High Accuracy Ultraviolet Index of Refraction Measurements Using a Fourier Transform Spectrometer

We have constructed a new facility at the National Institute of Standards and Technology (NIST) to measure the index of refraction of transmissive materials in the wavelength range from the visible to the vacuum ultraviolet. An etalon of the material is illuminated with synchrotron radiation, and the interference fringes in the transmittance spectrum are measured using a Fourier transform spectrometer. The refractive index of calcium fluoride, CaF₂, has been measured from 600 nm to 175 nm and the resulting values agree with a traditional goniometric measurement to within 1 × 10⁻⁵. The uncertainty in the index values is currently limited by the uncertainty in the thickness measurement of the etalon.