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

Significance: 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.

Aim: 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.

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

Results: 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.

Conclusion: 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.