Quantifying the properties of two-layer turbid media with frequency-domain diffuse reflectance.

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Abstract
Noncontact, frequency-domain measurements of diffusely reflected light are used to quantify optical properties of two-layer tissuelike turbid media. The irradiating source is a sinusoidal intensity-modulated plane wave, with modulation frequencies ranging from 10 to 1500 MHz. Frequency-dependent phase and amplitude of diffusely reflected photon density waves are simultaneously fitted to a diffusion-based two-layer model to quantify absorption (μa) and reduced scattering (μs′) parameters of each layer as well as the upper-layer thickness (l). Study results indicate that the optical properties of two-layer media can be determined with a percent accuracy of the order of ±9% and ±5% for μa and μs′, respectively. The accuracy of upper-layer thickness (l) estimation is as good as ±6% when optical properties of upper and lower layers are known. Optical property and layer thickness prediction accuracy degrade significantly when more than three free parameters are extracted from data fits. Problems with convergence are encountered when all five free parameters (μa and μs′ of upper and lower layers and thickness l) must be deduced.