Evaluation of reflectivity of metal parts by a thermo-camera

Abstract

All objects, apart from black-bodies, reflect a part of the incident radiation. The reflected fraction of incident radiation is termed as „reflectivity‟. The reflectivity of surfaces is often a function of the wavelength, the material, the quality of the surface, and the incident angle of the radiation. In order to evaluate the specular reflectivity of a surface at small incident angles, a method, based on thermal radiation measurement by a thermo-camera, was developed. A quasi black-body, a so-called „Hohlraumstrahler‟, was realized and used as thermal radiation source. The temperature of the source was continuously monitored by temperature probes at 3 different locations inside the source, while the radiation flux of the source was measured by a thermo-camera. During a calibration process, the temperature data from the probes and the radiation flux measured by the thermo-camera were simultaneously recorded and evaluated. In a next step, the radiation from the source was reflected by the samples and measured by the thermo-camera along with the temperature data from the probes. Using the calibration data, the reflected radiation flux could be calculated. The surface profiles of the samples were also measured by a stylus instrument. The calculated reflectivities were evaluated together with the surface roughness of the samples. The results showed good coincidence with the theoretical model of Bennett and Porteus [1].