Thermal infrared imaging of conifer leaf temperatures: Comparison to thermocouple measurements and assessment of environmental influences

Thermal infrared (TIR) imaging provides a potentially powerful tool for ecological and biometeorological research, but the accuracy of TIR imaging of plant surface temperature in a variable outdoor environment has been poorly characterized. This study evaluated the accuracy of TIR measurements of a conifer tree that were made using the thermal camera's factory default settings, and developed a post-collection data processing method to correct measurements for ambient conditions that depart from the factory default settings. We conducted a 42-day measurement campaign in Corvallis, Oregon during spring 2014 that focused on the canopy temperature of a noble fir (Abies procera) and included the simultaneous collection of meteorological data and leaf temperature measurements with an array of thermocouples. We then developed empirical functions for temperature corrections that better accounted for emissivity, and atmospheric and reflected temperature. The original TIR temperatures using the factory settings averaged 8.6 ± 5.6 °C (−4.2 to 25.4 °C in range), and the original TIR temperatures differed from the thermocouple temperatures by 1.4-1.6 K. The TIR temperatures were generally accurate, except for a low temperature bias at leaf temperatures below 5 °C. TIR temperature corrections reduced this bias to a <1.0 K mean error relative to the thermocouples. Our results show that TIR measurements can be corrected to more accurately quantify canopy temperature, and that the importance of this correction increases under more extreme ambient conditions, such as colder periods.