The balance between the energy that the leaf receives and returns determines the leaf temperature. Factors that influence leaf temperature are:
The leaf receives in a greenhouse energy through the sun and heat radiation from the heating. The leaf loses energy by radiation and evaporation. Transfer by air, both of leaf to air and air to leaf. This depends on the difference in temperature. The leaf temperature will follow the surrounding temperature but is never exactly equal. With an active plant which is evaporating leaf temperature usually is somewhat lower than the surrounding temperature. A sudden increase in leaf temperature to over surrounding temperature can point to water stress. Due to water shortage the stomata in the leaf closes which prevents evaporation. The leaf temperature rises as a result, because the leaf is not able to cool himself. There is risk that the leaves will burn. A low evaporation rate can also occur in a small humidity deficit of the greenhouse air. Experiences in the aircokas greenhouse with active cooling through air humidification shows that under normal circumstances the leaf temperature is higher than the surrounding temperature without the crop to suffer from stress. At night due to a cold greenhouse roof the temperature of the upper plants can fall well below the surrounding temperature. A screen reduces this effect. A leaf temperature which is lower than the surrounding temperature at high humidity can lead to condensation.
Leaf temperature can be measured with an Infra-Red camera. The plant temperature camera uses infrared to measure plant temperature and is doing this without any plant contact. It can measure both small and large surfaces.