The basic idea of nondestructive testing is that a sample is characterised by its response to a certain kind of excitation. The excitation may be an electric or elastic field with a time pattern described by a step, a pulse, or a sine wave type. The field can be applied locally to characterise the area around a certain sample spot. A raster scan image is then performed by measuring many spots one after the other.
That was the typical situation for thermal wave imaging derived originally from photoacoustic spectroscopy where the thermal response to a modulated optical input is monitored. Though this technique was soon extended to imaging, there were serious restrictions to the size of the sample. Photothermal detection removed this problem. However, it became clear that the depth range needed for many applications required such low modulation frequencies that it took a long time to generate an image. To give an example, to look 0.1 mm deep into polymers one needs about 1 Hz even with signal phase. This may be acceptable for a one point measurement, but not for one pixel of an image. The only way to avoid a long image generation time at low modulation frequencies is to use a multiplex technique where all pixels are dealt with in parallel. This can be achived by generating and monitoring the thermal wave everywhere in the sample at the same time. So one can use a lamp to illuminate the whole sample periodically and a thermography camera to observe the resulting temperature modulation. Phase and magnitude at each pixel are calculated in the simplest way from four thermographic images (S1 to S4) taken during one modulation cycle so that there is a constant phase angle of 90 degrees between subsequent images. Due to the speed of modern thermography cameras, images may be recorded during one modulation cycle at low frequencies. However, for sinusoidal modulation one can apply suitable averaging techniques ending up again in the four raw images given above which are particularly easy and fast to evaluate.
It is obvious that this kind of modulation thermography provides a phase angle image which is not affected by local variations of thermal wave excitation (e.g. due to inhomogeneous illumination or optical structure of the surface), infrared emission coefficient, or local change of the average temperature. It displays basically the time shift between the remote heat injection and the remotely observed thermal response in the stationary oscillation. So it provides information on thermal structures to a depth given by about twice the thermal diffusion length which can be adjusted via the modulation frequency. That is why multiplex photothermal imaging or lockin-thermography is relevant for applications.
The thermal wave source can be a laser, a lamp (equipped with suitable filters to avoid cross-talk between excitation and detection), a beam of heated or cooled air (e.g. hot air gun), or even a combination of some of these sources. While these exitations are outside the sample one can also think about selectively activating heat sources within the sample. Examples for this kind are resistive heating or mechanical loss angle heating at high frequency (ultrasound) which results in a dark-field method.
In the measurement setup of lock-in thermography a commercial thermography system (AGEMA 900) is coherently coupled to a thermal wave source which is operated in such a way that a sinusoidal temperature modulation results. This is achieved by automatically evaluating the themographic data and by using a self-learning program which controls the power supply. After several modulation cycles to etablish a stationary situation the data acquisition starts. Time for measurements is typically below 3 minutes for frequencies between 1 Hz and 0.03 Hz.
For NDT purposes, we investigated several samples with layered structures using lamp and ultrasound heating. The method showed its potential for these kind of inspections, being especially favourable for the detection of impact damages and delaminations in composites, cracks in ceramic components, and hidden corrosion areas in Al-airplane parts.
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