Radiography in Modern Industry - Kodak

Penetrameters and Visibility of DiscontinuitiesIt should be remembered that even if a certain hole in a penetrameter is visible on the radiograph,a cavity of the same diameter and thickness may not be visible. The penetrameter holes, havingsharp boundaries, result in an abrupt, though small, change in metal thickness whereas a naturalcavity having more or less rounded sides causes a gradual change. Therefore, the image of thepenetrameter hole is sharper and more easily seen in the radiograph than is the image of thecavity. Similarly, a fine crack may be of considerable extent, but if the x-rays or gamma rays passfrom source to film along the thickness of the crack, its image on the film may not be visiblebecause of the very gradual transition in photographic density. Thus, a penetrameter is used toindicate the quality of the radiographic technique and not to measure the size of cavity that canbe shown.In the case of a wire image quality indicator of the DIN type, the visibility of a wire of a certaindiameter does not assure that a discontinuity of the same cross section will be visible. The humaneye perceives much more readily a long boundary than it does a short one, even if the densitydifference and the sharpness of the image are the same.Viewing And Interpreting RadiographsThe examination of the finished radiograph should be made under conditions that favor the bestvisibility of detail combined with a maximum of comfort and a minimum of fatigue for the observer.To be satisfactory for use in viewing radiographs, an illuminator must fulfill two basicrequirements. First, it must provide light of an intensity that will illuminate the areas of interest inthe radiograph to their best advantage, free from glare. Second, it must diffuse the light evenlyover the entire viewing area. The color of the light is of no optical consequence, but mostobservers prefer bluish white. An illuminator incorporating several fluorescent tubes meets thisrequirement and is often used for viewing industrial radiographs of moderate density.For routine viewing of high densities, one of the commercially available high-intensity illuminatorsshould be used. These provide an adjustable light source, the maximum intensity of which allowsviewing of densities of 4.0 or even higher.Such a high-intensity illuminator is especially useful for the examination of radiographs having awide range of densities corresponding to a wide range of thicknesses in the object. If theexposure was adequate for the greatest thickness in the specimen, the detail reproduced in otherthicknesses can be visualized with illumination of sufficient intensity.The contrast sensitivity of the human eye (that is, the ability to distinguish small brightnessdifferences) is greatest when the surroundings are of about the same brightness as the area ofinterest. Thus, to see the finest detail in a radiograph, the illuminator must be masked to avoidglare from bright light at the edges of the radiograph, or transmitted by areas of low density.Subdued lighting, rather than total darkness, is preferable in the viewing room. The roomillumination must be such that there are no troublesome reflections from the surface of the filmunder examination.Radiography in Modern Industry 94