Radiography in Modern Industry - Kodak

and the like being introduced between film and screen during darkroom handling. The timeconsumingprocess of loading and unloading cassettes and film holders is avoided.Roll FilmsIn the radiography of circumferential welds in cylindrical specimens, in the examination of thejoints of a complete frame of an aircraft fuselage, and the like, long lengths of film permit greateconomies. The film is wrapped around the outside of the structure and the radiation source ispositioned on the axis inside allowing the examination of the entire circumference to be madewith a single exposure. Long rolls of film are also convenient for use in mechanized exposureholders for the repetitive radiography of identical specimens or for step-and-repeat devices inwhich radiation source and film holder move in synchronism along an extended specimen.Handling Of FilmX-ray film should always be handled carefully to avoid physical strains, such as pressure,creasing, buckling, friction, etc. The normal pressure applied in a cassette to provide goodcontacts is not enough to damage the film. However, whenever films are loaded in semiflexibleholders and external clamping devices are used, care should be taken to be sure that thispressure is uniform. If a film holder bears against a few high spots, such as occur on an ungroundweld, the pressure may be great enough to produce desensitized areas in the radiograph. Thisprecaution is particularly important when using envelope-packed films.Marks resulting from contact with fingers that are moist or contaminated with processingchemicals, as well as crimp marks, are avoided if large films are always grasped by the edgesand allowed to hang free. A convenient supply of clean towels is an incentive to dry the handsoften and well. Use of envelope-packed films avoids these problems until the envelope is openedfor processing. Thereafter, of course, the usual precautions must be observed.Another important precaution is to avoid drawing film rapidly from cartons, exposure holders, orcassettes. Such care will help materially to eliminate objectionable circular or treelike blackmarkings in the radiograph, the results of static electric discharges.The interleaving paper should be removed before the film is loaded between either lead orfluorescent screens. When using exposure holders in direct exposure techniques, however, thepaper should be left on the film for the added protection that it provides. At high voltage, directexposuretechniques, electrons emitted by the lead backing of the cassette or exposure holdermay reach the film through the intervening paper or felt and record an image of this material onthe film. This effect is avoided by the use of lead or fluorescent screens. In the radiography oflight metals, direct-exposure techniques are the rule, and the paper folder should be left oninterleaved film when loading it in the exposure holder.The ends of a length of roll film factory-packed in a paper sleeve should be sealed in thedarkroom with black pressure-sensitive tape. The tape should extend beyond the edges of thestrip 1/4 to 1/2 inch, to provide a positive light tight seal.Identifying RadiographsBecause of their high absorption, lead numbers or letters affixed to the subject furnish a simplemeans of identifying radiographs. They may also be used as reference marks to determine thelocation of discontinuities within the specimen. Such markers can be conveniently fastened to theobject with adhesive tape. A code can be devised to minimize the amount of lettering needed.Lead letters are commercially available in a variety of sizes and styles. The thickness of theletters chosen should be great enough so that their image is clearly visible on exposures with themost penetrating radiation routinely used. Under some circumstances it may be necessary to putRadiography in Modern Industry 98