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In scanning probe microscopy laser interferometers are usually used formeasuring the position of the probe tip with a metrological traceability. Asthe most of the AFM setups are designed to work under standard atmosphericconditions the changes of the refractive index of air have an influence tomeasured values of the length with 1.0e-4 relatively. In order to achievebetter accuracies the refractive index of air has to be monitored continuouslyand its instantaneous value has to be used for compensating the lengthsmeasured by all of the interferometric axes. In the presented work we developeda new concept of an electronic unit which is able to monitor the refractiveindex of air on basis of measurement of ambient atmospheric conditions:temperature, humidity, pressure of the air and the CO2 concentration. The dataprocessing is based on Ciddor equation for calculating the refractive index ofair. The important advantage of the unit is a very low power consumption of theelectronics so the unit causes only negligible temperature effects to themeasured environment. The accuracy of the indirect measuring method employed bythe unit was verified. We tested the accuracy in comparison with a directmethod of measuring refractive index of air based on an evacuatable cell placedat the measuring arm of a laser interferometer. An experimental setup used forverification is presented together with a set of measurements describing theperformance. The resulting accuracy of the electronic unit falls to the 4.1e-7relatively.

Automatic unit for measuring refractive index of air based on Ciddor equation and its verification using direct interferometric measurement method