IQI-Sensitivity and Applications of Flat Panel Detectors and X-Ray Image Intensifiers A Comparison

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1 IQI-Sensitivity and Applications of Flat Panel Detectors and X-Ray Image Intensifiers A Comparison Dr. Matthias Purschke/ Ulf Reimer, Agfa NDT Pantak Seifert GmbH und Co. KG, Bogenstr. 4, 96 Ahrensburg, Germany (Tel.: , m.purschke@roentgenseifert.de) Abstract A new generation of x-ray sensitive detectors, amorphous silicon (a-si) digital imaging systems or so-called Flat Panel Detectors, is available and will enable further developments in the radioscopic practice and in system engineering. This paper deals with a comparison of the IQI sensitivity of two kinds of Flat Panel Detectors and the usual Image Intensifier. The Flat Panel Detectors under discussion are common detectors for industrial applications. Introduction The quality of a radioscopic examination depends as much on what technology has to offer in terms of detectors and system design as it does on the training level of the inspector, a fact, by the way, that is also true of all other NDT fields. Today, a new generation of detectors, amorphous silicon (a-si) digital imaging systems or so-called Flat Panel Detectors, will enable further developments in radioscopic practice and system engineering. The experience with Flat-Panel-Detectors has shown some advantages due to the dynamic range of up to 6 bit. The requirements on digital imaging systems and the suitability for radioscopic weld inspection will be discussed. Radioscopic System Set-Up and Applications The X-ray source illuminates the test object. The attenuated radiation is captured by the input screen of an X-ray sensitive detector. The geometric resolution (or rather the resulting unsharpness) in the image is set by the geometric magnification. The relation between Focus-to-Detector distance (FDD) and Focus-to-Object distance (FOD) determines the geometric magnification of the image. An image sensor transforms the attenuation image into a visible X-ray image. The set-up of a radioscopic system with an X-ray sensitive Image Intensifier is shown in figure. X-ray image FDD visible image FOD video signal digital signal Monitor image X-ray tube specimen image intensifier digital image processing / evaluation optics and camera Figure. Radioscopic System Set-up with Image Intensifier a 6 de Junho de / June to 6 Rio de Janeiro - RJ - Brasil

2 During the past few decades, radioscopy with Image Intensifiers has firmly established itself in a wide range of industrial applications that of necessity depend on instant evaluation and - in the case of continuous inspection - also on a high degree of automation. In fact, radioscopy with image Intensifiers, in conjunction with digital image processing and automatic image assessment, constitutes state of the art by now as far as the continuous inspection of castings is concerned //. Generally speaking, radioscopic systems are used in industrial applications for the continuous inspection of objects since these systems allow for flexible settings of the incident beam direction and the inspection perspective as well as for on-line viewing of the radioscopic image. The output signal of a flat panel detector is a digital signal. Due to this fact a suitable data acquisition unit is required. Compared to an Image Intensifier the advantage is that an additional camera is no longer necessary. Consequently, the digitisation of the video signal with the usual image degradation is not necessary, too. The set-up of a radioscopic system with a flat panel is shown in figure. attenuation image FDD digital signal monitor image FOD Data acquisition and digital image processing test sample flat panel detector image processing system X-ray tube Figure. Radioscopic System Set-up with Flat Panel monitor Specifications of Different Detectors So far, the usual radioscopic detector system has consisted of a combination of image intensifier and CCD camera. In Table, two a-si RID detectors with different sensitivities, namely AL- and AL-, and one a-si Pax Scan detector with two different operational modes, namely real-time and high-resolution, are compared with an image intensifier. Table clearly demonstrates the marked differences of the two detector types in terms of their technical properties. On account of its superb dynamic range and 6-bit grey value resolution, the RID detector is preferably suited for the inspection of light alloy castings. The Pax Scan detector when operated in the high-resolution mode, provides ideal conditions for the inspection of seam welds on account of its excellent spatial resolution. Furthermore, the Pax Scan detector can be operated in the real-time mode with / frames per second; provided, its resolution is reduced through pixel binning. With its dynamic range and -bit grey value resolution, this detector could even be considered an alternative to the image intensifier when the inspection of castings is concerned.

3 Specification Image Intensifier & CCD Camera RID AL RID AL Pax Scan HR ) Pax Scan RT ) Scintillator CsJ Lanex (CsJ) ) Lanex (CsJ) ) Lanex (CsJ) ) Lanex (CsJ) ) Input Screen Size (active) Diameter: 9" ( mm) x mm x mm 44 x 9 mm 44 x 9 mm Pixel Matrix - x x 9 x 6 9 x 6 Pixel Pitch - 4 µm 4 µm 7 µm 4 µm Spatial Resolution.8 -. Lp/mm,4 Lp/mm,4 Lp/mm,9 Lp/mm,9 Lp/mm Dynamic Range 4 4 A/D Conversion 8 bit 6 bit 6 bit bit bit Frame Rate / fps 7 fps 7 fps fps / fps Weight 6 kg 8 kg 8 kg 8 kg 8 kg ) High-Resolution Mode, ) Real-Time Mode, ) optional Table. Specifications The next few paragraphs outline how image quality indicators (IQIs), in accordance with European Standard EN 46-part (Wire Penetrameters), have been used to investigate the performance capability of the different detectors by comparison with the image intensifier. IQI Sensitivity IQI sensitivity 4,,,, IQI Sensitivity Aluminium mm Al RID AL (4 ms) RID AL ( ms) Vistalux Image Intensifier Vistalux II & digital enhance. Figure. Comparison Image Intensifier / RID Detector

4 Figure shows that the RID detector reaches the IQI sensitivity of an image intensifier with digital enhancement techniques almost independently of the read-out rate. This fact confirms, in principle, the detector s qualification to the radioscopic test of light alloy castings. Figure 4 shows the IQI sensitivity of the Pax Scan detector in the real-time and highresolution modes. In the real-time mode, the detector does not reach the IQI sensitivity of the image intensifier with digital enhancement techniques. In the high-resolution mode, however, the detector achieves a better IQI sensitivity than the image intensifier with digital enhancement techniques. IQI sensitivity measurements for ferrous materials have revealed that this high-resolution mode implies great advantages in seam weld inspection (Figure ). IQI Sensitivity Aluminium Pax Scan (RT) IQI-sensitivity (%),,, Pax Scan (HR) Vistalux Image Intensifier Vistalux II & digital enhance. mm Al Figure 4. Comparison Image Intensifier / Pax Scan Detector (Al) IQI Sensitivity Fe IQI sensitivity (%) 6 4 Vistalux Image Intensifier Vistalux II & digital enhance. Pax Scan (RT) Pax Scan (HR) mm Fe Figure. Comparison Image Intensifier / Pax Scan Detector (Fe) Yet it has to be pointed out that those IQI sensitivity measurements do not allow any conclusions as to the actual flaw detectability. The practical relevance of the IQI sensitivity has been demonstrated in further measurements.

5 Practical Relevance Figure 6 shows that the RID detector offers marked advantages in an actual inspection task that tackles light alloy castings with a range from to mm. The prefiltering required of the image intensifier results in an IQI sensitivity that is clearly lower than that documented in the preceding measurements. Prefiltering is needed for the RID detector as well, but only to a moderate degree in order to prevent saturation. The IQI sensitivity, per se, is still far superior on account of the detector s considerable dynamic range. In practical terms this means that the RID detector offers distinct advantages by comparison with the image intensifier, especially in the field of fully automated (unattended) inspection of castings. Of course, the aforesaid applies all the same to visual inspection. However, it should be pointed out that a typical computer display will handle 8 bits of grey scale information at the most. For visual radioscopic examinations this implies an approach completely different from that with an image intensifier, a fact that has to be taken into account. IQI sensitivity in practice (step wedge -mm Al) IQI sensitivity (%) 4, 4,,,, VISTALUX II & digital enhance. RID AL (4 ms) Al (mm) Figure 6. IQI sensitivity in practice (aluminium step wedge mm) When a Pax Scan detector is operated in the high-resolution mode to inspect welds of a mm nominal (single-wall technique) such inspection proves that the standard specifications of e.g. EN 444, EN 46, EN 4 and EN 68 can be met without any problem with W4 and platinum duplex wire 9 D (see Table ) //. A geometrical magnification of about.4 is required for the Pax Scan detector to meet the requirements of European standard EN 68 Pt. Radioscopic testing. The Pax Scan detector s high resolution mode allows the inspection of a weld section of 6 mm. The examination by means of an image intensifier with digital enhancement techniques would require not only higher geometrical magnification by a factor of but as well as zooming of the input screen. Furthermore digital image enhancement techniques has to be applied. The weld section which can be inspected is 64 mm. Thus the Pax Scan detector s high-resolution mode allows the inspection of a substantially longer weld section. The resulting economic advantages are obvious; longer seam weld sections are inspected in less time.

6 Inspection Class SA SB Inspection Class System Class SC SC System Class penetrameter wire no. duplex no. wire no. duplex no. penetrameter / mm / mm,-, W7 D W9 D -,,-, W6 D W8 D,-,,-, W 9D W7 D,-4,,-7, W4 8D W6 D 4,-6, 7,- W 7D W 9D 6,-8, - W 7D W4 9D 8,- - W 7D W 9D - - W 7D W 9D - -4 W9 6D W 9D - 4- W8 6D W 9D -4-8 W7 4D W9 9D 4-6 Table. Radioscopic System performance for metallic materials (EN 68 Pt. ) Conclusions In radioscopic inspections, modern a-si detectors represent an alternative to the usual image intensifiers. This statement applies to the inspection of both welds and castings. Practical experience and the measurement results presented here have shown that it is imperative to always choose the detector best suited for a certain application task. By comparison with the image intensifier that still constitutes a very flexible multi-purpose detector, the use of the most suitable a-si detector for a specific inspection task will often yield better results in terms of the IQI sensitivity. But then an a-si detector is a much higher priced item than an image intensifier, a fact to be considered when choosing a specific application approach. References // H.Schulenburg, M.Purschke: Advances in the Automatic Evaluation of Radioscopic Images, Symposium CT&IP, Berlin, March 999 // M.Purschke: Radioscopy New Perspectives Gained through European Standardization, 7th ECNDT, Copenhagen, 6-9 May 998