Effects of the environment on refractive index of air in long gauge block interferometer

The environment parameters, i.e. temperature, pressure, humidity and CO2 concentration affecting the refractive index of air play an important role on the long gauge block measurement using interferometer. In this paper, the environment parameters in a measurement chamber had been continuously investigated more than 24 h. The refractive index was then calculated by using an updated Edlén equation. The stability of the refractive index of air in the experiment is 0.1 × 10−6 with the measurement uncertainty of 4.32 × 10−8 at the confidential level k = 2. While the sensitivity of the pressure of air was small, it was an evidence that the pressure of air mainly contributed to the stability of the refractive index of air due to its large variation. Furthermore, the calculated refractive index of air was employed to correct the length of long gauge blocks. The correction for 1 m long gauge block in interferometer system is 0.270 mm, 0.269 mm and 0.267 mm which are respectively based on the three laser wavelengths at 532 nm, 633 nm and 780 nm.

► The system used for long gauge block measurement using interferometer.
► The refractive index was evaluated by using an updated Edlén equation.
► The environment parameters were obtained 24 h in measurement chamber.
► Air pressure is the main factor of the refractive index of air.
► The correction from refractive index for 1 m long gauge block is about 270 μm.