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Refractometry Using a Helium Standard



Jack A. Stone Jr., Alois Stejskal


The refractive index of helium at atmospheric pressure can be calculated from first principles with a very low uncertainty, on the order of 10^-10. Furthermore, the low refractive index of helium puts minimal demands on the pressure and temperature measurements required to determine the refractive index of a given sample of helium gas. Therefore helium can serve as a practical, theory-based standard of refractive index that might be used in place of air for ultra-high accuracy interferometric length measurements. Because its index of refraction is known, helium can also be used to characterize and correct errors in a gas refractometer. We have built two refractometers based on a laser locked to the transmission maximum of a Fabry-Perot cavity, and we use helium to measure and correct pressure-induced distortions of the refractometers. (We have also characterized other sources of error in our Fabry-Perot refractometers, such as errors associated with the effect of humidity on the mirror coatings.) As a proof-of-principle of the helium-correction technique, we have used our refractometers 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. The good agreement suggests that many potentially troubling sources of uncertainty can be overcome. These measurements are sufficiently encouraging that one might speculate on the possibility of developing pressure standards based on refractometry.
Proceedings Title
2004 NCSL International Workshop and Symposium
Conference Dates
July 11-15, 2004
Conference Location
Salt Lake City, UT
Conference Title
Metrology: The Process of Providing a Good Measurement


helium, index of refraction, refractometer


Stone, J. and Stejskal, A. (2004), Refractometry Using a Helium Standard, 2004 NCSL International Workshop and Symposium, Salt Lake City, UT (Accessed April 17, 2024)
Created July 1, 2004, Updated February 19, 2017