Visual Effects of Surface Emissivity in Thermal Imaging

Abstract

The article discusses the impact of surface emissivity on the thermograms, obtained from a thermal imaging camera during the measurements of construction objects. The study was carried out as an analysis of the digital and thermal images for three selected materials types of building finishing such as: Wood outside doors, cement-lime plaster and glass façade. The images obtained from the exterior and from the interior of the buildings were compared and analyzed in terms of the spectrum intensity within the range of ambient temperature. The result of the conducted research shows how important it is to use both side images for such details as locks, door knobs and handles to properly assess the optional light reflections, especially on glossy surfaces. The influence of the most significant factors on the surface emissivity, such as direction of emission θ, surface temperature TS, radiation wavelength λ, time τ, was discussed on the basis of the experiment. The measurements were made using a Flir T440bx thermal imaging camera, while for the analysis of thermal images and the generation of graphs, the Flir Tools+ 6.4.18039.1003 software was used. For all tested materials, the emissivity value was estimated using a camera and black insulating tape characterized by a known emissivity value ε = 0.98. The study of materials with different emissivity under the reference conditions helped to identify the influence of material reflectivity on the obtained temperature spectrum values and to correctly perform the research. The temperature ranged between 269–293 K, but in particular measurements, the range was reduced. The thermal images reveals additional unexpected details of insulation discontinuity, the indication of which is necessary for building modernization. The wood door joinery research showed the leakage, which disqualifies them from use in low energy buildings in much more definite way because the temperature range resulting from heat outflow approaches even 15 K. The use of glass wind insulation boards can eliminate the wood door icing, which occurs at a temperature of about 268 K, and at the same time, increase the temperature of the shielded door by about 10 K.