Finite element simulation of light transfer in turbid media under structured illumination

Authors: HU, DONG - Zhejiang University; Lu, Renfu; YING, YIBIN - Zhejiang University

Submitted to: Applied Optics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2017
Publication Date: 7/20/2017
Citation: Hu, D., Lu, R., Ying, Y. 2017. Finite element simulation of light transfer in turbid media under structured illumination. Applied Optics. 56(21):6035-6042.

Interpretive Summary: Spatial frequency domain (SFD) imaging is a relatively new optical technique for measuring and mapping the optical absorption and scattering properties of biological tissues, which can, in turn, be used for assessing the composition and condition of the tissues. While the technique looks promising for measuring food and agricultural products, it is prone to errors in measurement. To help develop an improved, practical SFD technique for food measurement, it is important to understand and quantify light transfer in turbid media under specific illumination patterns. In this research, we applied a numerical method, called finite element (FE), to simulate light transfer in turbid media under sinusoidal patterns of illumination over a large range of spatial frequencies. FE analysis was performed for 50 simulation samples with different combinations of the optical absorption and scattering properties, and the results were then compared with those obtained by an analytical method and Monte Carlo simulation - a standard, yet time-consuming statistical technique. Results showed that the FE method provided reasonable results compared to the analytical method and Monte Carlo simulation, when the scattering coefficient for the samples was at least 10 times the absorption coefficient. The performance of the FE analysis was also influenced by the relative value of the optical absorption and scattering properties to the spatial frequency of the light illumination. Overall, the FE method is effective for modeling light transfer in turbid media and can be used to explore the effects of crucial parameters for the SFD technique for measuring optical properties of food and agricultural products.

Technical Abstract: Spatial-frequency domain (SFD) imaging technique allows to estimate the optical properties of biological tissues in a wide field of view. The technique is, however, prone to error in measurement because the two crucial assumptions used for deriving the analytical solution to diffusion approximation cannot be met perfectly in practical applications. This research was mainly focused on modeling light transfer in turbid media under the normal incidence of structured illumination using finite element method (FEM). Finite element simulations were performed for 50 simulation samples with different combinations of optical absorption and scattering coefficients for varying spatial frequencies, and the results were then compared with analytical method and Monte Carlo simulation. Relationships between diffuse reflectance and dimensionless absorption and dimensionless scattering coefficients were investigated. The results indicated that FEM provided reasonable results for diffuse reflectance, compared with the analytical method. Both FEM and analytical method overestimated the reflectance when the ratio of transport coefficient to spatial frequency was greater than 2, and underestimated the reflectance when the ratio was smaller than 2. Larger ratios of the reduce scattering coefficient to absorption coefficient yielded better estimations of diffuse reflectance than did that of smaller than 10. The reflectance increased nonlinearly with the dimensionless scattering, whereas the reflectance decreased linearly with the dimensionless absorption. It was also observed that diffuse reflectance was relatively stable and insensitive to the reduced scattering coefficient when the dimensionless scattering was larger than 50. Overall results demonstrate that FEM is effective for modeling light transfer in turbid media and can be used to explore the effects of crucial parameters for the SFD imaging technique.