Pulse Phase Thermography
Pulse phase thermography (PPT) is a heating and data analysis method that can be
used to reduce the effects of non-uniform heating (Maldague et al. 2002). In this method,
a high energy heat pulse is applied using a photography flash and thermal images are
recorded while the surface cools. The discrete Fourier transform (DFT) is computed for
the temperature vs. time response of each pixel in the series of thermal images using the
following:
1 27don 2 lnd
F,...N -1 2= [ COSI N -isin 2N- ] (6-13)
N,,L \ IX N N _
N = Number of time steps
Tn = Temperature value of pixel at each time step
The output of the DFT includes real and imaginary components that vary as a
function of frequency. The phase angle is determined at each frequency interval using
the following:
k0...N1 arctan Ime() (6-14)
Im(Fk) = Imaginary component of DFT output
Re(Fk) = Real component of DFT output
This procedure generates a new series of phase images in the frequency domain.
The first value in the DFT output, Fo, corresponds to the average value of the temperature
response (dc-component). The corresponding phase angle, Do, is zero. The second value
in the DFT, Fi, corresponds to the lowest frequency. The lowest frequency value can be
determined as follows:
1
S= 1- (6-15)
tN