Low-level absorption in the visible and near infrared has been reported for single crystal o-ray sapphire and a limited set on other crystalline based material. A survey of reported measurements in regions of high transparency for such materials is presented.
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M. E. Thomas, "Low-level background absorption in durable window materials," Proc. SPIE 10179, Window and Dome Technologies and Materials XV, 101790B (3 May 2017);