Infrared reflectometry of skin: Analysis of backscattered light from different skin layers.

We have recently reported infrared spectroscopy of human skin in vivo using quantum cascade laser excitation and photoacoustic or photothermal detection for non-invasive glucose measurement . Here, we analyze the IR light diffusely reflected from skin layers for spectral contributions of glucose. Excitation of human skin by an external cavity tunable quantum cascade laser in the spectral region from 1000 to 1245cm-1, where glucose exhibits a fingerprint absorption, yields reflectance spectra with some contributions from glucose molecules. A simple three-layer model of skin was used to calculate the scattering intensities from the surface and from shallow and deeper layers using the Boltzmann radiation transfer equation. Backscattering of light at wavelengths around 10µm from the living skin occurs mostly from the Stratum corneum top layers and the shallow layers of the living epidermis. The analysis of the polarization of the backscattered light confirms this calculation. Polarization is essentially unchanged; only a very small fraction (<3%) is depolarized at 90° with respect to the laser polarization set at 0°. Based on these findings, we propose that the predominant part of the backscattered light is due to specular reflectance and to scattering from layers close to the surface. Diffusely reflected light from deeper layers undergoing one or more scattering processes would appear with significantly altered polarization. We thus conclude that a non-invasive glucose measurement based on backscattering of IR light from skin would have the drawback that only shallow layers containing some glucose at concentrations only weakly related to blood glucose are monitored.

Optical properties of porcine dermis in the mid-infrared absorption band of glucose.

The optical properties of skin in the mid-infrared range are not known, despite their importance for e.g. non-invasive glucose monitoring. In this paper, transmission, absorption, scattering, and reduced scattering coefficients are quantified using a custom-built goniometer based on a quantum cascade laser operated at the glucose absorption band at a wavelength of around 9.7 µm. The measurements show a strong dominance of absorption and moderate contributions from scattering. The scattered radiation is dominated by single scattering events in the forward direction (g = 0.967) within the range of the investigated dermis layer thicknesses of up to 50 µm, whereby the fraction of multiple scattering is expected to increase with the layer thickness.