3 May 2017 Refractive index of infrared-transparent polycrystalline alumina
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Abstract
The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by

𝑛𝑛2 βˆ’ 1 = (A+B [𝑇𝑇2βˆ’π‘‡π‘‡20]) +Dλ2 /λ2βˆ’(λ1+C [𝑇𝑇2βˆ’π‘‡π‘‡20])2 + λ2βˆ’λ22

where wavelength λ is expressed in μm, To = 295.15 K, A = 2.07156, B = 6.273Γ— 10-8, λ1 = 0.091293, C = –1.9516 Γ— 10-8, D = 5.62675, and λ2 = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 Γ— 10-5 K-1 in the range 296–498 K.
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Daniel C. Harris, Linda F. Johnson, Lee R. Cambrea, Lawrence Baldwin, Meghan Baronowski, David E. Zelmon, William B. Poston, John D. Kunkel, Mark Parish, Marina R. Pascucci, John J. Gannon, Tzu-Chien Wen, "Refractive index of infrared-transparent polycrystalline alumina", Proc. SPIE 10179, Window and Dome Technologies and Materials XV, 101790F (3 May 2017); doi: 10.1117/12.2257580; https://doi.org/10.1117/12.2257580
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