A Double Wavelength Laser Doppler System to Investigate Skin Microcirculation

A new experimental laser Doppler setup has been designed to discriminate between total and superficial skin blood flow. This selectivity is based on the use of two wavelengths with different penetration depths into the skin. An argon ion and helium-neon laser are mounted on the same optical bench and are stabilized by an optical feedback loop. A single optical fiber directs the beams to the skin and collects the reflected light back to a photodetector, the signal of which is sampled and Fourier transformed to give a frequency power spectrum. Several models of light scattering by the skin are examined, and a single Lorentzian function is found to be the best fit for our experimental power spectra. Flow parameters have been thus measured for several in vitro and in vivo situations. In vitro calibration indicates that there is a constant ratio between the frequency responses at both wavelengths used. The decrease of this ratio encountered in in vivo measurements is attributed to different depths of investigated skin microcirculation according to the incoming wavelengths.