Pulsed infrared radiometry for remote monitoring of thermal properties or thickness of thin films

We have developed a single-ended pulsed radiometry technique for the remote measurement of thermal diffusivity, absolute absorption coefficients, thickness, or layered structure of thin films, which can be free-standing or on a substrate. Our technique relies on the use of a short-pulsed N2 laser beam to heat up rapidly the sample surface (e.g., by a few degrees Celsius), and the detection of the shape of the transient IR thermal radiation that is backscattered from the sample surface. No sample damage occurs, since the laser pulse (< 1-mJ energy and 8-nsec duration) is gently focused to a spot size of ~4-mm diam at the sample. We have developed exact analytical solutions to permit us to analyze the observed IR signal profile to derive the desired parameters of the sample. We have applied this single-ended pulsed photothermal radiometry (PPTR) technique to various cases of metal films, polymer films, and paper (including paper currency) and have demonstrated several important advantages of the PPTR technique for material testing applications; namely, it is noncontact, fast, nondestructive, and applicable to materials with an inaccessible backside. This should be compared with traditional methods1 of measuring thermal diffusivity; these methods are usually double-ended (i.e., requiring transmission probing) or require the attachment of electrical or thermocouple leads to the sample.