Thermal properties of plant leaves are influenced by the internal structure, which contains a substantial amount of water per a unit of area. Owing to this reason we may contactlessly, thermographically explore individual plants due to universality, accuracy and high resolution of IR thermography. Yet exact measurement is very much dependent on the conditions of the environment that influence the thermal properties of the visualised plant. As a result, if we want to compare thermographic data obtained in different time intervals, we need to calibrate the measuring system in dependence on the weather. Thermographic research and observation may be used in all agricultural products and processes in which heat is generated or, vice versa, heat is lost, either in the space or with time. The potential options of the use of thermography in agriculture include monitoring of three nurseries, planning and inspection of irrigation systems, detection of salinization, detection of pathogens and diseases in plants and crops, yields estimations and evaluation of fruit ripeness.
The traditional methods of measuring vegetative parameters of plants are reliable but they are time-consuming and labour intensive and they manage to cover only a limited/restricted area. Distant measuring via a thermal camera provides more accurate data on vegetative parameters when compared to traditional techniques due to the fact that the data may be obtained from a large continual area and it may be repeated quickly and easily at regular intervals. The intensity of radiation (or the ability of plants to radiate, absorb and reflect thermal radiation) depends on the properties of a leaf surface of the cover (orientation and surface structure), yet the amount of radiated energy also depends on the pigmentation of leaves, their width, arrangement of cells (structure) and the content of water in the leaf tissue. Green plant leaves typically show very low reflectivity and permeability in a visible radiation spectrum (0.4-0.7µm) due to strong absorption of photosynthetic pigments. Contrary to that, in the infrared area reflectivity increases owing to the content of mesophyll in cell walls which differs in individual parts of the leaf) and small absorption of infrared radiation by pigments. As a result, the structure of leaves is the decisive element when analysing this radiation. Spectral properties (in the area of short IR waves – i.e. 1.1-2.5µm) are strongly influenced by the content of water in the leaves tissue, reflectivity is relatively high, however, it decreases with a decreasing content of water in tissues.
Tree nurseries – places in which we may grow plants up to a certain size, defined in advance, at which they may be further replanted or distributed. Plants may be grown in open fields, tunnels or greenhouses. Early detection of the quality of sowing is an efficient instrument to detect seeds capable of germinating and seeds that are not able to do so. The traditional methods of the inspection of germination, viability are time-consuming, destructive and labour intensive. Infrared imaging technique seems to be a possible alternative to traditional methods. We have ascertained a high degree of correlation between the temperature of seeds and their germination. The lower temperature the seeds have, the better their condition is and the better they will germinate. This issue is probably connected with the degradation of cell membranes during the ageing of seeds and, therefore, a larger ability to radiate thermal radiation. On the basis of temperature changes, we may differentiate not only the degrees of quality of seeds, but also the quality of the process of germination and growth of seeds, seedlings and plants in tree nurseries.
Irrigation is necessary for agricultural production, predominantly, in the areas with insufficient or irregular amounts of the rainfall. The studies show that the intensity of the thermal radiation of plants is very sensitive to the lack of water. The reaction of plants to so-called water stress is that the pores on leaves close, which increases the temperature and, therefore, also IR radiation emitted by the plant.
Attack of Diseases
During the infection of plants by pathogens, the physiological state of the tissue significantly changes, we detect the changes in photosynthetic processes, pores conductivity, intensity of salicylic acid transpiration or accumulation. Some colour-distinct diseases of leaves may be detected by classical optic methods, yet this method may not be applied in the cases where a bottom part of the leaf is attacked or the attacked part of the leaf does not differ in terms of colour from the remaining part of the leaf. In these cases, the method of applying infrared thermography seems to be useful and reliable, when the illness of leaves is demonstrated by an increase or decrease in temperature by ca. 0.5-1.3°C, depending on the kind of pathogen and the extent of the attack of the leaf.
This application requires necessary monitoring of the plant surface, recording measured data and its subsequent analysing. The placement of the Workswell WIRIS System on the drone enables recording a large area, for example a field or orchard, saving data as radiometric images and analysing it easily by the supplied software. The system can communicate with the GPS locator, which significantly helps when comparing data from individual localities in time. High sensitivity of a thermal sensor in a thermal camera is essential to distinguish even small temperature differences. The Workswell Wiris Systemii thermal camera may achieve sensitivity up to 0.03°C and thus it is also able to detect the initial stage of attacking leaf surface.
Plant Yield Estimation
Thermal camera systems may be also utilised in the area of plant yield estimation and detection. Specifically, the thermal imaging camera systems have been successfully applied in measuring the amounts of fruit on fruit trees. We have verified that we may differentiate the fruit from the remaining part of the tree via IR radiation as during the night the temperature of fruit is by ca. 1.6°C higher than the temperature of leaves, whereas during the day the temperature differs by less than 0.5°C. This noticeable difference enables us to determine the amount of fruit or predict a future yield. In the case of individual trees (for example in experimental orchards of research institutions), we may perform these analyses with a manual thermal camera. If we need to determine yields at vast plantations, a thermal camera system for drones will be of a valuable assistance as it enables flying over a large area during a very short time, easy repeating of measuring in regular intervals and identifying every tree via the GPS information that is assigned to images (in the case of connecting an external GPS receiver).
Fruit Ripeness Evaluation
The evaluation of fruit ripeness is usually carried out via visual inspection (very often destructive). Such methods require a large contribution of human labour and the results are more or less subjective with respect to difficult repeatability and time intensity of the data record for each observer. Again, thermography as a technique is appropriate to be used in this environment as it is known that the resistance of the fruit surface against water vaporisation increases with the physiological age. At the moment of ripeness, this value reaches the maximum value, which may be detected via the thermal camera.
The overview stated above shows that the Workswell Wiris System (in connection with a drone) may be definitely utilised in agriculture. The major positives of the system include high sensitivity of the camera thermal sensor (up to 0.03°C) and resolution up to 640x512px. The thermal camera is calibrated in terms of temperature, therefore the maximum possible accuracy of measured values is ensured. A possibility to set a manual range of the temperature scale is a great advantage when compared to competitive products. Thus we may achieve the same chromaticity of individual temperatures for all images, the colour range will not change automatically with the change of temperatures of surroundings or subjects. Every moment, the user will recognise the temperature of the observed subject according to the colour (and the scale at the side part of the image).
The system may be fully controlled by two switches on a standard RC transmitter, all the functions may be controlled during the flight, therefore, we may, for instance, change emissivity if the drone flies over different surfaces and materials (construction sites, fields, plants, bodies of water,…). Saved data (both videos and images) are fully radiometric, which means that they preserve the information on the temperature in addition to colourful images. Such images may be processed in the supplied software, in which we may additionally change the setting (emissivity, reflected temperature, range, temperature scale, etc.), save data regarding the temperature in the csv format or convert a video to the AVI format.