Start Date

2016 12:00 AM

Description

In flash thermography, the surface of a test piece is excited by a pulse of light from a xenon flash lamp that is powered by a capacitor discharge. Typically, the lamps and power supplies are commercial units designed for photography applications. The equipment most often used for thermography has nominal energy output of 4-6 kJ and duration 3-5 msec. However, the actual optical output is more complex, since the energy rating pertains to the electrical discharge of the capacitors, and does not account for efficiency in converting electrical current to light, or resistive losses in the transfer of energy from the power supply to the lamp. Furthermore, the nominal flash duration that is usually cited for thermography refers to the full width, half maximum duration, and does not convey the fact that the flash pulse, which contains both visible and IR components, has an extended IR tail that continues for tens of milliseconds after the visible (white light) flash. In previous work, we introduced a hardware device to truncate the duration of the flash so that the lamp output more closely resembles a rectangular pulse, with durations as low as 200 microseconds. In addition to the flash duration, the relationships between the onset of the flash and the integration and readout functions of the IR camera focal plane are critical, and can be exploited to great advantage in some measurement applications. In this work, we compare performance of normal flash performance with truncated performance where the onset time has been optimized, with examples including thickness measurement of thermal barrier coating (TBC) and depth measurement of subsurface features in aluminum.

Language

en

File Format

application/pdf

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Jan 1st, 12:00 AM

Excitation Duration and Onset Time Effects in Active Thermography

In flash thermography, the surface of a test piece is excited by a pulse of light from a xenon flash lamp that is powered by a capacitor discharge. Typically, the lamps and power supplies are commercial units designed for photography applications. The equipment most often used for thermography has nominal energy output of 4-6 kJ and duration 3-5 msec. However, the actual optical output is more complex, since the energy rating pertains to the electrical discharge of the capacitors, and does not account for efficiency in converting electrical current to light, or resistive losses in the transfer of energy from the power supply to the lamp. Furthermore, the nominal flash duration that is usually cited for thermography refers to the full width, half maximum duration, and does not convey the fact that the flash pulse, which contains both visible and IR components, has an extended IR tail that continues for tens of milliseconds after the visible (white light) flash. In previous work, we introduced a hardware device to truncate the duration of the flash so that the lamp output more closely resembles a rectangular pulse, with durations as low as 200 microseconds. In addition to the flash duration, the relationships between the onset of the flash and the integration and readout functions of the IR camera focal plane are critical, and can be exploited to great advantage in some measurement applications. In this work, we compare performance of normal flash performance with truncated performance where the onset time has been optimized, with examples including thickness measurement of thermal barrier coating (TBC) and depth measurement of subsurface features in aluminum.