Patent ID: 6501974
Filing Date: 2002-12-31
Classification: A61B

Abstract:
A method for calibrating an apparatus intended for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject and being provided with emitter means for emitting radiation at a minimum of three different wavelengths, the method comprising the steps of:(a) according to the Lambert-Beer model, determining a theoretical measurement signal for each wavelength used in the apparatus, (b) according to the Lambert-Beer model, determining an invariant which is a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two measurement signals, the weighted sum being theoretically independent of the relative concentrations of said substances in the blood of the subject, (c) calculating a first value for said invariant by means of said theoretical measurement signals, (d) performing an in-vivo measurement on a living tissue with the apparatus, whereby in-vivo measurement signals are obtained for each wavelength, (e) applying a transformation on said in-vivo measurement signals for transforming said signals to the Lambert-Beer model, whereby transformed in-vivo measurement signals are obtained, (f) calculating a second value for said invariant, the second value being calculated similarly to the first value, except for the replacement of said theoretical measurement signals by the transformed in-vivo measurement signals, (g) comparing the second value with the first value of the invariant, and (h) calibrating the apparatus on the basis of a difference between said first value and said second value.