In determining the sensitivity of radiographic testing of welded joints by groove standard, computer analysis of brightness is usually carried out along a longitudinal line drawn in the middle of an X-ray image of the standard. It is suggested that a brightness curve of the line outlined along the standard should not be analyzed but the brightness should be determined by averaging the gray levels of the line segments parallel to the smaller side of the standard. To determine points belonging to the line segments, methods for indicating the boundaries of an object in an image and Hough transforms are used. The radiographic testing sensitivity is determined taking into account the Weber-Fechner law as well as values of the grayscale of grooves and their background calculated by the proposed technique.