Effects of a finite spectral bandwidth light source in time-resolved diffuse spectroscopy

We discuss the spectral distortions occurring when time-resolved diffuse spectroscopy is performed illuminating with a spectrally wide source. We show that the spectral region within the source bandwidth that exhibits the lowest absorption will dominate the resulting time-resolved curve, leading to significant distortions on the retrieved absorption spectrum (including shifts in peak positions). Due to the nonlinear behavior of the light attenuation due to absorption, this effect becomes more pronounced when including longer and longer photon path lengths. First, a theoretical treatment of the problem is given and then the distortion is described by timeresolved reflectance simulations and experimental measurements of lipid and water samples. Finally, a spectrally constrained data analysis is proposed to overcome the distortion and improve the accuracy of the estimation of chromophore concentrations from absorption spectra. Measurements on a lipid sample show a reduction of the error from 30% to 6%. © 2009 SPIE-OSA.