Case Studies 03

Non-Destructive Testing Methods

Photo-thermal diagnostics are non-destructive testing methods complimentary to conventional methods such as ultrasound and X-ray inspection. Photo-thermal methods have advantages especially for near-surface (a few micrometers to a couple of millimeters) detection with a variable depth range, compared to conventional techniques. In this family of methods a beam of energy (typically a laser), intensity-modulated at a certain frequency, is focused onto the sample surface. The resulting periodic heat flow due to the absorbed optical energy in the material is a diffusive process, producing a periodic temperature distribution which is called a “thermal wave”.

Photo-thermal applications include measurement of optical and thermal properties of a specimen as well as other aspects of the specimen which modify the temperature distribution such as thermal boundaries and defects in the sample. This requires the measurement of the periodic temperature at an external boundary of the specimen. There are several detection methods available to monitor the temperature. At CADIPT we use the IR radiometric detection method and in some cases where both thermal and optical measurements are required we use pyroelectric sensors (PVDF films). Pyroelectric-based photo-thermal devices and instruments for specialty analytical research projects are designed and built. The signal from the detector is sent to either (a) lock-in amplifier (b) box car integrator, or (c) FFT analyzer, depending on the measurement requirements. Other “conventional” photo-thermal instrumentation systems (spectroscopic, Mirage effect, photoacoustic) are also available. (reference: UofToronto)