While exposed to the sun’s radiation, plant surfaces increase in temperature . Infrared sensors monitor the intensity of IR radiation emitted from the plant’s surface which can then be displayed in artificial colours. Actual temperature is then inferred by using measurements and transformations which take emissivity, object properties and geometry into account.
Plant heat emission depend largely on transpiration and stonger transpiration cools down the leaf surfaces, while lower transpiration leads to increases in temperature. Transpiration depends on water availability and stomatal activity.
 Jones, H. G. and Rotenberg, E. 2001. Energy, Radiation and Temperature Regulation in Plants. eLS. http://onlinelibrary.wiley.com/doi/10.1038/npg.els.0003199/abstract