Absolute refractometry for the NIST Watt Balance

Optical setup for an absolute refractometer with refractive index tracking using a Wu-type interferometer.
Optical setup for an absolute refractometer with refractive index tracking using a Wu-type interferometer.

This research is funded by National Institute of Standard and Technology (NIST), which aims to correct the refractive index error for interferometric velocity measurement in Watt Balance experiments. For decades, the refractive index of air has been estimated using the Edlen Equation, or other variants, by continuous measurements of the environmental conditions in the area (temperature, pressure, humidity, etc.), which are limited based on their underlying correction algorithms to approximately several parts in 10^8. We propose a refractometer configuration based on Wu-type interferometer, which measures and tracks the two pairs of optical path length change in a variable length vacuum cell, with the aim of achieving absolute refractive index measurements with uncertainty of several parts in 10^9, while simultaneously allowing for refractive index tracking at high speeds once the absolute refractive index is known.