Monte Carlo simulation for quantification of light transport features in apples

Light interaction with turbid biological materials involves absorption and scattering. Quantitative understanding of light transport process and features in the fruit is critical to designing better optical systems for inspection of food quality. This paper reports on the quantification of light transport in the apple fruit in the visible and short-wavelength near-infrared region using Monte Carlo simulations. The absorption and reduced scattering coefficients (μa and μ′sμ′s, respectively) of 600 ‘Golden Delicious’ apples were determined over the spectral range of 500–1000 nm using a spatially resolved hyperspectral imaging method coupled with a diffusion theory model. The μa and μ′sμ′s values were used in Monte Carlo (MC) models to simulate light transport in the fruit tissue. MC simulation models were validated against the diffusion theory model and experimental data. The patterns of diffuse reflectance, internal absorption, and light penetration depth were determined using typical values of μa and μ′sμ′s for the apples. Simulation results showed that up to 96.4% of the photons were absorbed under the maximum absorption condition, while 75.9% photons exited as diffuse reflectance for the maximum scattering case. The optimum sensing range under our imaging system setup was found to be 1–11 mm for ‘Golden Delicious’ apples. Fruit tissue with a larger μa value absorbed light energy rapidly in short depth and radial distances, whereas light in the tissue with small μ′sμ′s values tended to propagate forward to the deeper area of the sample. Light penetration depths in ‘Golden Delicious’ apples, defined as the depths at which the incident light was reduced by 99%, were in the range of 0.43–8.67 cm over the 500–1000 nm spectral range, with a majority of the samples (approximately 68%) in the range of 0.81–4.48 cm. Pigments and water in the fruit tissue greatly influenced light penetration depth.