Patent ID: 8625100

Claim:
A method for determining a measured variable of a measured medium, comprising the steps of: bringing the measured medium in contact with an indicator or an indicator mixture, whose absorption spectrum has a first, and a second, wavelength range, which essentially do not overlap; activating a first light source for emitting a first light signal with a wavelength from the first wavelength range and a second light source for emitting a second light signal with a wavelength from the second wavelength range, the intensity of the first light signal being modulated by a first periodic signal, and the intensity of the second light signal being modulated by a second, periodic signal, the first periodic signal and the second periodic signal having equal frequencies; at least a part of the first light signal and at least a part of the second light signal propagate as first and second measurement light signals along a measuring path and are transformed on the measuring path by optical interaction with the indicator or the indicator mixture; registering a total intensity of the transformed first and second measurement light signals providing the first periodic signal with a first phase difference relative to the second, periodic signal; ascertaining a second phase difference between the total intensity of the transformed first and second measurement light signals and the first or the second periodic signal; and determining the measured variable with application of the second phase difference.