Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Using Helium as a Standard of Refractive Index: Correcting Errors in a Gas Refractometer



Jack A. Stone Jr., Alois Stejskal


The refractive index of helium at atmospheric pressure can be determined from ab initio calculations in combination with careful pressure and temperature measurements. Therefore helium can serve as a theory-based standard of refractive index; it might be used as a medium of known refractive index for high-accuracy interferometric length measurements, or it can be used to characterize and correct errors in a gas refractometer. We have used helium to correct for pressure-induced distortions of two refractometers built by us, where both refractometers basically consist of a laser locked to the transmission maximum of a simple Fabry-Perot cavity. As a proof-of-principle of the helium-correction technique, we have used our device to measure the molar refractivity of nitrogen and we find reasonable agreement with previous measurements. When our two refractometers simultaneously measure the refractive index of a common nitrogen sample, we find that the two systems agree with each other within a few parts in 10^9.
No. 3


helium, index of refraction, length metrology, refractive index, refractometer


Stone, J. and Stejskal, A. (2004), Using Helium as a Standard of Refractive Index: Correcting Errors in a Gas Refractometer, Metrologia (Accessed April 21, 2024)
Created June 1, 2004, Updated February 19, 2017