INTERCOMPARISON OF DETERMINATIONS OF THE REFRACTIVE INDEX OF AIR
Length measurement by means of laser interferometer is becoming common practice in industry. The length is calculated from L = (k. lambda-v)/N where k is the number of counted fringes over L, divided by a constant (generally two or four), lambda-v is the vacuum wavelength and N is the refractive index of the medium. The refractive index of air varies with composition, pressure and temperature. The variations were studied by Edlen who has produced a formula giving N as a function of wavelength, pressure, temperature, humidity and carbon dioxide content. In one series of measurements the difference between NPL and PTB varied from 0.2 and 1.8 x 10**-8 and occasionally reached 3.8 x 10**-8. In addition there was very good agreement with the calculated values. The difference between NPL and THE was of the order of 5.10-8. Commercial interferometers provide automatic correction for pressure, temperature and humidity but the influence of CO-2 or other gases is neglected. For accurate measurement this is not sufficient. Metrology laboratories have therefore constructed refractometers for the direct measurement of refractive index with an accuracy better than 10**-7. For the intercomparison of their measurements, the refractometers of the four participating laboratories (NPL, PTB, VSL & THE) were placed in a thermally controlled cabinet installed at THE and equipped with sufficient instruments to measure temperature to the nearest 10**-2k, pressure to the nearest 20Pa, humidity and CO-2. The participants had calculated their uncertainties as NPL + 2.2.10**-8 PTB + 1 .10**-8 THE + 4 .10**-8 VSL + 6 .10 There were many fluctuations between measurements which in addition did not always coincide with the calculated data. This was felt to be caused by very small variations in the atmosphere measured by the individual refractometers. Overall however the results showed excellent agreement (5.10**-8 or better) and confirmed the uncertainty estimates of the individual laboratories.
Bibliographic Reference: EUR 10838 EN (1986) MF, 71 P., BFR 150, BLOW-UP COPY BFR 350, EUROFFICE, LUXEMBOURG, POB 1003
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Record Number: 1989125048300 / Last updated on: 1987-07-01
Available languages: en