'Dual-reference' method for high-precision infrared measurement of leaf surface temperature under field conditions.

Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high-precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote-sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg . A novel 'dual-reference' method was developed to increase the accuracy of infrared needle-leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf-to-air temperature differences of needle-leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively. Routine high-precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf-scale processes and their dynamics.

Method for accurate measurement of infrared emissivity for opaque low-reflectance materials.

Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented.