Patent ID: 7728984

Claim:
Method for evaluating a measured parameter ( 12 ) with a measuring cell ( 5 ) comprising a cavity ( 11 ) which generates an optical path length difference (d Gap ) for light which changes by correspondingly following a variation of a measured parameter ( 12 ), the method comprising: introducing light ( 1 ) into the cavity ( 11 ) from a white light source ( 2 ) with the aid of an optical waveguide ( 4 ) via a coupler ( 3 ) disposed in the path of the optical waveguide ( 4 ); coupling out at least a portion of the light ( 1 ′) reflected back from the cavity ( 11 ) into the optical waveguide with the aid of the coupler ( 3 ) and conducting the reflected light ( 1 ′) to an optical spectrometer ( 6 ); determining the optical spectrum of the reflected light ( 1 ′) in the spectrometer ( 6 ) and generating a spectrometer signal ( 8 ); conducting the spectrometer signal ( 8 ) to a computing unit ( 9 ); directly converting the spectrometer signal ( 8 ) using the computing unit ( 9 ) into an interferogram I(d); determining from the intensity progression within the interferogram, the location of the particular extremal amplitude value (I extremal ) that directly represents the particular value of the optical path length difference (d Gap ) in the cavity which comprises the measured parameter ( 12 ); the cavity ( 11 ) comprising two mirrors ( 19 , 19 ′) spaced apart by a geometric mirror distance (d geo ), at least one of the mirrors ( 19 , 19 ′) being partially transmissive; a portion of the light being reflected back and forth between the mirrors ( 19 , 19 ′) so that the optical spectrum is determined by the optical path length difference (d Gap ); and wherein the geometric mirror distance (d geo ) is calculated directly and absolutely in a selected physical unit of length, from the optical spectrum without circuitous routes via correlations.