Non-Contact Thermal Wave Imaging Using Infrared Radiation

A Non-Contact thermal wave imaging technique based upon the detection of infrared radiation is described. An amplitude modulated electron beam periodically heats the sample and is raster scanned. The amplitude and phase of the modulated infrared radiation emitted by the heated portion of the sample is collected, detected, and used to form amplitude and phase images. Both pyroelectric and liquid nitrogen cooled photoconductive infrared detectors have been used. The collection optics consists of an ellipsoidal collector with the sample at one focus and the detector at the other. The collector can also be cooled. Since the emitted infrared radiation is a function of the thermal and geometric properties of the sample, the images are able to display subsurface geometrical and thermal inhomo-geneities. Examples will be given of each. Model calculations show dependence of the infrared signal amplitude and phase on material properties, geometry, and experimental parameters. The method requires no contact to the sample, requires no special sample preparation, and appears to offer resolution, contrast and scan rates similar to other forms of thermal wave imaging. Modulation frequencies ranging from 10 Hz to 1 MHz have proven feasible. This imaging technique may find applications in subsurface flaw detection and the bonding within laminated structures.