The citrus industry, one of the most important fruit industries in China, is currently focusing on nondestructive internal quality grading. Near-infrared (NIR) spectroscopy has been widely used to provide accurate analysis of the material components of agricultural products, owing to its advantages of convenience and efficiency. Among different measurement setups, optical fiber probes are frequently used as a key optics accessory for collecting the NIR spectrum. The Monte Carlo (MC) method offers an accurate description of light propagation in the fruit tissue using a multilayer sample model, which provides the theoretical basis for the design of a more effective optical fiber probe in fruit-quality inspection. However, in the MC model, the target sample is usually described as a combination of multiple semi-infinite turbid mediums, which simplifies the structural characteristics of the tissue. Moreover, the structure and geometry of optical fiber probes are also ignored in most cases, which may lead to deviations in the simulation results. This study develops a more detailed description of light propagation in citrus tissues by analyzing the photon transmission near the optical fiber probe as well as the surface boundary of the sphere sample and suggests a probe design for reflectance-based detection.
Based on the characteristics of citrus fruits, a three-layered optical model in the shape of a sphere is established, which includes flavedo, albedo, and vesicle layers with different absorption coefficients, scatter coefficients, refractive indices, and anisotropy factors. A simulation system of citrus-quality detection is developed, comprehensively considering the injection near the source fiber, the photon track near the surface boundary, and the information collected by the detector fiber. The corresponding changes are implemented in the general MC code for the simulation of transmission characteristics in the citrus tissue, including normalized relative diffuse reflectance, average motion pathlength of photons, and the percentage of effective photons in vesicles. Then, MC models with specific parameters of the optical fiber probe, including the numerical aperture and radius of both the source and detector fibers, and the source–detector distance are established for MC simulation.
Based on the simulation results at a wavelength of 800 nm, the guiding principle of the optical fiber probe design suitable for citrus spectrum acquisition is determined. As shown in Fig. 4, the normalized relative diffuse reflectance increases with an increase in the radius of the detector fiber and decreases with an increase of source-detector distance, while other parameters show no significant influence. The average motion pathlength of the received photons increases with increasing source-detector distance (Fig. 5). In this case, the acquired spectrum carries more internal information about the citrus sample. For the percentage of effective photons in vesicles, a large detector fiber radius of 250 μm or more is recommended to achieve a stable result (Fig. 6) as well as a longer source-detector distance. Based on the simulation results, a new structure of “4 in 9 out” coaxial fiber probe is designed to direct light emitted by the light source and to receive the feedback signal (Fig.7). This versatile fiber probe achieves a collecting efficiency of 3.24% and a percentage of effective photons of 0.07% in MC simulation.
Nondestructive inner quality inspection techniques play an important role in the fruit industry. In this study, light propagation through the citrus tissue is simulated by the MC method to determine the relationships between fiber probe geometry parameters and the detected optical signal. A three-layered media model in the shape of sphere is established for further simulation and analysis. The optical fiber probe parameters that are closely related to NIR spectrum measurement are explained and introduced into the general MC model, and corresponding simulations are performed to determine the basic designing principles of the fiber probe in citrus tissues. Based on the simulation results, the radius of the detector fiber probe and the source-detector distance can be optimized to obtain more information of interest, while other parameters including the radius of the source fiber and the numerical aperture show limited impact on the simulation results. To achieve higher collection efficiency, a larger detector radius and smaller source-detector distance are recommended. Moreover, the photons received by the detector fiber are found to carry more information on the inner citrus tissue when the source-detector distance increases. A versatile optical fiber detection structure is designed with a large detector fiber radius and multiple source-detector distances to increase the level of received information of the citrus sample. The MC simulation result of the fiber probe indicates that the photons from the vesicle layer can be efficiently collected and the sensitivity of citrus inner quality detection is further improved, which provides a theoretical reference for designing a new detection accessory of nondestructive quality evaluation by NIR spectroscopy.
