Inspection of transparent polymers by photothermal detection of ultraviolet-laser generated thermal waves

Abstract

The concept of photothermal wave imaging has been adapted to the nondestructive inspection of transparent polymeric samples by specific generation of thermal waves. Utilization of light sources according to the absorption properties of the material secured pure surface heating which is necessary for sensible measurements. Thickness profiles of thin films made of poly(ethylene terephthalate), which are transparent in the visible spectrum, could be measured using a pulsed KrF laser. Examples for the nondestructive inspection of complex textile samples such as coated fabrics are presented.The photothermal analysis (PTA) of optically generated thermal waves is a powerful tool for nondestructive, contactless inspection and evaluation of intrinsic properties of a sample.1,2 The method has been used for the observation of dynamic processes in adhesives, for the inspection of coatings on metal and of carbon fiber composites. A modulated light source, e.g., a chopped HeNe- or Ar+ laser, is used for local, modulated heating at the sample’s surface resulting in a time- and space-dependent temperature modulation which propagates through the material. In the case of a homogeneous layer heated only in a small volume at the surface the solution of the heat diffusion equation gives a heavily damped thermal wave T(x,y,z;t). In a one-dimensional model3 only the direction z perpendicular to the surface is considered and we obtain