Finite element modeling of light propagation in fruit under illumination of continuous-wave beam

Abstract

Abstract. Spatially-resolved spectroscopy provides a means for measuring the optical properties of biological tissues, based on analytical solutions to diffusion approximation for semi-infinite media under the normal illumination of infinitely small size light beam. The method is, however, prone to error in measurement because the actual boundary condition and light beam often deviates from that used in deriving the analytical solutions. It is therefore important to quantify the effect of different boundary conditions and light beams on spatially-resolved diffuse reflectance in order to improve measurement accuracy by the technique. This research was aimed at using finite element method (FEM) to model light propagation in fruit, subjected to the normal illumination by a continuous-wave beam of infinitely small or finite size. Three types of boundary conditions [i.e., partial current (PCBC), extrapolated (EBC) and zero (ZBC)] were evaluated and compared against Monte Carlo simulations, which provided accurate results in fluence rate and diffuse reflectance. The effect of beam size was also investigated. Overall results showed that FEM provided as accurate results as analytical method when an appropriate boundary condition was applied. ZBC did not give satisfactory results in most cases. FEM-PCBC yielded better fluence rate at the boundary than did FEM-EBC, while they were almost identical in predicting diffuse reflectance. Results further showed that FEM coupled with EBC simulated spatially-resolved diffuse reflectance under the illumination of finite size beam effectively. Large size beam introduced more error, especially within the region of illumination. Research also confirmed an earlier finding that a light beam of less than 1 mm diameter should be used for estimation of optical parameters. FEM is effective for modeling light propagation in biological tissues and can be used for improving the optical property measurement by spatially-resolved technique.