NDTnet 1998 Aug, Vol.3 No.8

Abstract

Computed radiography (CR) with Luminescence Imaging Plates (IP) has become a routine method in medical applications. It is a new medium for filmless radiography. Since the last five years several tests were performed to check this method for industrial NDT [11. ASTM already issued a proposal for a standard. IP's are exposed similar to radiographic films. Colour centres formed during x-ray exposure can be detected directly by a LASER scanner without any development process. After read out the IP's can be erased and are reusable up to 1000 times.

The image quality achieved by the NDT-film system classes is the basis for the comparison and evaluation of the CR by IP's. The exposure process and the handling of the IP's is optimized with the goal to achieve the same or better image quality than film systems. Furthermore, exposed films will be digitized for comparison of film digitization with direct IP-scans. Similar factors influence the image quality due to optical scattering, limitations of data processing and monitor properties. Investigations are presented characterizing the contrast sensitivity, dynamic range and linearity, considering the spectral response of the IP-combinations with front screens of lead and steel. Signal-to-noise ratios depending on the radiation dose for different energies have been determined from the evaluation of step wedge images. By using thicker lead screens than for x-ray films the same specific contrast could be attained like for film. Longer exposure times than for medical applications yield S/N ratios comparable to good NDT-film system classes. These results depend from the scanner type and the IP-type. For the performed measurements systems from AGFA and Fuji-Film were tested.

Additionally, the obtained inherent unsharpness was measured depending on the energy. These measurements were based on the double wire IQI CERL B, a Siemens star and MTF calculations from a Pt edge. The values are presented in relation to the exposure time and the contrast sensitivity. The unsharpness of about 80 µm for the best systems may restrict the utilization for fine crack detection (welding), but the high dynamic may open new advantages in areas which require the inspection of large wall thickness ranges. Nevertheless the systems are suitable for weld inspection in the enlargement mode. The image quality is clearly better than attained by usual radioscopic systems which are applied for weld inspection already. The new standard proposal (CEN TC138 WG1 SWG "Radioscopy") for radioscopic weld inspection may open the way for new techniques of CR.From the obtained results first guidelines for CR with IP's are derived. The different properties of available systems enable their usage for different application areas. The main industrial sectors for radiography are wall thickness / corrosion measurements, casting and weld inspections. The obtained system parameters of IP-CR are evaluated for their applicability to these sectors. Corresponding applications on the basis of IP's are presented.

[1]U. Ewert, J. Stade, U. Zscherpel, M. Kaling; Proceedings of the 14th World Conference on NDT, pp. 1347-50, Dec. 8-13, 1996, New Dehli, India.

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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