According to first equation an interferometer’s unit of measure in length measurement is laser’s wavelength.
Where: N – number of pulses,
lambda – light wavelength.
From definition a wavelength depends on laser’s frequency f and the speed of light v in the measuring path.
Where: n – the refraction coefficient.
If the measurement is done in vacuum, than v= c = 3*1G m/s. The speed of light in a medium other than vacuum (e.g. air, water) is lower and is described as Normally the refraction coefficient n is a complex variable or even a tensor, but for less accurate calculations it is simplified to a constant. The air coefficient depends mostly on the pressure P, temperature T and humidity H. The dependence , for the air was empirically determined by Edlen and is described as
From the above equations one may obtain the refraction coefficient dependences on T, P and H in usual conditions (T=293K, P=1000hPa, H=50%):
It is worth to notice that the most critical parameter is the temperature, because its change influences the coefficient n more than changes in the pressure and much more than changes in the humidity.