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.
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