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Estimation of Refractive Index in MIR range from the Reflectance Measurements for IR Optics Materials

반사율 측정에 의한 적외선 광학재료의 중적외선 굴절률 추정

  • Jin, Doo-han (Corporation TAAD) ;
  • Jeong, Kyung-Seok (Department of Mechanical Engineering, Korea University of Technology and Education)
  • 진두한 (주식회사 타드) ;
  • 정경석 (한국기술교육대학교 기계공학부)
  • Received : 2020.03.16
  • Accepted : 2020.06.05
  • Published : 2020.06.30

Abstract

An optical arrangement has been set inside a photo-spectrometer to measure the reflectance of IR optics materials in mid IR range. The optical arrangement consists of equally spaced 4 gold coated full reflecting mirrors with the incidence angle of 45°. Baseline beam intensity IB has been measured while the beam proceeds through the 4 mirrors. Reflectance of a mirror has been estimated from the IB. And the beam intensity IS with the specimen in the optical path has been measured with the 4th mirror replaced with the specimen. Reflectance of the specimen has been estimated from the value of IS/IB. Then the estimated reflectance has been put in Fresnel equation relating reflectance and refractive index(RI) to estimate the RI of the material. Measurement has been made for sapphire, germanium, magnesium fluoride, and zinc sulfide. The estimated RI of the materials are closely matching with reference data and the maximum difference less than 2% over the wavelength range 3-5㎛ for all materials tested. As an FT-IR photo-spectrometer with a broadband wavelength infrared light source is used, this method has the advantage of measuring the refractive index at multiple wavelengths in a single measurement.

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