Photothermal radiometry is suited for non-contacting inspection of subsurface defects, but the imaging speed is very low, especially at the low modulation frequency required for a large depth range. The low inspection speed limits the industrial applications of this method. However, the scan method can be improved by combining it with the thermographic technique. This "phase sensitive modulation thermography" or "lock-in thermography" described some time ago [1-3] is based on the same measurement principle as the point by point photothermal radiometry, but the measurement time is much shorter due to the parallel thermal wave excitation and detection. In this mode of multiplex operation the thermography system is synchronised with the excitation source. The calculation of phase and amplitude at each pixel is performed with software that simulates the lock-in technique of signal analysis

Unlike pulse thermography, various sources can be used for thermal wave generation in material. The choice of the kind of source depends on the properties of the investigated samples. Radiation (light) and hot air can be modulated to be used as external sources. In this case thermal waves are generated on the sample surface and propagate into the material [4]. But thermal waves can be excited inside the sample as well using various mechanisms: for electric conductive materials a modulated electric current is useful. Polymers and damaged ceramics have usually a high mechanical loss angle. When these materials are under cyclic mechanical load part of elastic energy is converted to heat. The high stress concentration in a defect area results in an increase of the hysteresis effect and therefore in selective defet heating. As the thermal waves are generated directly in the defect region and move to the sample surface, they reveal the defects directly [5] like in other "dark-field" techniques.

The applicability of phase sensitive modulation thermography was demonstrated on various samples, e. g. composite samples with delaminations, impact damages, voids, and cracks. The results show that this method is suitable for remote in-service inspection of aerospace components and other layered or laminated materials. REFERENCES

1. P. K. Kuo, Z. J. Feng, T. Ahmed, L. D. Favro, R. L. Thomas and J. Hartikainen, "Parallel Thermal Wave Imaging Using a Vector Lock- in Video Technique", Photoacoustic and Photothermal Phenomena (Edited by P. Hess and J. Pelzl) Springer-Verlag, Berlin, pp. 415 (1988)
2. J. L. Beaudoin, E. Merienne, R. Danjoux and M. Egee, "Numerical system for infrared scanners and application to the subsurface control of materials by photothermal radiometry", Infrared Technology and Applications, SPIE Vol. 590, pp. 287 (1985)
3. G. Busse, D. Wu and W. Karpen, "Thermal wave imaging with phase sensitive modulated thermography", J. Appl. Phys. 71, 8, pp. 3962(1992)
4. D. Wu, R. Steegmuller, W. Karpen and G. Busse, "Characterization of CFRP with lock-in thermography", Review of Progress in Quantitative Evaluation, Vol. 14 (Edited by D. O. Thompson and D. E. Chimenti) Plenum Press, New York, pp. 439 (1995)
5. J. Rantala, D. Wu and G. Busse, "Amplitude Modulated Lock-in Vibrothermography for NDE of Polymers and Composites", Research in Non-destructive Evaluation (in print).