Patent ID: 7420689

Claim:
A method for determining the refractive index and/or compensation of the influence of refractive index during interferometric length measurements with the aid of an interferometer ( 13 , 13 ′) to which there are applied at least two measuring beams (v 2 , v 3 ) having at least defined frequencies approximately at a harmonic ratio to one another, and at whose output phases for the at least two measuring beams (v 2 , v 3 ) are evaluated, the interferometric phases being multiplied in an interferometrically fashion corresponding to the harmonic ratio of the frequencies of the measuring beams (v 2 , v 3 ) and at least one phase difference of the phase values thus formed being examined, characterized in that at least one of the measuring beams (v 3 ) is of variable frequency, and in that from the phase difference formed a control signal is formed in order to vary the frequency of the variable frequency measuring beam (v 3 ) and is used to control the frequency such that the phase difference vanishes.