NDTnet 1998 Aug, Vol.3 No.8

Abstract

Storage phosphor based luminescence imaging plates (IP) for computed radiography are a new medium for filmless radiography. This technology is based on the principle that phosphors can be developed in such a way that they will retain the latent image information until the moment that they are stimulated by laser light. The light emission is a function of the received dose. The information of the scanning laser beam and the by a photomultiplier collected amount of light is electronically processed and displayed on a monitor.

The special advantage of this technique is its higher sensitivity than classical film radiography and the potential for automated for automated image processing due to the digital read-out of the IIP's. Former investigations have shown [1] that the available W's achieve the same specific contrast like NDT films with an IP type and reader dependent inherent unsharpness. Corrosion and wall thickness inspections need usually an unsharpness of 100 - 200 µm. This is a typical value for medical scanners. NDT -IP's with low inherent structure noise, scanned with 100 µm pixel resolution, are the optimum medium for that application.

Computed radiography has been tested in comparison to film radiography under routine conditions in a petrochemical plant with Ir-192. The image quality is evaluated visually and by a comparison with digitised films.
The corrosion inspection and wall thickness measurement of pipes is performed in the classical way on the monitor. Automated tools and procedures for measurement, documentation and archiving are presented in comparison to the analysis of digitised films. The measurement of the wall thickness is based on the projection technique by correction with the known magnification factor. In the central area of pipes and armatures the corrosion depth is measured by analysis of the IP dose values. New algorithms are presented which calculate the wall thickness and corrosion depth profiles by curve fitting of simulated profiles with measured projection profiles. Other algorithms are discussed which estimate the wall thickness in projection direction by a complex geometrical transformation into the real wall thickness. Also a much simpler algorithm which is based on a local calibration provides reliable values. Here no geometrical information about the pipe beside its rotational symmetry is needed.Furthermore archiving and the integration of test reports with image presentation is enhanced.

[1] Ewert et al. , Materialprüfung 37 (1995) 11-12 pp. 474-478

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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