Index of refraction of germanium.

Measurements of the index of refraction of a sample of high-quality, single-crystal germanium using the minimum deviation refractometry method are presented for temperatures near 22°C and for wavelengths in the range 2 to 14 µm. The standard uncertainty for the measurements ranges from ${1.5} \times {{10}^{ - 5}}$1.5×10-5 to ${4.2} \times {{10}^{ - 5}}$4.2×10-5, generally increasing with wavelength. A Sellmeier formula fitting the data for this range is provided. Details of the custom system and procedures are presented, along with a detailed analysis of the uncertainty. These results are compared with previous measurements.

Progress on vacuum-to-air mass calibration system using magnetic suspension to disseminate the Planck-constant realized kilogram.

The kilogram is the unit of mass in the International System of units (SI) and has been defined as the mass of the International Prototype Kilogram (IPK) since 1889. In the future, a new definition of the kilogram will be realized by fixing the value of the Planck constant. The new definition of the unit of mass will occur in a vacuum environment by necessity, so the National Institute of Standards and Technology (NIST) is developing a mass calibration system in which a kilogram artefact in air can be directly compared with a kilogram realized in a vacuum environment. This apparatus uses magnetic suspension to couple the kilogram in air to a high accuracy mass balance in vacuum.

Deformation-free form error measurement of thin, plane-parallel optics floated on a heavy liquid.

We describe a novel method for measuring the unconstrained flatness error of thin, plane-parallel precision optics. Test parts are floated on high-density aqueous metatungstate solutions while measuring the flatness error with an interferometer. The support of the flat optics by the uniform hydrostatic pressure at the submerged face of the flat optic eliminates flatness errors caused by mounting forces. A small, well characterized flatness error results from the bending of the floating flat by the hydrostatic pressure gradient at the edges. An equation describing the bending of thin, flat plates floating on a liquid is derived, which can be used to correct the flatness measurements of arbitrarily shaped plates. The method can be used to measure flatness errors of both nontransparent and transparent parts, and it is illustrated with flatness measurements of photomask blanks and substrates for extreme ultraviolet lithography. The refractive index of a saturated aqueous lithium metatungstate solution was measured at 632.8 nm and was found to be close to the refractive indices of several low thermal expansion optical materials.