Characterization of an infrared detector for high frame rate thermography

Abstract

The use of a commercially available photodetector based infrared thermography system, operating in the 2–5 µm range, for high frame rate imaging of temperature evolutions in solid materials is investigated. Infrared photodetectors provide a very fast and precise means of obtaining temperature evolutions over a wide range of science and engineering applications. A typical indium antimonide detector will have a thermal resolution of around 4 mK for room temperature measurements, with a noise threshold around 15 to 20 mK. However the precision of the measurement is dependent on the integration time (akin to exposure time in conventional photography). For temperature evolutions that occur at a moderate rate the integration time can be relatively long, enabling a large signal to noise ratio. A matter of increasing importance in engineering is the behaviour of materials at high strain rates, such as those experienced in impact, shock and ballistic loading. The rapid strain evolution in the material is usually accompanied by a temperature change. The temperature change will affect the material constitutive properties and hence it is important to capture both the temperature and the strain evolutions to provide a proper constitutive law for the material behaviour. The present paper concentrates on the capture of the temperature evolutions, which occur at such rates that rule out the use of contact sensors such as thermocouples and electrical resistance thermometers, as their response times are too slow. Furthermore it is desirable to have an indication of the temperature distribution over a test specimen, hence the full-field approach of IRT is investigated. The paper explores the many hitherto unaddressed challenges of IRT when employed at high speed. Firstly the images must be captured at high speeds, which means reduced integration times and hence a reduction in the signal to noise ratio. Furthermore, to achieve the high image capture rates the detector array must be windowed down, therefore there is a compromise made between the extent of the full-field imaging and the temporal resolution of the image capture. In the present work a maximum image capture speed of 15 kHz was achieved with a detector array of 64 × 12 elements and an integration time was 60 µs. Results from initial work on woven E-glass/epoxy tensile specimens are presented.