Dependency of crossover point on absorption changes in bilayer diffusion reflection measurements

A better understanding of diffusion reflection (DR) behavior may allow it to be used for more noninvasive applications, including the development of in vivo non-damaging techniques, especially for medical topical diagnosis and treatments.

For a bilayer opaque substance where the attenuation of the upper layer is larger than the attenuation of the lower layer, the DR crossover point ( $C_p$ ) is location where the photons coming from the bottom layer start affecting the DR. We aim to study the dependency of the $C_p$ on absorption changes in different layers for constant scattering and top layer thickness.

Monolayer and bilayer optical tissue-like phantoms were prepared and measured using a DR system. The results were compared with Monte Carlo simulations.

There is an agreement between the experiments and the simulations. $C_p$ correlates with the square root of the absorption coefficient ratio of the lower layer to the top layer.

The experimental findings support and validate the theoretical prediction describing the dependency of the $C_p$ on the square root of the ratio of the layers' absorption coefficients. In addition, a secondary breaking point is suggested to be observed experimentally at the entrance to the noise area.