Radiography in Modern Industry - Kodak

Figure 33: The sharpness of the radiographic image depends on the contact between theintensifying screens and the film. In the radiographs of a wire mesh test object, the one(left) shows the fuzzy image produced by poor film-screen contact; the other (right), madewith good contact, resulted in a sharp image.As a rule, the mounting of screens is done by the x-ray dealer, who is equipped to provide thisservice in accordance with the manufacturer's recommendations. If the screens are mounted bythe purchaser, care must be exercised to avoid physical unevenness that would result from anythick or uneven binding material. The adhesive must not cause discoloration of the screens--evena small degree of discoloration will reduce their effective speed--nor can the adhesive be such asto cause fogging of the film.Care of ScreensFluorescent light from intensifying screens obeys all the laws of light and cannot pass throughopaque bodies as do x-rays. To prevent extraneous shadows caused by absorption of thefluorescent light by foreign matter during exposure, dust and dirt particles must not be allowed tocollect between the film and screen surfaces, and stains on the screens must be avoided.Cleanliness of the order desirable for handling film and screens is sometimes difficult to maintain,but much can be done by stressing its need and eliminating carelessness.Whenever it can he avoided, fluorescent intensifying screens should not be exposed to the fullintensity of the primary beam when making a radiograph. In extreme cases, in which very highintensity primary x-radiation falls directly on the screen-film combination, the screens maybecome discolored or an afterglow, which will show up on subsequent radiographs, may beproduced. If the specimen more than covers the screen area or if proper marking is provided,there is no difficulty from this source.As a matter of routine, all cassettes should he tested periodically to check the contact betweenthe screens and the film. This can be done easily by mounting a piece of wire screening (any sizemesh from 1/16 inch to 1/4 inch is satisfactory) so that it lies fairly flat. The cassette is thenloaded with a film, the wire screening is placed on the exposure side of the cassette, and a flashexposure is made. If there are areas of poor contact, the result will be as shown in the figureabove, left. If there is proper contact, the shadow of the wire mesh will be sharply outlined (SeeFigure 33, right).Close-up viewing of wire-mesh contact tests can be a very fatiguing visual task. A better andmore comfortable way is for the observer to stand about 15 feet from the illuminator and look forthe dark areas of the pattern, which are indicative of poor contact. Viewing is even easier and thedark areas show up more definitely if the radiograph is viewed at an angle of about 45 degrees,or if a thin sheet of light-diffusing material is put over the radiograph. Wearers of glasses oftenfind it advantageous to remove them while viewing tests.Radiography in Modern Industry 47