Radiography in Modern Industry - Kodak

covers for microradiographic film holders because their absorption for soft x-rays is likely to behigh.The source-film distance usually ranges from 3 to 12 inches. The choice of tube voltage or K-emitting element must be based on the character of the specimen and may be quite critical if thefinest radiographic results are to be obtained from a particular specimen. The type of film or platemust be selected to meet the degree of magnification to be used in examining the radiograph.It is of little value to specify exact exposure techniques in microradiography, but the followingexample may serve as a useful guide: Specimen, plate of aluminum-base alloy 1/4 mm thick;exposure, 20 kV, 25 mA, 12-inch source-film distance; time, 11/2 minutes using the slowestavailable industrial x-ray film.Because of the relatively high absorption by air of the very soft x-radiation used inmicroradiography, the x-ray intensity decreases with distance from the focal spot more rapidlythan calculations based on the inverse square law would indicate (See "Milliamperage-DistanceRelation").In conventional microradiography, the specimen and film must be in close contact because thesubsequent optical enlargement of the radiograph demands that geometric unsharpness (See"Calculation Of Geometric Unsharpness") be minimized. By the use of special x-ray tubes havingfocal spots about 1 micron (0.00004 inch) in diameter, the source-specimen distance may bemade very small and the specimen-film distance some tens of times greater. This results in ageometric enlargement in the original microradiograph that can often be viewed directly withoutany intervening optical or photographic steps.Enlargement of MicroradiographsIf a microradiograph must be enlarged more than 50 diameters, conventional photomicrographictechniques and equipment must be used. However, it is seldom necessary to enlarge industrialmicroradiographs more than 50 diameters, Such enlargements may be prepared by a techniqueknown as photomacrography. The term photomacrography refers to enlargements of about 50diameters or less, made with a single lens, as opposed to photomicrography, which impliesgreater enlargements using a compound microscope. Photomacrographic equipment is relativelyinexpensive, and is often readily available in plant photographic departments.Lenses and CamerasThe simplest method obtaining moderate enlargements of microradiographs, and one thatfrequently gives satisfactory results, is to use a conventional photographic enlarger. A microfilmenlarger embodying a "point source" of light and condenser illumination is preferred, but anyphotographic enlarger of good quality may be used. The microradiograph is placed in thenegative carrier of the enlarger and a sheet of photographic paper or film is placed on the easel.As in all photographic enlarging, it is important that the enlarger be rigid; if the negative carrier ofthe enlarger vibrates with respect to the easel during the exposure, the enlarged image will beblurred.For enlargements up to 50 diameters, best results are obtained if the specialized equipment--cameras, lenses, and specimen holders--available for photomacrography is employed. Severaloptical manufacturers make lenses specially for this purpose. View cameras or those speciallydesigned for enlarged-image photography are most convenient, although an enlarger with a filmadapter back can also be used.In addition, short focal-length camera or enlarger lenses can be employed if they are used"backward"--that is, with the front of the lens toward the film in the camera and the back of thelens toward the microradiograph being reproduced. The disadvantage of using these lenses isthat the aperture settings are inside the camera and can be changed only by removing the lensRadiography in Modern Industry 166