If you are an engineer accustomed to creating optics for the visible region, then the world of infrared optics (especially other than near-IR) can seem quite foreign. Rather than falling back on optical glass for most of your needs, you now have a choice of transmissive materials that range from substances you can eat (sodium chloride) to materials that transistors are made of (silicon, germanium) to things that you give to your beloved (diamond, sapphire).
Many of these materials are difficult to work with. The salts (of which sodium chloride is but one of many) are soft and also absorb water from humid air. Many IR materials (including cadmium-based semiconductors and arsenic-based glasses) are toxic; others are chemically reactive. Still others are mechanically tough to shape.
But the benefits are worth the trouble. Many IR materials have high transmission over vast ranges: 0.17 to 8 μm for calcium fluoride; 0.6 to 16 μm for zinc selenide; mid-IR out to greater than 100 μm for diamond. And some are incredibly wear-resistant and work well in harsh environments.
Spinel
One of these materials is Spinel (magnesium aluminate), an optically clear, nonbirefringent polycrystalline ceramic that transmits from 0.20 to 5.8 μm, is highly impact and penetration resistant, and has a high melting point of 2135 C. It is especially well-suited for aircraft and other vehicle-mounted windows and domes for IR optical systems.
Spinel has traditionally been hard to make in quantity and at a reasonable cost, but this is changing: ArmorLine Corporation has created a large facility dedicated to the high-volume production of Spinel. Now, the company is talking about the successful transition from R&D to manufacture of Spinel optics as large as 50 x 30 in. in size.
This transition to manufacture, as well as a detailed summary of the properties and uses of Spinel, is the topic of an upcoming (June 13, 2013) Laser Focus World webcast. The presenter, John Voyles, plant engineering manager at ArmorLine, will detail the hot-press process and other equipment created to fabricate large Spinel optics, as well as the latest results.
To find out more and to register, go to: http://www.laserfocusworld.com/webcasts/2013/06/Spinel-Ceramic.html
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.