In infrared thermographic tests, background reflection from the sun or neighboring structures is one of the serious problems that lead to false detection of flaws. Generally, the background reflection appears as a hot spot on the thermal image, and can lead to faults during inspection. To avoid this, the background reflection must be removed from the thermal image. Metals, owing to their high reflectivity, are especially susceptible to background reflection during conventional infrared thermographic tests. In this study, the authors have developed a blackbody film made of polyvinyl alcohol that has a high emissivity in the infrared range and a high transmissivity in the visible range. At first, the authors measured the spectral emissivity of the film by using a Fourier transform infrared spectrometer. A measuring equation was proposed for the film that has high emissivity and low transmissivity. Further, the measurements revealed that: a higher degree of saponification leads to higher emissivity; it has a sufficiently high emissivity; and a long infrared range. Next, the authors carried out infrared thermographic tests on a stainless steel plate having background reflection. The results show that the film is effective as a material with low emissivity.
Since typical interior materials have high-emissivity surface of about 0.9, far-infrared generated from radiant heater is absorbed once in interior walls and then reradiated infrared from the warmed walls reaches human body surfaces. In the case of low-emissivity (low-E) interior surface of about 0.1, generated far-infrared is reflected at interior wall surface and reaches human body surfaces without warming processes of walls. Thus, it is expected that human body feels warmth quickly and/or heating load can reduce owing to the decrease in set temperature by forming the room covered with low-E materials. Therefore, we studied effects of the low-E interior surface material on interior thermal environment using radiant heating by comparing two real-size experimental rooms with the same thermal insulation: one is covered with a low-E material, aluminum foil (Low-E room), and the other is covered with a normal interior material (Std-E room).
When warm up the rooms using electrically heated carpet as radiant heating equipment, the globe and air temperatures of the Low-E room increased rapidly and showed about 0.8℃ higher temperatures than those of Std-room in 30 min. The radiant temperatures of the wall and ceiling of Low-E room increased immediately with the increase in the temperature of the carpet by turning on, while slow increase in those of Std-E room. Thus, the increase in globe and in air temperatures of the Low-E room may be because the whole Low-E surface, except for floor, of the room become pseudo radiant heating surface.
In this study we used aluminum foil as low-E materials. However, this material is not suitable for interior due to high specular reflection. Therefore, novel materials with low specular reflection in the visible range and low emissivity in the far -infrared range are also developing at present.