As mentioned previously, as a result of beam spread and attenuation, echo heights observed from equivalent defects decrease with increased distance. Consequently, a technique known as distance amplitude correction (DAC) is commonly employed to adjust signals generated at different distances for comparison purposes. This technique consists of generating a DAC curve that essentially indicates that a smaller echo at a greater distance may have similar properties to a larger echo at a lesser distance.
With straight beam transducers, blocks with flat bottom-hole specimens typically are used to generate the DAC curve. However, generating the same curve with an angle transducer is typically completed using a specimen with side-drilled holes. Regardless of the technique used to generate a DAC curve, the material used in the calibration block should be the same as the material in the test specimen due to potential differences in attenuation characteristics.
Figures 15a and 15b conceptually illustrate how a DAC curve would be generated for an angle beam transducer. In figure 15a, a side-drilled hole is shown in a test block that can be scanned with four different scanning patterns. Note that one could double the number of points on the DAC curve by using a second equivalent side-drilled hole at a different depth. When the echo signals are plotted together, the DAC curve shown in figure 15b results. This curve is referred to as "100 percent DAC." This means that for an equivalent defect in the test specimen, the echo signal will fall on this line. Smaller or larger defects in the test specimen will lie below or above the 100 percent DAC curve, respectively. The most accurate way to assess these defects is to repeat the DAC curve generation with a series of diameter holes. The result will be a series of curves that should allow for more accurate defect assessment. figures..

Neutron radiography provides a very efficient tool for investigations in the field of non - destructive testing as well as for many applications in fundamental research. A neutron beam penetrating a specimen is attenuated by the sample material and detected by a two dimensional imaging device. The image contains information about material and structure inside the sample because neutrons are attenuated according to the basic law of radiation attenuation. Contrary to X – rays, neutrons are attenuated by some light materials, as i.e. hydrogen, boron and lithium but penetrate many heavy materials. Neutrons are able to distinguish between different isotopes and neutron radiography is an important tool for studies of radioactive materials.
At the Atominstitute der österreichischen Universitäten (ATI) neutron radiographic investigations are performed for more then 35 years. The detectors mainly used are converter / film assemblies. However, these detectors are limited regarding their sensitivity, dynamic range and linearity. Due to rapid development of detector and computer technology as well as deployments in the field of digital image processing, new technologies are nowadays available which have the potential to improve the performance of neutron radiographic investigations enormously. Therefore, the aim of this work is to identify and develop two and three dimensional digital image processing methods suitable for neutron radiographic and tomographic applications, and to implement and optimize them within data processing strategies.