Predicting food component content is one of the important research directions in the field of food science and engineering, and it is also an important indicator for evaluating food quality. With the increasing attention to food quality and safety issues, the prediction of food component content is of great significance for quality control in the food industry and food safety monitoring. Traditional prediction of food component content is usually carried out through chemical analysis and physical measurement techniques, which have disadvantages such as long detection times and high costs, and may be influenced by environmental factors and sample variability. They cannot meet the requirements for non-destructive, rapid prediction of multidimensional food component content. Therefore, the development of non-destructive, rapid, and multidimensional detection methods is crucial. Hyperspectral imaging technology is a non-destructive, rapid, and high-precision technology that can obtain a large amount of spectral information from food. Deep learning, as a powerful machine learning method, has the ability to handle large-scale data and extract complex features. With the development of hyperspectral imaging technology and deep learning, an increasing number of studies are combining the two for application in non-destructive food detection. This paper reviews the application of hyperspectral imaging technology combined with deep learning in food quality analysis, including nutrient analysis, traceability identification and maturity assessment. Secondly, the research progress of hyperspectral imaging technology combined with deep learning algorithms in data preprocessing, model building and evaluation is summarized to provide reference for improving the accuracy and efficiency of food analysis.
In this paper, a high transmission metasurface for subwavelength focusing of terahertz waves was proposed. A full 2π phase coverage with high transmission at target frequency was designed by varying the lengths of the cross resonators. The high transmission characteristics of the resonators were analyzed and the performance of the focusing was also discussed. The results show that the maximum transmission of the resonators reaches 0.92 and the terahertz wave can be focused with at the focusing point of a full width at half-maximum of 143 μm, which agrees well with the full width at half-maximum of 139 μm obtained by Huygens' principle. This device with the characteristics of flexible, thin and easy-integration exhibits the potential applications in THz imaging and communications, and also can be extended in the design of other planar THz components easily.
Optically tunable negative refractive metamaterials composed of electric response split-ring resonator (eSRR) and magnetic response split-ring resonator (mSRR) are theoretically investigated in terahertz (THz) region. A negative refractive band is achieved when both the eSRR and mSRR are fabricated on one substrate. Meanwhile, the optically tunable response is realized by filling the photo conductive semiconductor GaAs in the capacitive regions of eSRR. The electric response frequency varies with the pump laser fluence, therefore, the electric response frequency is controlled to overlap with the magnetic response frequency, and the negative refractive of the metamaterials can be controlled flexibly. The extracted constitutive parameters illuminate that a bandwidth 60GHz of negative refractive is realized when eSRR and mSRR combine together (with no GaAs), while a negative band width 30GHz is realized at the same structure parameters (with GaAs) when the pump laser fluence increases to 0.4mJ/cm2. Furthermore, the transmission spectra changes from dual band to single band with the variation of pump laser fluence.
Hyperspectral imaging technique and artificial neural network were used to investigate the feasibility of the nondestructive prediction for firmness and soluble solids content (SSC) of “Red” and “Green” plums. And the standard normal variation (SNV) was adopted to preprocess original spectral reflectance of region of interests. Then 5 and 28 characteristic wavelengths were selected from 256 full wavelengths by the methods of successive projection algorithm (SPA) and competitive adaptive reweighted sampling (CARS), respectively. An error back propagation (BP) network model was proposed based on selected characteristic variables to predict firmness and SSC of plums. The SSC prediction accuracy of CARS-BP model in calibration set (rc = 0.989, RMESC = 0.451 °Brix) was slightly higher than SPA-BP model (rc = 0.978, RMESC = 0.589 °Brix), while the SSC prediction accuracy of SPA-BP model in prediction set (rp = 0.964, RMESP = 0.778 °Brix) was slightly higher than CARS-BP model (rp = 0.955, RMESP = 0.851 °Brix).
In general, the terahertz metamaterial filter based on the complementary structure mainly has the fixed resonant frequency and asymmetric resonant frequency edges. In this paper, a thermally control terahertz narrow bandpass filter consisting of a periodic array of complementary wires embedded with thermosensitive semiconductor indium antimonide (InSb) has been proposed. Due to the structure of the filter is relatively simple, it can be more easily fabricated by lithograph technology compared with traditional terahertz metamaterial filter structure. Furthermore, its performance has also been analyzed based on the effective medium theory and the Drude model. The results show that the resonant frequency shifts from 1.16THz to 2.11THz with the increasing of the temperature from 160K to 360K, and the blueshift of resonance frequency as large as 81% can be implemented. Meanwhile, the reflectivity at the resonant frequency almost tends to be zero and the transmissivity spectrum at the resonant frequency exhibits sharp and symmetric edges. The filter can be applied to THz imaging system, and can effectively improve the imaging quality as a result of well characteristics of dynamic tuned filer and relatively simple structure.
Considering the two-photon light shift (TPLS) effect and the Doppler shift effect, the model of the output frequency of the optically pumped THz gas laser has been built up. The influence of the TPLS and the Doppler shift effects on THz output frequency stability has been analyzed theoretically and numerically. Results indicate that, increasing the pump laser power may degrade the THz output frequency stability to some extent. When only considering the TPLS, with certain pump laser power, the THz frequency shift increases first and then decreases with the increasing of the pump laser frequency offset. In addition, the THz frequency shift tends to decrease gradually with the increasing of the gas pressure and the operating temperature. However, further considering the influence of the Doppler shift effect simultaneously, the THz frequency shift tends to increase nearly linearly with the pump frequency offset. The results provide reference to improve output frequency stability of the optically pumped THz gas laser.
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