What causes radiographic unsharpness?

Answer 1

Radiographic unsharpness, or geometric unsharpness (Ug) as it is widely known in the NDT (Non-destructive Testing) industry, can be measured and controlled. Ug is related to the geometry of the radiographic technique and simply put, is the amount of 'blur' present in a radiological image. The primary factors contributing to Ug in the radiographic technique are: 1. A excessively large focal spot (point from which the usable radiation beam emanates). In X-ray tubes, this is the area where high speed electrons are focused onto the target, resulting in the generation of photons. In gamma-ray sources, focus is the actual physical size of the radioactive material. Typical focus for an industrial X-ray tube (10 mA) may range from 4 to 7 mm. Gamma-ray source size is often about 1/8". 2. Excessive test specimen (object) to detector distance, as related to focal spot size (FSS). There are mini- and micro-focus X-ray systems available that can be used to magnify the image of the test specimen (up to 100x or more), while limiting Ug. Some of these micro-focus X-ray systems may have a focus of less than 10 microns. Gamma-ray sources are manufactured to have as high of a specific activity (radiation output per unit of mass) as possible, thus limiting physical size of the source material and resultant exposure time. Ug can be minimized by using as small a focal spot as possible or available, keeping the test specimen in contact with or as close to the detector as possible, or using a longer source to film distance (which will increase exposure time). These factors can be adjusted to suit the inspection application, or to achieve compliance with a specification. Also, any motion of the test specimen, detector or source that may occur during exposure will cause unsharpness, but this situation is not related to geometry. Industrial radiography specifications limit acceptable Ug from .04" down to .002", depending upon material thickness and service application (aerospace component vs. automotive part) of the test specimen. Ug for any radiographic technique can be easily calculated using the following formula: Ug=FSS x ODD/SOD FSS=Focal spot size ODD=Object to detector distance(source side of object to detector distance) SOD=Source to object distance