Comparison of four measurement methods for acquiring maize hemispherical directional brightness temperature with a crane based thermal camera system

As one of the ultimate approaches to clearly define and understand directional features of canopy thermal radiation, ground level experimental studies of directional brightness temperature (DBT) have been concerned for many years. In this study, a recent crane based thermal camera system developed by INRA-Avignon of France is presented, and then four measuring methods for hemispherical DBT observations are introduced. These methods are based on different measurement principles: (1) view the same surface from different angle by moving the crane and platform to assure the sensor always toward the target; (2) view different part of the field to collect DBT of different angles by means of rotating the camera to different directions instead of moving the camera; (3) view different part of the field by moving the camera along the row direction; and (4) scan the field with a high imaging frequency while the camera moves along the crane bar vertical to the row direction. During the comparison of these methods, special emphasis is on the analysis of temporal and spatial variations of maize canopy brightness temperature distribution, from which the main errors of these methods are analyzed. The study also presents common shortcomings and limitation of the field observations with this system, such as the less capacity of hot spot observation and the sensitivities to field environmental influences. Finally, recommendations for optimal field measurement of hemispherical DBT with the system are given from an application-oriented viewpoint.