Meller to be ‘polished’ down to the aspect ratio and optical quality required for the end-user’s payload. “It’s a proprietary method to a large degree, but essentially it uses a high-speed double-sided polishing process to manufacture each window, with an emphasis on how parallel the two sides of each part come out,” said Meller’s technical director Ted Turnquist. “An extremely flat and parallel set of window faces is critical for minimising transmitted wavefront error – for maintaining a good image coming through it with EO, IR or lidar sensors.” Typical fabrication of sapphire windows involves ‘continuous ring’ polishing, a single-sided process that takes far longer for payload glass and even more so for sapphire – roughly 14-18 weeks compared with the five or six weeks with the Meller approach. It is also more expensive for each given unit of production. The new technique can be used on circular, dome and kidney- shaped windows among others. “Our windows typically measure less than 10 angstroms in surface roughness, and 10-5 scratch- dig, a key metric for optics in minimising image scatter,” Meller Optics’ business development manager Dale DeJoy added. Various coatings can also be applied after manufacture, although for EO cameras through to MWIR wavelengths, the sapphire alone tends to provide sufficient transmissibility, as well as suitable durability for military or industrial-grade UAVs working in potentially hazardous environments. Cutting time for sapphire Sensors Sapphire covers for sensors can now be made in a few weeks