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Abstract
Increased use of infrared thermometers (IRT) for measuring plant temperatures has shown the need for determining IRT positions so measurements can be used with more reliability. The lack of controlled devices has been an obstacle for the correct evaluation of such positions. At the same interpretation and evaluation of these measurements has been done with subjective approaches that sometimes are not accurate.
This study was conducted to determine variations on composite radiative temperatures (CRT) taken at different positions and to quantify the relationship between sunlit and shaded parts of the canopy and CRT.
A robotic system which controls positions accurately was used for obtaining CRT over sorghum (Sorghum bicolor (L.) Moench) at Mead, Nebraska during the summer of 1991. An image acquisition and analysis system was used to collect images of the CRT scene viewed by the IRT on some days of that summer.
The variations of CRT were related to solar radiation levels and to the position of the sun with respect to the sensor. They were also influenced by the structure of the canopy and the row direction (N-S) used in this study. The largest variations occurred at high radiation levels. On the other hand the images collected allowed the identification of sunlit and shaded parts which helped to establish their relationship with CRT. High positive correlations (r $\approx$ 0.80) for shaded soils and CRTs were found which indicated the warming effect that they can have on CRT measurements. High negative correlations (r $\approx -0.90$) for sunlit vegetation and CRTs showed the cooling process occurring in plants.
Some of the data showed responses that were not easy to interpret. Such responses may be attributed to the size of the area sampled and its lack of representativeness of whole canopy conditions. Increasing the area sampled can help to overcome this problem. Sample areas with similar orientations and structures will allow more credible comparisons. Narrow field of view IRTs should be used with caution.
