Errors in diffuse optical absorption spectroscopy of two-layered turbid media due to assuming a homogeneous medium

We investigate the errors in diffuse optical absorption spectroscopy of two-layered media when assuming a homogeneous medium as the reference model. Using analytical solutions of the radiative transfer equation, we obtain spatially resolved reflectance (SRR) curves in a two-layer model and fit these to determine absorption and scattering coefficients under the usually assumed single-layer simplification. Our results demonstrate two major findings: systematic errors in fitted absorption are a likely source of the so-called background absorption and that the degree of such errors is highly sensitive to the optical properties of the two layers and to the method of cost function calculation within the fitting method. Our findings provide an expectation of the errors that one may expect during in vivo chromophore concentration measurements of layered superficial tissue when using a single-layer homogenous model.