Dual Wavelength Method for Molecular Difference Absorption Measurements in Turbid Media

Abstract

A dual wavelength differential absorption method for the detection of inclusions in turbid media is proposed. The method is based on the measurement of the transmitted intensity differences of the two diode lasers tuned to selected wavelengths within a broad absorption band of an inclusion of interest. The strategy for the optimum selection of the diode laser wavelengths and initial adjustment of the detection scheme is developed. It is shown that small deviations in the wavelengths from an optimum position result in a considerably deteriorated background suppression. The method is demonstrated by the detection of two dyes in a small volume inside a phantom filled with a turbid medium with scattering and absorption coefficients similar to those of biological tissues. The type and relative concentrations of the dyes were chosen so that the optical properties at the laser wavelengths are similar to those of a mixture of oxy- and deoxy-hemoglobin. The proposed method is insensitive to inclusions in the test object with spectrally independent scattering and absorption but detects the changes of the dye composition of a few percent inside a small sub-volume. The technique can be useful for the detection of the physiological status and changes in biological tissues.