Thermography offers noncontact, wide area detection of subsurface defects, and can be used as an alternative or complement to conventional inspection technologies.

Active and passive thermography
IR thermography can be divided into two approaches, the passive approach and the active approach. The passive approach tests materials and structures which are naturally at different (often higher) temperature than ambient while in the case of the active approach, an external stimulus is necessary to induce relevant thermal contrasts.
In many industrial processes temperature is an essential parameter to assess proper operation and passive thermography aims at such measurement. Important applications of the passive approach are in production, predictive maintenance, medicine, fire forest detection, building thermal efficiency survey programs, road traffic monitoring, agriculture and biology, medicine, detection of gas (by mean of absorbing tracer gas) and in nondestructive testing (NDT). In all these applications, abnormal temperature profiles indicate a potential problem to take care of. Interestingly for some applications, knowledge of the workpiece fabrication and operation combined with proper thermal modelling opens the door to quantitative extraction of information such as for instance the remaining thickness of refractories, etc.
Contrary to the passive approach, in the active approach, an external stimulus is required to generate relevant temperature differences not present otherwise. Known characteristics of this external stimulus (example: time t0 when it is applied) enable quantitative characterization such as for instance the depth of a detected disbond. Depending on the external stimulus, different approaches of active thermography have been developed, such as pulse thermography (PT), step heating (SH), lockin thermography (LT), vibrothermography (VT). The active approach finds numerous applications in NDT.
Advantages and difficulties of IR thermography
Each NDT technique has its own strengths and weaknesses. In the case of IR thermography, the strengths are as follows:

• fast inspection rate (up to a few m2 at a time);
• no contact (no couplant needed although in some cases a ‘blackpainting’ step is required to perform the inspection so that strictly speaking, a 'contact' is thus present in this case;
• security of personnel (since there is no harmful radiation involved, however high power external stimulation - such as powerful flashes - require a shroud, case of pulse thermography;
• results are relatively easy to interpret (you 'see' what you are inspecting) since they are (often)obtained in image format, furthermore images can be processed to extract more information;
• wide span of applications;
• unique inspection tool for some inspection tasks (e.g. as in the case of some ceramic coatings hardly inspected by other NDT approaches or in the case of some maintenance surveys).

On the other hand, there are some difficulties specific to IR thermography:

• difficulty in obtaining a quick, uniform and highly energetic thermal stimulation over a large surface;
• effects of thermal losses (convective, radiative) which induce spurious contrasts affecting the reliability of the interpretation;
• cost of the equipment;