Optimizing Sharpness in Radiography
Obtaining the best quality image is essential for accurate results in X-ray radiography. One of the factors influencing sharpness in X-ray images is distance between X-ray source to part and part to film. This month's column identifies problems associated with current practices and suggests a simple solution which is practical and economical.
G.P. Singh
Associate Technical Editor
Problem
An important factor in the radiographic examination of material is the gap between the X-ray film and the area of the material being examined. In radiography performed with conventional X-ray tubes or isotopes, the correct relationship between source to part distance and part to film distance must be maintained; otherwise, geometric unsharpness (Ug) may increase to objectionable levels and prevent the radiograph from showing discontinuities present in the material being examined. Many inspection standards impose specific limits on geometric unsharpness.Geometrical unsharpness (Ug) of the radiograph is determined in accordance with the formula: Ug = Fd/D
F = Source size. The maximum projected dimension of the radiating source (or focal spot) in the plane perpendicular to the distance (D) from the weld or object being radiographed.
D = Distance from the source of radiation to the weld or object being radiographed.
d = Distance from the source side of the object being radiographed to the film.
Ideally, to assure the best possible image, the film should be in intimate contact with the area being examined. Correct film placement using conventional film holders is not always possible or economically practical. An example of this is manufactured gear assemblies where the web of the gear is electron beam welded to a small shoulder on a hollow gear shaft (Figure 1). Placing the part on a conventional film holder to radiograph the weld would position the weld too far from the film to allow proper imaging. Placing the film holder under the web of the gear and in contact with the weld is the acceptable solution. Using conventional film holders, ten or more exposures might be required to cover the circumference of the weld (Figure 2).
Correct film placement using conventional film holders is not always possible or economically practical.
Specially cut (contour cut) packaged films are available to match the radius of the parts to be examined, thus allowing a greater portion of the weld to be imaged in one exposure, may be purchased through equipment suppliers (Figure 3). These prepackaged films are ideal for production size lots of the same component, but are not always practical for small lots or urgently needed work.
Figure 1 - Typical web gear assembly.
Figure 2 - Web gear assembly side view
Figure 3 - Schematic of contoured radiographic film.
The simple solution for small jobs is a gasket cutter to contour cut your own film, and a simple, light-tight setup/exposure box with an opaque vinyl lid (Figure 4).
Figure 4 - Cut-away schematic of a custom setup/exposure box.A basic gasket cutter is similar to a beam compass, except that instead of having a tip for marking, it has a blade for cutting material to the radius to which it has been set. Gasket cutters are available through suppliers of engine overhaul tools and can be adapted to cut radii of various diameters.
The setup/exposure box can easily be made from available materials and should include some means for positioning parts and film. Conventional size prepackaged or bulk packaged film can be contour cut in the darkroom and loaded, with the part, the image quality indicator (IQI,) and the required film identification into the setup box. Lead screens, as required for intensification of primary radiation or scatter control, can be contour cut with the gasket cutter and used as needed. The cover is then placed on the box and the box taken to the X-ray area for exposure. After each exposure, the box is taken to the darkroom for film processing and additional setup until all exposures are completed on the part(s).
For large parts, the X-ray area itself could be converted into a darkroom and the contour cut film carried from X-ray area to film processing area in a light tight envelope or film manufacturer's packaging box.
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