T.O. 33B-1-16-53degree of distortion. Alignment is particularly critical for crack detection. Since discontinuities revealed inradiographic images are usually identified by their shape, images free of distortion are very important in radiographicinterpretation. Where complex structures are encountered in aircraft inspection, it is often impossible to locate theparts in the most desirable position, and sometimes inspection is facilitated by planned distortions. Interpretation ofdistorted images is not impossible, but the film reader needs to visualize mentally the geometry of exposure andsubstitute visualization of the distorted image with the projection of the image by the radiation source. This abilityrequires practice and experience.6.7.2.5 ImageUnsharpness.Image unsharpness is the term usually applied recognizing the fact that there is always unsharpness to some degree andthat perfect image sharpness is unattainable. The amount of geometric image unsharpness is due to size of the sourceof radiation and relative distances as shown in Figure 6-23. The distance on the film over which an edge is spread isknown as the penumbral shadow or the geometrical unsharpness, Ug. The value of Ugdoes not enter into othercomputations; it sets the upper limit for Ft/d. The value must be determined experimentally. The equation todetermine unsharpness is:Figure 6-23. Possible Geometric Distortions.Ug= Ft/dwhere:F = maximum dimension of the focal spott = distance from the source side of the test object to the filmd = distance from the source to object6.7.2.5.1In considering geometrical unsharpness, recognize the value of new microfocus X-ray sources and the potential forgeometric magnification. Figure 6-24 is a nomogram to assist in solving this equation for various geometrical
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
