2.1. Pulse ThermographyPulse thermography (PT) consists simply in the stimulation of the object (under evaluation) and monitoring of its surface temperature variation during the transient heating, or cooling, phase. Heating is selleck chem generally performed by lamps, flash lamps, scanning lasers, or hot air jets. Cooling can be practically attained by cold air jets. Of course, air jets (hot or cold) can be used only on a massive surface since jet impingement may damage delicate objects, for example, artworks. For the case of slabs, analysis with PT can be performed in two different modes: transmission and reflection. In the transmission mode, the infrared camera views the rear face, that is, the face opposite to the heating/cooling source.

However, since the opposite side is not always accessible and/or available, the reflection mode, for which both heating (cooling) source and camera are positioned on the same side, is mainly applied. The thermal energy propagates by diffusion under the surface while the infrared camera monitors the temperature variation over the viewed surface. Obviously, for a uniformly heated surface, the temperature distribution is uniform in case of a homogeneous material. The presence of a defect at a certain depth interferes with the heat flow causing local surface temperature variations. The visibility of a defect can be evaluated by the following parameter DT [14]:DT=|��T||��Ts|=|Ts?Td||Ts?Tr|,(1)where Td is the temperature over a defective zone, Ts is the temperature in a sound zone, and Tr is a reference temperature.

More specifically, Tr is the temperature of the sound material before starting transient heating, or cooling, Tr = Ts(t=0) (i.e., the temperature of the first thermal image taken at t = 0s in the time sequence). Indeed, the quantity Ts ? Tr = ��Ts plays an important role because it indicates the optimal temperature variation to which the material has to be subjected for good defect visibility. The defect detection is limited by the signal-to-noise ratio (SNR) [15] or by the noise equivalent temperature difference (NETD) of the infrared detector. In addition, the surface finish is of great importance since variations in surface roughness, cleanliness, uniformity of paint, and other surface conditions can cause variations in the emissivity coefficient and affect the temperature measurement.

These drawbacks are overcome using lock-in thermography.2.2. Lock-In ThermographyIn this work, lock-in thermography is performed with halogen lamps and is simply referred to as LT. The energy, generated by halogen lamps, is delivered to the object surface in the form of periodic thermal waves. Batimastat The thermographic system is coherently coupled to the thermal wave source which is operated in such a way that a sinusoidal temperature modulation results. This modulation is obtained from a nonlinear electrical signal produced by the lock-in module which also allows for frequency variation.