The technique of lock-in thermography was originally developed at MPI Halle for investigating solar cells. In that application, a solar cell is subjected to current pulses leading to a surface temperature modulation in positions where the current flows.
Since water absorption and desorption (evaporation) in a plant tissue also generates heating and cooling, we have used the lock-in thermography technique also for imaging this water absorption activity. We have constructed a special thermostatted measurement cell implying an IR-transparent window and some gas stream facilities. By periodically switching the gas stream between "wet" and "dry" we periodically switch between water absorption and evaporation in the tissue in this cell. Thus, the local magnitude of the water absorption activity in the material can be imaged. The "amplitude" image is a measure of the amount of this activity, and the "phase" image is a measure of the time delay between humidity change and absorption, respectively desorption.
Fig. 1: Thermography of water absorption in plant tissue (Heat in air, oscillating humidity)