Analysis of laser light propagation in kiwifruit using backscattering imaging and Monte Carlo simulation

The propagation of laser light in kiwifruit (Actinidia deliciosa) tissue was measured by backscattering imaging and modelled with the Monte Carlo (MC) method. The parameters of the vision system (8 bit/channel camera, 27.42 μm/pixel resolution) and the laser module (785 nm, 45 mW, Ø1 mm) were utilized in simulation. The required number of the photons was optimized with time-resolved MC model. The injected photon pulse travelled further than the beam radius and the calculated intensity fell below the noise level of the camera within 1 ns time. This short pulse contains 2.49 × 10⁸ photons and its application reduced computation load compared to the amount emitted within the integration time of 0.5–8.3 ms. The statistical effects of the optical properties of the tissue, scattering coefficient (μ_s) absorption coefficient (μ_a) and anisotropy factor (g), on photon flux was evaluated within ±20% range relative to expected mean values of μ_a = 0.9 cm⁻¹ and μ_s = 40 cm⁻¹. The anisotropy factor was taken into account using the Heyney–Greenstein phase function and was adjusted to g = 0.8 ± 20%. Because individual significance of each optical property was also analysed, scattering (μ_s) and transport corrected reduced scattering coefficients (μ^′_s=1−g]μ_s) must be distinguished. The multi-factor ANOVA test pointed out the highest importance (p < 0.001) of the anisotropy factor amongst scattering and absorption coefficients.In the kiwi backscattering images, rotation of the intensity profiles was observed as a result of changing anisotropy. The measured and calculated profiles were compared to estimate the anisotropy factor of kiwifruits. Significant difference (p < 0.01) was found between anisotropy of premium quality and overripe pieces with respect to the fruit texture properties.