Spectral distortions in near-infrared spectroscopy of turbid materials

A liquid suspension consisting of a mixture of H2O, D2O, and polystyrene latex microspheres was used to study the effects of multiple scattering on the near-infrared (800-1600 mm) spectrum of a pure absorber (H2O) in a turbid medium. This simple experimental model enabled us to isolate and explain the spectral distortions introduced by variations in the optical pathlength of scattered photons. We observe the following: (1) Reflectance spectra measured with the detector positioned close to and far from the point of illumination have distinctly different sensitivities to background scattering variations. Within a certain range of detector positions, the use of spectral derivatives to correct for multiplicative scattering effects is most effective. (2) The wavelength dependence of the scattering background of the log(1/R) spectrum depends not only on particle size but also on the separation between the source and detector probes. And (3) the ratio of the magnitudes of the spectral peaks caused by absorption within the background medium and absorption within the scattering particles decreases as multiple scattering increases. We explain these observations in the context of photon-diffusion theory and point out their significance with respect to the design of diffuse-reflectance spectrometers. Photon diffusion theory proves to be valuable for interpretation of diffuse spectra, but it fails to account for spectral distortions introduced by low-order backscattering at close source-detector separations.