Balanced bright and dark field illumination for remote visual testing to detect cracks on pressure vessel of nuclear reactors

Remote visual testing by video cameras is the standard process for inspecting the pressure vessel of nuclear reactors. The inspectors need to find stress corrosion cracking of around several tens of µm wide on a display monitor observed by remote wired cameras in the reactor pressure vessel filled with cooling water. The inside surface of the vessel structure has typically been blasted or grinded during its construction process for smoothing any excess weld metal, and as such, it might contain partly specular areas and partly rough areas. We found that the cracks on such surfaces are hardly distinguished from the boundary of these areas under normal scattered illumination, since the specular area reflects more of the light from the light source outside the path to the camera than the light scattered out by the rough area. To resolve this issue, we propose using two illumination light sources simultaneously, i.e., the bright field illumination and the dark field illumination, to balance the contrast to cracks from both areas. The results of experiments in a simplified condition showed that there was an improvement in the visibility of the cracks using this method.