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A discussion of some of the basic principles underlying the radiographic process is desirable if the specific applications of the method are to be thoroughly understood. The present paper deals with the relations existing between the various factors entering into the production of a successful radiograph.
An important part of the radiographic equipment is the detecting device consisting of the photographic film or paper, and the intensifying screens. The factors of contrast, latitude, speed, and definition must be properly related to use this device to its best advantage. The X-ray source also contains several variable factors which the operator must adjust with a view to obtaining the best conditions for the particular problem at hand. The reaction of the X-ray beam with the object results in the X-ray image. Not only must this image be intense enough to operate the detecting device, but also it must show the necessary variations in intensity to reveal the interior structure of the object. Contrast, latitude, and definition are important here as in the detecting device for the two work together in rendering a picture of the object.
The question of sensitivity of the method is of considerable importance, and to apply the method successfully the operator should know the size of the smallest detectable defect. There are many factors which modify the sensitivity and they should be controlled as closely as possible. Finally, the question of proper shielding to protect the film from extraneous X-ray effects forms a vital part of the correct radiographic technique and must be carefully worked out in each specific case.
The principles discussed are fundamental to the process, and by employing them properly, the operator can produce a radiograph which will satisfactorily portray the interior structure of the object he is examining. At the same time, he will be assured that the defects larger than a certain minimum size will definitely appear if they are present so that the method may be applied with considerable confidence.
Norton, John T.
Professor of Metallurgy, Massachusetts Institute of Technology, Cambridge, Mass