Measurements of the Refractive Index of Air Using Interference Refractometers

Comparisons have been carried out between interference refractometers built in different countries. Individual measurements of the refractive index of air have been made using air from the same sample volume. Direct comparison of refractometers was realized by coupling the instruments to the same air-inlet system. In order to compare the measured results the refractive index was also calculated from accurate measured values of pressure, temperature, humidity and CO2 content, using Edlen's formula. Most of the individual measurements show an agreement within 1 part in 107 while direct comparisons show an agreement within 5 parts in 108. Over a period of five days an increase in CO2 content of 400 ppm per day was measured in a 300 m3 laboratory room. The results show the great importance of correcting calculated results for the CO2 content of the air.