Effect of Solar Azimuth and Infrared Thermometer View Direction on Measured Soybean Canopy Temperature1

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Abstract
Measurements of radiative canopy temperature taken with infrared thermometers (IRTs) are strongly influenced by amount of crop cover and amount of viewed soil background. This influence can be minimized by making off‐nadir measurements so that mostly vegetative surface is viewed. But off‐nadir measurements vary with the relative azimuthal positions of the sun and the IRT due to the measurement of true variations in the individually sunlit and shaded canopy elements. The geometrical relationship between solar azimuth and IRT view azimuth on measured canopy temperatures was investigated for soybeans grown at two Nebraska locations in 1982. Soils at the two locations were a Typic Argiudoll and a Typic Ustipsamment. Canopy temperature as measured by the IRT declined linearly as the difference between the solar azimuth and IRT view azimuth increased from 0° to about 110°. As this difference increased to angles greater than 110°, the viewed canopy temperature remained fairly constant at about 0.30°C below the average of the canopy temperatures measured at the four cardinal directions. The declining linear relationship seen for angles between 0° and 110° was found to be more strongly defined during the vegetative growth stages than during the reproductive growth stages. This was attributed to declining heliotropic response of soybean [Glycine max (L.) Merr.] leaves as the canopy ages.
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