Thermal image evaluation of crop diseases in the incubation period.

BACKGROUND: Early blight is one of the main diseases that causes serious pepper yield and quality reduction. The identification of the presymptomatic features during the incubation period is of great research significance, in that scientific prevention measures in the incubation period may effectively reduce the loss of peppers. RESULTS: The present study confirms the feasibility of the identification of presymptomatic features in pepper early blight by infrared thermography during the incubation period. The infrared thermal images of pepper blades were captured during the whole incubation period in our experiment. Evaluation parameters such as temperature uniformity (U) and the variation coefficient (C) were established by infrared thermal images for the blade temperature field. Evaluation parameters such as temperature uniformity U and variation coefficient C of pepper blades change during the time from being inoculated to significant morbidity. For cases of stabbing inoculation, there were no relatively obvious symptoms in visible light images until 96 h after inoculation, whereas some presymptomatic features appeared in infrared thermal images at 60 h after inoculation. Based on the uniformity changes (δU) and the variation coefficient changes (δC) in the temperature field distribution, the characteristic polynomial (CP)-GBDT model has been established by optimizing the gradient boosting decision tree (GBDT) model. Compared with the GBDT mode, the CP-GBDT model accuracy increased from 87.5% to 91.7% on the test set. CONCLUSION: Evaluation parameters such as temperature uniformity U and variation coefficient C can be used to effectively characterize the presymptomatic features of pepper early blight by infrared thermography during the incubation period. The results of the present study can provide a reference for the detection of crop diseases in the incubation period. © 2024 Society of Chemical Industry.

Broadband negative-refractive index terahertz metamaterial with optically tunable equivalent-energy level.

An optically tunable terahertz negative-refractive index metamaterial (NIM) is proposed. The NIMs are composed of two aluminum rings and two photosensitive ring-shaped silicon apertures coaxially coated on the both sides of Teflon substrate. The NIMS are also designed to realize wide incident angle, polarization insensitivity, and tunability. Similar to the real atom, the unit cell of NIMs is equivalent to the Teflon nucleus surrounded by top and bottom resonator electrons, which indicates that the equivalent-energy level of NIMs can be dynamically controlled by the resonator electrons, once the scale of substrate nucleus is fixed. Using the LC-circuit model, the dynamic control of the equivalent-energy level of NIMs is studied in detail. Simulation results indicate that the transmission of NIMs is tuned from lowpass to highpass when the conductivity of silicon is increased, and the corresponding phase at lower frequency can be continually tuned. Correspondingly, the negative refractive index of NIMs represents dynamically tunable property, and the tunable negative refraction is simulated by classical wedge prism model. Besides, the phase flow indicates that the direction of phase velocity of NIMs is negative for the single-negative index.

First-order nonparaxial scalar theory of surface and bulk scattering for high-quality optical coatings.

A novel nonparaxial scalar theory is presented to calculate the angular scattering that is due to interface roughnesses or bulk inhomogeneities in a high-quality optical coating. Based on the empirically modified Beckmann-Kirchhoff surface scatter model, this theory in surface scattering and bulk scattering predicts similar formulas for the angular scattered intensity, and at the same time provides new understanding and insight into multilayer scattering phenomena. It is worth noting that the derived expressions are in the same form as those given by the typical vector methods. Based on comparisons of the surface and bulk models with the corresponding typical models for several multilayer designs, the novel theory is demonstrated to be valid for multilayer coatings even with large incident and scattering angles.

[Study on Output Frequency Stability for Optically Pumped THz Lasers].

The frequency stability of the terahertz (THz) radiation source is one of the key issues which directly affect its application regarding to THz imaging, Radar detection, Coherent communication and so on. Based on two-photon light shift effect, the physical model for the frequency shift of optically pumped THz lasers has been built up and the analytical formula for calculating the frequency shift has been derived. Taking the optically pumped methanol(CH3OH) for example, the absorption line of methanol under different pressure has been given, the influence of the bump laser frequency offset and the power of pump laser on the frequency stability of output THz laser have been analyzed quantitatively while the effect of the pressure in THz cavity on the THz laser frequency drift has also been discussed. The results show that THz laser frequency drift increases with the increasing of the bump power, whereas it decreases with the increasing of pressure in THz cavity. The maximum of THz laser frequency drift reaches under the condition that the bump laser frequency drift is equal to a quarter of the absorbed line width of the operating gas when the bump laser frequency offset locates within a certain range. In practice, in order to improve the output frequency stability of THz laser, not only the appropriate working conditions (pressure, temperature) should be chosen, but also the measure for controlling the bump laser frequency shift within a certain range should be taken.

A broadband tunable terahertz negative refractive index metamaterial.

A strategy to greatly broaden negative refractive index (NRI) band, reduce loss and ease bi-anisotropy of NRI metamaterials (MMs) has been proposed at terahertz frequencies. Due to the symmetric structure of the MM, the transmission and refractive index are independent to polarizations of incident radiations, and a broadband NRI is obtainable for the range of the incident angle from 0° to 26°. In addition, THz MMs' properties such as transmission, phase and negative refraction exhibit a real-time response by controlling the temperature. The results indicate that the maximum bands of the negative and double-negative refraction are 1.66 THz and 1.37 THz for the temperature of 40 °C and 63 °C, respectively. The figure of merit of the MMs exceeds 10 (that is, low loss) as the frequency increases from 2.44 THz to 2.56 THz in the working temperature range, and the maximum figure of merit is 83.77 at 2.01 THz where the refractive index is -2.81 for a given temperature of 40 °C. Furthermore, the negative refraction of the MMs at the low loss band is verified by the classical method of the wedge, and the symmetric slab waveguide based on the proposed MM has many unique properties.

A simple digital control system with field-programmable gate array for stabilization of CO2 laser output.

A simple digital control system was designed to stabilize CO2 laser based on digital signal processing with the Field-Programmable Gate Array (FPGA) controlling chip and Very-High-Speed Integrated Circuit Hardware Description Language program. In this system, the control parameters were easily determined by software real-time control, and the control circuit was also designed compactly. In addition, the theoretical analysis on the stabilization of CO2 laser output characteristics was presented based on the photoacoustic effect, and the corresponding experiments on the stabilization of CO2 laser output characteristics were further performed. The results show that the output power of CO2 laser is stabilized at the peak with a relative power stability of 2.71%. Furthermore, the frequency of CO2 laser 9P(36) line is stabilized at the center of the laser gain curve with a relative stability of (1.57 ± 0.37)×10-8. This system has a potential of further improvements by optimizing the algorithm and choosing higher-speed signal processor.