Patent ID: 7473897

Claim:
An apparatus for performing transmission measurements on sample tubes, the apparatus comprising: a first light source that generates light with a first near-infrared range wavelength as a first wavelength component; a second light source that generates light with a different and second near-infrared range wavelength as a second wavelength component; an optical device for mixing the first and second wavelength components of the two light sources and for guiding these wave-length components into an optical channel; the optical channel guides the mixed first and second wavelength components to a sample tube having a sample in such a way that the different wavelength components penetrate the sample tube with the sample at exactly the same location; a movement unit for generating a relative movement between the optical channel and the sample tube with the sample, a receiver for detecting the intensity of the first and second wavelength components after penetration of the sample tube with the sample and for generating detection signals for the first and second wavelength components; converters for converting the detection signals for the first wavelength component into first digital measurement values (X) that describe the degree of transmission at the first wavelength, and for converting the detection signals for the second wavelength component into second digital measurement values (Y) that describe the degree of transmission at the second wavelength; and an analysis system for analyzing the measurement values by producing a quotient from the first digital measurement values (X) and the second digital measurement values (Y); and for determining a first derivative of the quotient; wherein the corresponding measurement values (X) and (Y) are obtained with respect to position during relative movement between the sample tube with the sample and the optical channel, so that the derivative of the quotient is obtained from corresponding spatial coordinates, which enables the apparatus, following the processing and analysis of the measurement values (X) and (Y), to determine the position of at least one boundary surface between two phases or density zones of the sample or the filling level in the sample tube.