Challenges In Using An Analog Uncooled Microbolometer Thermal Camera To Measure Crop Temperature

It has been long known that thermal imaging may be used to detect stress (e.g. water and nutrient deficiency) in growing crops. Developments in microbolometer thermal cameras, such as the introduction of imaging arrays that may operate without costly active temperature stabilization, have vitalized the interest in thermal imaging for crop measurements. This study focused on the challenges occurring when temperature stabilization was omitted, including the effects of focal-plane-array (FPA) temperature, camera settings and the environment in which the measurements were performed. Further, the models for providing thermal response from an analog LWIR video signal (typical output from low-cost microbolometer thermal cameras) were designed and tested. Finally, the challenges which typically occur under practical use of thermal imaging of crops were illustrated and discussed, by means of three cereal showcases, including proximal and remotely based (UAV) data acquisition. The results showed that changing FPA temperature greatly affected the measurements, and that wind and irradiance also appeared to affect the temperature dynamics considerably. Further, it is found that adequate settings of camera gain and offset were crucial for obtaining a reliable result. The model which was considered best in terms of transforming video signals into thermal response data included information on camera FPA temperature, and was based on a priori calibrations using a black-body radiation source under controlled conditions. Very good calibration (r(2)>0.99, RMSE=0.32 degrees C, n=96) was obtained for a target temperature range of 15-35 degrees C, covering typical daytime crop temperatures in the growing season. However, the three showcases illustrated, that under practical conditions, more factors than FPA temperature may need to be corrected for. In conclusion, this study shows that thermal data acquisition by means of an analog, uncooled thermal camera may represent a possible, cost-efficient method for the detection of crop stress, but appropriate corrections of disturbing factors are required in order to obtain sufficient accuracy.