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Traditional farming methods have many problems, such as high cost, low efficiency and environmental pollution. The transformation from extensive traditional agriculture to information-based precision agriculture is the inevitable trend of agricultural development. Precision agriculture (PA for short) can quickly obtain and analyze crop information, which is a necessary condition for its development. Infrared thermal imaging technology has the advantage of rapid response, and can obtain crop thermal image information without contact and damage by hand or machine. In addition, among all crop monitoring indicators, the surface temperature of crops is considered to be the fastest response indicator, which can detect the stress of crops before the crops appear visual symptoms. Therefore, infrared thermal imaging technology is considered to be one of the most promising technical means in the development of precision agriculture.
Drought, freezing injury and infectious diseases will affect the physiology of crops, some of which make the change of crop surface temperature very significant compared with the temperature of normal crops. Therefore, infrared thermal imaging technology can be used to monitor some physiological conditions of crop growth or crop storage, and help to realize intelligent and information-based management in agricultural monitoring.
This paper introduces the basic principle and common image processing means of thermal infrared imaging technology, summarizes the research and application progress of infrared thermal imaging technology in agriculture at home and abroad (including water stress, infectious disease monitoring, freezing stress, yield prediction and other applications), and analyzes the difficulties and future development trend of infrared thermal imaging technology in the development and application of agriculture.
Infrared thermal imaging technology is widely used in agriculture. The following are mainly introduced and discussed from water stress monitoring, infectious disease monitoring, freezing stress monitoring, yield measurement and other aspects, as shown in the table.
Water stress monitoring
Water has high specific heat capacity, stable chemical properties, high solubility and huge latent heat of vaporization, which lays the physical foundation of transpiration. Crops are very sensitive to water content, and water has a significant impact on their growth trend. At the same time, crop water stress is also an important indicator of farmland irrigation scheduling and yield prediction. Therefore, in agricultural thermal infrared monitoring, water stress is one of the important monitoring indicators.
Infectious disease monitoring
Disease monitoring plays an important role in agriculture. If the disease is found to cause physiological changes in crops in the early stage, prevention and treatment can be carried out to reduce its impact on yield. Generally, crop diseases can be divided into infectious diseases and non infectious diseases. Non infectious diseases are usually caused by environmental factors, such as water stress. Infectious diseases are diseases caused by pathogenic organisms infecting the host. For infectious diseases, if the changes of leaf surface can be observed in the traditional visual monitoring methods, most crop diseases have been in a serious period at this time.
In virus monitoring, infrared thermal imaging technology can obtain the temperature change of the infected part of the virus with time, and compare it with visible light pictures to verify the feasibility of applying infrared thermal imaging technology to early detection. Infrared thermal imaging technology has a good effect in early virus monitoring, which provides a more timely monitoring means for early virus monitoring based on machine vision than visible light monitoring.
Freezing stress monitoring
Freezing injury stress refers to that when the crop is below 0 ℃ for a long time, the water in the plant changes solid due to low temperature and forms ice nuclei, resulting in the loss of physiological activity. The formation and development of ice crystals in the plant are also related to a bacterium, which eventually leads to the death of the plant. Observing the formation of ice nuclei and the characteristics of freezing transmission with a thermal imager is conducive to better explore the essence of crop freezing stress. Observing the survival condition of crops after recovering to normal temperature is a common method to detect the frost resistance of crops. The characteristics of local freezing injury stress of wheat after heading were studied by thermal imager. The ice nucleation process of stems and ears of two kinds of wheat was observed by thermal imaging at 7 temperatures below 0 ℃. The results showed that the two kinds of wheat reached the limit of withstanding freezing stress at -5 ℃, which was more frost resistant than that at other temperatures, and it was considered that most varieties of wheat should have similar characteristics. In the remote sensing monitoring based on multivariate data, the combination of infrared thermal imaging remote sensing inversion technology and other remote sensing technologies can be used to monitor freezing stress.
Production forecast
The sampling survey and production measurement method based on statistics has high precision prediction results, but this method requires a lot of human and material resources, and is inefficient and cost-effective. The efficiency can be significantly improved by obtaining the infrared thermal imaging information characteristics of crops for yield measurement.
In yield prediction, because the characteristics extracted by infrared thermal imaging are related to a certain stress, the essence of yield prediction based on infrared thermal imaging technology is to study the relationship between predicted yield and stress, which is usually water stress. Some scholars first used infrared thermal imaging technology to extract eigenvalues to measure yield, and used stress degree day (SDD) to schedule irrigation volume, and found that this index was negatively correlated with yield. Assuming that the index measuring water stress is used as the characteristic value of yield prediction, the second research point is at which growth stage of crops to obtain the characteristic value. Using infrared thermal imaging technology to study different transgenic maize, it was found that there was a negative correlation between water stress index and maize yield at the grain formation stage, and the correlation reached a significant level.
Other aspects
Storage monitoring
In order to ensure the quality and safety of finished grain in the storage process, it is necessary to monitor the condensation in the finished grain warehouse to prevent the occurrence of condensation. The temperature, humidity and micro air velocity in the storage pile are three key factors that directly affect the occurrence of dewing. Under the same humidity, with the change of warehouse temperature, condensate will be separated out, which will affect the grain storage ecosystem. When studying the mathematical model of heat transfer in storage reactor, the temperature field in the warehouse is observed by thermal imager, and the process of heat transfer in the warehouse and the flow mode of the gas field are analyzed. Then use the vertical diaphragm to separate the grain in the bin, and check the uniformity of heat in the bin through its temperature field. In the study of wheat grain pile coupling model and dewing prediction, the temperature field picture presented by the thermal imager is used for image graying and binarization processing, the edge points of the heating area are found and its area is calculated, the changes of the heating area are obtained by comparing the pictures before and after, and then the natural convection phenomenon of the gas in the silo is inferred.
Pesticide droplet deposition effect
Based on this principle, the deposition effect of pesticide droplets can also be measured by infrared thermal imaging technology. When studying the influence of UAV spray parameters on droplet deposition, the thermal imager is used to obtain the temperature difference change information of field temperature before and after spray, so as to calculate the temperature difference change rate of rice canopy, which is an indicator of droplet deposition.
Pest monitoring
The monitoring of insect pests plays an important role in agricultural applications. The feeding behavior of phytophagous insects can change the manifestation of the host plant and cause a series of physiological and biochemical stress reactions of the host plant, so as to change the physical and chemical information connection between the host plant and the same or different insects that feed on the plant later or at the same time, Intraspecific and interspecific interactions of phytophagous insects are largely achieved by host plants.
At present, the key of infrared thermal imaging technology is to monitor the surface temperature of the object in the study of early means of monitoring pests. In essence, the changes in physiological characteristics of crops caused by pests are diseases caused by insects, such as water stress caused by the transmission of viruses carried by insects, infection or sucking juice. Therefore, the significance of infrared thermal imaging technology for pest monitoring lies in its auxiliary role.
Defoliant effect
Defoliant has the effect of destroying chloroplast membrane, which will reduce the physiological activity of crop leaves and reduce the transpiration rate. In order to observe the effect of defoliant, a thermal imager can be used to monitor the canopy temperature before and after application for efficacy evaluation. The results showed that the sunny and cloudless noon was the best time for monitoring, which preliminarily proved the feasibility of infrared thermal imaging technology to evaluate the efficacy.
From the current research, the monitoring of infrared thermal imaging in agriculture begins to move towards the route of combining UAV remote sensing, which is also the development trend in the future. Although the application of infrared thermal imaging technology in agriculture is still in the development stage, as the most promising information acquisition technology in precision agriculture, it has played an important role in agricultural production, crop monitoring and resistance detection. In precision agriculture, accurately and effectively obtaining the infrared thermal imaging characteristics of crops is still the key and difficult point of agricultural precision management and operation. The breakthrough of related technologies and methods is of great significance to realize precision agriculture, information agriculture and intelligent agricultural management.
