Thermal-wave imaging system using a thin-film pyroelectric bimorph sensor element

Abstract

In photothermal, or thermal-wave, imaging, a laser beam is scanned across the front surface of a sample mounted on a temperature-sensitive sensor. A rastered image of the sample’s thermoacoustic properties is then built up by detecting the associated heat pulse propagation through the sample. Sensor materials most used in such thermal-wave imaging systems are conventionally ceramics such as lead zirconate titanate and single crystals such as triglycine-sulphate and lithium tantalate. Recently, polymer films such as polyvinyl fluoride and polyvinylidene fluoride have found increasing use,1,2 due to their flexibility, robustness, and relatively low cost. This work describes a thermal-wave imaging system based on a pyroelectric bimorph detector element. The sensor is based on a novel bilayer laminate configuration of pyroelectric polymer films, which has been shown to have a higher theoretical responsivity than for an equivalent singleelement film.3,4 Specific detectivity values for our sensor are approximately twice the value reported for the equivalent singlefilm element. This gain in detectivity continues up to about 100 Hz for polyvinylidene fluoride bimorphs; at higher laser modulation frequencies the thermal attenuation becomes larger and the specific detectivity decreases.