In infrared thermographic testing, a background reflection is one of the main causes of a false detection. The authors developed an image processing program that can reduce the effect of the background reflection from a thermal image. The program mainly consists of two parts. The first part is the correction of emissivity which depends on a face angle. The emissivity is changed not only by a material property and a surface roughness but also by a face angle. Especially in a test for a high building and a floating roof tank, the thermal image includes a wide range of face angle. In order to solve this problem, by using the theoretical equation of emissivity, the angle correction of emissivity is carried out for each pixel in the infrared image. It is confirmed that the first part of this program can correct efficiently the emissivity change for a flat plate and a curved surface which have a wide range of face angle respectively. The second part of the program is the reduction of a background reflection. The emissivity change causes the reflectivity change and, as a result, this changes the effect of the background reflection. To reduce this effect, the background heat source is measured separately and then is subtracted from the infrared image. At this time, the specular reflectivity should be used. Consequently, this program can reduce the effect of reflection from the background heat source and extract the radiation of the flaw part from the complex thermal image.
Ceramic tiles are widely used for building walls. False detections are caused in inspections by infrared thermography because of the infrared reflection and angle dependence of emissivity. As the first problem, ceramic tile walls are influenced from backgrounds reflection. As the second problem, in inspection for tall buildings, the camera angles are changed against the height. Thus, to reveal the relation between the emissivity and angles is needed. However, there is very little data about it. It is impossible to decrease the false detection on ceramic tile walls without resolving these problems; background reflection and angle dependence of emissivity. In this study, the angle problem was investigated. The purpose is to establish a revision method in the angle dependence of the emissivity for infrared thermography. To reveal the relation between the emissivity and angles, the spectral emissivity of a ceramic tile at various angles was measured by FT-IR and infrared thermographic instrument. These two experimental results were compared with the emissivity-angle curves from the theoretical formula. In short wavelength range, the two experimental results showed similar behavior, but they did not agree with the theoretical curve. This will be the subject of further study. In long wavelength range, the both experimental results almost obeyed the theoretical curve. This means that it is possible to revise the angle dependence of spectral emissivity, for long wavelength range.
In order to efficiently inspect very wide area of concrete structure wall, an infrared thermographic testing with a distance heating was developed in this study. The researched subjects were the following three; 1. Improvement of radiant heating efficiency, 2. Development of distance heating method and 3. Development of data analysis method against nonuniformity of a heating and/or a wall absorptivity. In this paper, we focus on the first issue. In order to investigate about combinations between the spectral emissivity of radiant heater and the spectral absorptivity of concrete, three different types of radiant heater, a near infrared type, a far infrared type and blackbody type, were used to heat concrete specimens. As a results, both a blackbody type and a far infrared type, e.g. a ceramics heater and a blackbody coated heater, can heat a concrete wall more efficiently than a near infrared type, e.g. a halogen lamp heater and a xenon lamp heater. This is because the spectral absorptivity of concrete is higher in a far infrared region than in a near infrared region. We find that the efficiency of the heating process may be improved by choosing a heater whose radiation is concentrated near wavelengths at which the structure to be heated exhibits maximal absorptivity. The efficiency of the concrete heating process may be easily improved simply by covering the surface of a near infrared heater with a blackbody surface coating to mimic the radiation characteristics of a blackbody.