Traditional radiography is used as an NDT technique in many industries. However, its effectiveness as an archival format is hindered by large storage space requirements, difficulty of access, and deterioration of film over time1. Digitization of existing analog X-Ray collections is a means of improving radiograph archiving. The objective of this paper is to examine the relationship between optical density (OD) and discrete code values (CV) for the purpose of evaluating performance of X-Ray digitization systems. Aim values for the EPRI test target are specified in OD units2, so in order to ascertain whether these aim values are being preserved in the digitization process, OD must be converted to CV equivalent for image analysis. In this study, the conversion was accomplished by tabulating aim OD values of the stepped density feature of the target against the CV measured from the same locations in digital images. The results indicate that although the relationship between OD and discrete CV is nonlinear on a global level, it does exhibit linearity on a local level. Thus, we can define linear transformations from OD to CV for various ranges of OD. This transformation allows the EPRI specifications provided in OD units to be used to specify benchmarks for digital image analysis of the same target.

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