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Optical Density vs. Discrete Code Value Metrics for X-Ray Digitization Workflow Evaluation

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.

1. Frost and Sullivan, 2012, Industrial X-Ray Film Market: Will Emerging Economies Sustain Demand for Film Radiography?, 9840-30, Frost and Sullivan North America, 2. ASTM Standard E1936-03, 2003, Standard Reference Radiograph for Evaluating the Performance of Radiographic Digitization, ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/C0033-03, 3. GE Inspection Technologies, 2005, GE Film Digitizer FS50/FS50B Technical Specification Document, General Electric Company, 4. GE Inspection Technologies, 2007, Industrial Radiography: Image Forming Techniques, General Electric Company, 5. BAM Berlin, 2003, Evaluation of the LASER Scanner “Array 2905-HD”, Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany, 6. ISO Standard 12233:2000, Photography – Electronic still-picture cameras – Resolution
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