Active Infrared Thermography
Infrared thermography involves passive and active
In the first case, the test object releases heat. Observing and analysing the cooling process, it is possible to reveal defective areas which are detectable due to the presence of thermal anomalies. For instance, the passive thermography procedure is used in industry applications that incorporate welded metal joints. 
In active thermography the test object is heated up by an external source.
The presence of a significant temperature difference pinpoints subsurface anomalies. In general, active thermography includes pulsed thermography (PT), pulsed phased thermography (PPT), Thermal Contrast (TC), step heating thermography (SH), lock-in thermography (LT) and vibrothermography (VT).
These techniques are briefly described in the following.
In pulsed thermography the test sample is heated up for a short period of time. This heating produces a thermal pulse that propagates into the sample by thermal diffusion.
During the pulse propagation an infrared camera is used to record temperature distribution on the sample surface. A subsurface defect, if present, modifies the diffusion heat flow so its location appears as an area with a temperature difference with respect to the surrounding area.
Pulsed phased thermography is a practical way to analyse data obtained from a PT experiment. A short pulse of energy is applied to the surface of the specimen to be analyzed using lamps or powerful photographic flashes. The temperature evolution of the surface is recorded using an infrared camera (IR). The measurement is made in the transient state. [3, 4]
The main drawback of this method is related to an important property of the Fourier transform. In fact, the Fourier basis function has infinite extensions along the time axis.
This means that the FT does not provide any information regarding the time evolution of spectral characteristics of the signal.
Among thermogram processing techniques, thermal contrast is often employed for enhancing defects detection, image quality and even for obtaining some quantitative approximation regarding the depth.
The different contrast definitions require the identification of a non-defective zone or “sound area” or, better, the cold image, and this is often a limitation because it demands a priori knowledge of the specimen. The definition of the sound area becomes a critical issue and, because it will always be an assumption, this is a strong limitation that provides inaccuracy to contrast methods. Some attempts have been proposed for avoiding this sound area knowledge and, even, overcoming other problems such as the non-uniformity of the thermal stimulation or the tilt of the pulse source with respect to the normal to the specimen surface.
Among them, the Differentiated Absolute Contrast (DAC) method is one of the simplest and easily implemented. 
In step heating the sample is constantly heated starting at a certain point of time. During the heating the temperature evolution is observed.
In lock-in thermography the sample is heated up periodically, generating thermal waves inside. For instance, a sine-modulated heating can be used. The resulting temperature response is recorded and is used for computation of phase and magnitude images.
Vibrothermography is based on an effect of direct conversion from mechanical to thermal energy. The inspected sample is subjected to mechanical excitation that causes a friction effect in places where the defects (delamination, cracks, etc.) are located. Due to friction, a certain amount of energy is released, pinpointing the defect location.
 X. Maldague, Theory and practice of infrared technology for nondestructive testing, Wiley & Sons, New York, 2001.
 Sergey Lugin, Pulsed Thermography, Algorithms for Efficient
and Quantitative non-destructive testing, VDM Verlag Dr. Muller, Saarbrucken,
 F. Galmiche, X. Maldague, Depth defect retrieval using the wavelet pulsed phased thermography, Proc. 5th Conference on Quantitative Infrared Thermography (QIRT), Eurotherm Seminar 64, D. Balageas, G. Busse, C. Carlomagno (eds.), Reims, France, July 18 – 21, 2000, 194 – 199.
 C. Ibarra-Castanedo, Quantitative subsurface defect
evaluation by pulsed phase thermography: depth retrieval with the phase,
Université Laval, 2005.
 D.A. Gonzalez, C. Ibarra-Castanedo, M. Pilla, M. Klein, J.M. Lopez-Higuera, X. Maldague, Automatic Interpolated Differentiated Absolute Contrast Algorithm for the Analysis of Pulsed Thermographic Sequences, Proc. 7th Conference on Qauntitative InfraRed Thermography (QIRT), Rhode Saint Genese, Belgium, July 5 – 8, 2004, H.16.1 – H.16.6.