Measurement the phase yields value for and
778 36 Thermography to Measure Water Relations of Plant Leaves
Figure 36.5: Parameter images for constant heating: a time constant τ; b tem
perature difference ∆T.
div =  

data from the same species.
Periodic flux method. A varying energy flux density jact(t) is applied
∂f (t)  (36.30)  

∂t  ◦ 

that is, a time derivative of a function in the time domain becomes a



ˆz(ω) 
(36.32)  
and  ���� 

(36.33) 
36.4 Measurements  779  



2π  
0  


2π  
0  
Figure 36.6:  Parameter images depicting the phase Eq. (36.33) for different 
In principle, a measurement of the phase ϕ(ω) yields a value for τ and, therefore, using Eq. (36.28), a value for (C/A)leaf.
The varying energy flux had the form of a rectangular function (the radiator was periodically switched on and off). As the Fourier spectrum of a rectangular function consists of all the odd harmonics, this can be considered as an application of several frequencies at once (with decreasing amplitude).