The influence of the outside conditions on the measurement accuracy

According to first equation an interferometer’s unit of measure in length measurement is laser’s wavelength.

interferometer formula

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.


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
formula3Where: n – a refraction coefficient.

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 ntph, 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%):

formulas 2

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.