Infrared emissivity measurement based on polarized reflection characteristics of non-transmissive insulators

Abstract

Emissivity is a crucial property in quantitative thermographic testing, a leading technique in nondestructive inspections for exterior wall diagnosis. We propose a method for measuring emissivity that exploits the angle dependence of polarized reflectivity. This approach involves measuring the reflected energy from a heat source near the Brewster angle using an infrared thermographic instrument equipped with an infrared polarizer. However, the emissivity measurements for insulators exhibited significant discrepancies when compared with values obtained via FT-IR, attributable to two main factors: diffuse reflection caused by surface roughness and approximation errors inherent in the simplistic theoretical formula. To address these issues, we applied a correction for specular reflectivity considering surface roughness and refined the theoretical formula by incorporating polarization theory and the extinction coefficient. This enhancement enabled accurate emissivity measurements for ceramic tiles with rough surfaces. Notably, this method only requires a heat source and does not necessitate precise temperature measurements with an infrared thermographic instrument, which is significant since standard instruments used in nondestructive inspections often fail to measure temperature accurately. Additionally, we achieved thermal imaging without background reflection through subtraction image processing, beneficial for field measurements. Consequently, this improved method is highly effective for nondestructive inspections.