Flash energy and duration issues in active thermography

Since its introduction in the 1980s, the field of Flash Thermography for Nondestructive Testing (FTNDT) has benefited from enormous advances in the underlying IR camera and computer technologies that enable it, while the flash excitation component has remained essentially unchanged. A typical FTNDT setup comprises a pair of helical or U-shaped xenon flashlamps designed for use in commercial photography, each powered by a bank of capacitors under computer control. A single flashlamp-power supply unit is often described as 4-6 kJ of energy with duration 2-3 msec, the Full Width Half Maximum (FWHM) duration of the flash. However, saturation of the IR camera detector may persist considerably longer due to the extended tail of the flash pulse. In high thermal diffusivity materials, the presence of saturation may mask features of interest, and limit access to early onset signals normally used for depth measurement of subsurface features. Saturation may be mitigated to some degree, but not entirely, by reducing flash energy (power supply voltage). However, more effective elimination of saturation is accomplished using a dedicated hardware device to truncate the duration of the flash pulse. In this paper, we compare the effect of varying flash energy by adjustment of power supply voltage and flash duration on detection of near surface features in an aluminum plate.