CHAPTER 3
NONDESTRUCTIVE EVALUATION USING
INFRARED THERMOGRAPHY
This chapter contains background information about infrared thermography (IRT)
and a review of previous research. The first section deals with the fundamentals of IRT
and describes some of the basic technology used in thermal imaging systems. The
following section addresses IRT as a nondestructive evaluation (NDE) technique.
Infrared Thermography Fundamentals
All objects at a temperature greater than 0 K emit electromagnetic (EM) radiation.
Furthermore, this radiation is emitted across a range of wavelengths. Max Planck
formally quantified the EM emissions of a blackbody (perfect emitter) in 1900 with the
following relationship describing intensity of the emitted radiation as a function of
wavelength and temperature of the object (Maldague 2001):
2he
I(A, T) = (3-1)
'[exp(hc/A DKT)-1l]
I = spectral radiance (W m2 sr-1 am)
S= wavelength of emitted radiation (itm)
T = temperature of the object (K)
h = Planck's constant (6.63 x 10-34 J s)
K = Boltzmann constant (1.38 x 10-23 J/K)
c = speed of light in a vacuum (m/s)
The wavelength at which the peak intensity occurs is given by the Wien
displacement law:
Amax = (3-2)
T