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Complementarity of a photon-counting system and radioscopy for inspection of cast aluminium components
NDT&E International 38 (2005) 239–250 www.elsevier.com/locate/ndteint
Complementarity of a photon-counting system and radioscopy for inspection of cast aluminium components Emmanuelle Cendrea,*, Vale´rie Kaftandjianb, Gwenae¨le Lecomteb, Kristian Kjaera b
a RISØ National Laboratory, Materials Research Department, P.O. Box 49, 4000 Roskilde, Denmark INSA-CNDRI Laboratory for Non Destructive Testing by Ionising Radiations, Bat. St Exupe´ry, 20 av. A. Einstein, 69621 Villeurbanne, France
Received 24 September 2003; revised 8 July 2004; accepted 26 July 2004
Abstract This study aims at establishing X-ray methods for inspection of cast aluminium components by combining two approaches, a radioscopic inspection, and a photon-counting system. Indeed, radioscopy is widely used in castings inspection for automatic defect detection and is an efficient method for characterising rather small thicknesses, typically less than 40 mm. However, cast components often show a high range of thicknesses and imaging the whole range is difficult. We propose to use photon-counting measurements to complement the radioscopic image for high thicknesses. Photon-counting is not an imaging tool, but the thickness sensitivity obtained from photon-counting measurements (number of X-ray photons transmitted through the object) is much better than what can be obtained by classical radioscopy for up to 60 mm of aluminium. For high thicknesses of aluminium, within small volumes where defects are believed to be critical, the photon-counting system allows getting information additional to that obtained from classical radioscopy images. q 2004 Elsevier Ltd. All rights reserved. Keywords: Radioscopy; Photon-counting; Thickness sensitivity; Aluminium casting inspection; X-ray characterisation
1. Introduction In the field of X-ray inspection, radiography is the usual reference in terms of image quality. However, in a number of industrial application areas [1], radioscopic devices are now preferred over film radiography [2]. Castings inspection is a typical field where a real-time imaging system is of great interest, especially because the optimal projection can be chosen while moving the object in front of the detector. Another interest is obviously the possibility of using pattern recognition methods for automatic defect detection and characterisation, which is reported by a number of recent communications [3–7]. However, these studies do not mention the difficulty of optimising the acquisition conditions in such a way to get an image of sufficient quality. In particular, cast components usually feature a complex shape with varying thicknesses, making it difficult to inspect. At present, thicknesses over * Corresponding author. Tel.: C45-46775822; fax: C45-46775758. E-mail address: [email protected] (E. Cendre). 0963-8695/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2004.07.008
40 mm are difficult to inspect by radioscopy and defect detection performance is low in this range. In this paper, we investigate the potential of a photoncounting system for inspecting high thicknesses of cast components, and we compare the sensitivity performance obtained with that of a classical radioscopic system using an image intensifier. This study is carried out in the frame of the EU-funded project QUME1. The overall objective of the QUME project is to implement an inspection technology to enable fast correction for manufacturing irregularities, causing structural imperfections, by integrating quantitative non-destructive techniques and intelligent data processing, to the manufacturing control. Since the photon-counting system is not an imaging tool, only a limited number of measurements are carried out, 1
QUME project: On-line process and quality optimisation for the manufacturing of cast metallic parts. European project G1RD-2000-00444. Partners: InnospeXion ApS (DK), Carl Bro A/S (DK), RISOE (DK), University Liverpool (UK), Monition Ltd (UK), ISQ (PT), Stampal Spa (IT), IFG (DE), INSA-CNDRI (FR), CSIRO (AU).
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within small volumes where defects are believed to be critical, and where a quantitative assessment of structural integrity is of high importance. Due to the sensitivity expected for the measurements, the photon-counting system requires a rigorous calibration, which is detailed in Section 2. In the same section, the sensitivity obtained on artificial defects show the potential of the method. In Section 3, the results obtained by radioscopy on the same artificial defects illustrate the difficulties of defect detection for large thicknesses. The first results obtained on a cast component with real defects are shown in Section 4 before concluding.
aperture. The slit system, composed of four tungsten plates (2 mm thick), is adjustable in size and stepper motors control the position of each plate (Fig. 1). Based on a Euro Card Bus (ECB), the ECB real-time computer, developed at Risø, generates steps for steppermotors, monitors limit switches and performs other real-time tasks. The ECB computer is supervised via a GPIB interface by a linux host computer. The acquisition software (running on the host computer) is the command-driven experiment control program TASCOM2 developed at Risø. 2.2. Calibration of the measurement chain
2. Photon-counting system 2.1. Experimental set-up The photon-counting system consists of an X-ray tube (Andrex 140 kVp, source size: 1.5!1.5 mm2), appropriate lead and tungsten shielding and collimators, a photoncounting detector, a multi-channel analyser (MCA) and computers. The counting detector is a NaI(Tl) Crismatec scintillation detector (crystal size 5 cm in diameter, 1 cm thick, with a 0.5 mm thick Al window) mounted on a Canberra 2007B preamplifier. The pre-amplified pulses from the NaI are fed into an amplifier (Canberra type 2012) whose output goes both to a single channel analyser (SCA), detecting the total counts without energy discrimination, and a MCA. The MCA was set to an ADC gain of 1k, a range of 2k, and the counts were read into an array of 1200 channels of 0.15 keV each. The photon-counting chain requires counting rates of less than 2!104 counts per second (cps) to avoid pulse pile-up. Appropriate count rates are obtained by using a sourcedetector distance of about 1 m and a detector aperture of 1 mm diameter. The sample, mounted on a three-axis translation and oneaxis rotation table, is 10 mm from the detector aperture, giving magnifications MZ1.01–1.07 for defects inside a 60 mm thick sample. The detector aperture consists of a hole of 1 mm in diameter drilled in a 4 mm thick tungsten circular plate, embedded in a lead plate (Fig. 1). The resolution at the level of the object is determined by the total unsharpness Wtotal related to the source size WS and the detector aperture WD as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðWS ðM K 1ÞÞ2 C WD2 =M Wtotal Z (1) Since MOw1, the detector aperture WD dominates. To limit the background signal due to air scattering, tight shielding and collimation is employed. The solid angle illuminated by the source is limited first by a slit system with an opening adjusted to 3!3 mm2, positioned between the X-ray tube and a 600!600 mm2 lead shield with a 10 mm
The photon-counting method is potentially very sensitive to small thickness or density variations, by detecting the small changes they would cause in the intensity of a fine pencil beam. Therefore, characterising the stability of the whole photon-counting chain, i.e. the detector intensity and energy stability and the X-ray tube stability, is essential for determining the capability of the system. 2.2.1. Detector efficiency and energy stability The stability of the efficiency of the detection system (detector intensity stability) and the stability of the energy calibration of the detection system (detector energy stability) were checked using a radioactive isotope of Co-57. Spectra of Co-57 were collected over 60 h using a counting time of 12,000 s. Fig. 2 shows an example of a Co-57 spectrum. The detector intensity stability is found to be very good, with a variation of intensity of less than G0.5% over 60 h (Fig. 3a). The peak position of the Co-57 isotope shifts slightly in energy, apparently systematically with time (Fig. 3b). If the photons were discriminated in energy, which is not the case in our study, an energy calibration would have to be repeated regularly. 2.2.2. Inherent energy resolution of the NaI(Tl) detector Unlike for example germanium detectors [8], scintillation detectors are low-resolution devices. In the case of our study, photons were counted without energy discrimination. The expectation from the counting system lies in its high intensity efficiency compared to the radioscopic system. The energy resolution of the NaI(Tl) detector was however evaluated as an indication. The 59.6 and 122.1 keV lines of respectively, Am-241 and Co-57 isotopes were used to determine an energy resolution of the detector of 13.2 keV (FWHM) at 60 keV and of 19.6 keV at 122.1 keV. This performance corresponds well to that reported for scintillating devices [9]. 2.2.3. X-ray source stability The stability of the X-ray tube (Andrex 140 kV) was evaluated over 18 h through 60 mm of aluminium at 91 kV 2
http://www.risoe.dk/afm/external/pska/Tascom_manuals.htm.
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Fig. 1. Schema of the photon-counting experimental set-up.
and 1 mA. Fig. 4 shows that the X-ray tube is rather unstable with intensity (number of photons) variations greater than 18% over time. The stability of the X-ray tube is essential for the photon-counting measurements. Indeed, the photon-counting technique in our study is expected to have a better sensitivity in thickness than radioscopy, in the order of a few percent, in order to be able to inspect high thicknesses of aluminium. The sensitivity in thickness is directly related to the detection of variations of intensity of the X-ray beam, due to thickness (or density) variations, assuming the incident X-ray intensity to be stable. In order to compensate the instability of the incident beam, the fluctuations of the X-ray tube were monitored and normalised out, by performing a reference measurement on known aluminium thickness, in-between each measurement performed. Moreover, the counting measurements, including reference measurements were repeated several times in order to assess the dispersion of the results.
has occurred. To guard against this, a relative tolerance level of tZ1% was chosen such that if the two reference counts ref Iiref and IiC1 differ by more than the tolerance t, the point is discarded. The 1% value corresponds to a variation of approximately four times the relative photonic noise. The criterion for discarding measurement points is represented by the following equation ref IiC1 K Iiref sðIÞ 4 Z pffiffi R t z4 ref ref I ðIiC1 C Ii Þ=2 I
(2)
with s(I) the photonic noise for an intensity I Due to time considerations for the inspection control and photon statistics, the total number of photons transmitted through the object and counted without discrimination in energy was used. Indeed, selecting an energy window might increase the sensitivity compared to using the whole energy spectrum, but reduces the photon statistics and increases the photon noise at the same time. Fig. 5 (left-side plots) shows the line scans crossing the three artificial defects. The reference counts, all at the same X position, cluster in the left side of the plots. The artificial
2.3. Results obtained on artificial defects 2.3.1. Acquisition conditions and data handling Artificial defects were used to assess the sensitivity in thickness of the photon-counting system. They consist of three holes of 1.5, 1.0 and 0.5 mm in diameter, drilled in a 2 mm thick aluminium plate stacked on a 60 mm thick aluminium block. Line-scans were carried out across the artificial defects, with measurement points every 0.25 mm. A counting time of 30 s and an X-ray tube high-voltage of 90 kV were used, and the measurements were repeated several times. During the scans, between each single counting measurement, a reference measurement of 30 s was done at a reference position. The intensity (number of photons) of the counting measurement is normalised by the average intensity of the two reference intensities measured before and after. The resulting normalised intensity may still be invalid if, at some undetermined time in-between the reference measurements, a large variation of the X-ray incident flux
Fig. 2. Co-57 spectrum measured with the photon-counting system: Co-57 peak at 122.06 keV; FWHM: full width at half maximum; MIDP: mid-point of the Co57 peak (one channel is equivalent to 0.15 keV).
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Fig. 3. Stability of the detection system assessed from Co-57 peaks measured over 60 h: (a) Intensity stability (total number of counts), (b) Energy stability (peak position: ‘MIDP’).
defects all correspond to a reduction of 2 mm in aluminium thickness (from 62 to 60 mm) and have diameters of 1.5, 1.0 and 0.5 mm. They appear quite clearly as peaks of higher intensity. However, variations of the incident X-ray flux, as witnessed by the reference counts, are sometimes as large as the intensity variations caused by the defects. The points to be discarded according to the criterion (2) are marked by vertical dotted lines. Fig. 5 (right-side plots) shows the resulting normalised intensity, with the discarded points indicated by vertical dotted lines. The peaks appear more clearly after normalisation, on a rather constant background. 2.3.2. Peak parameters and influence of defect size From the normalised profiles, several parameters were extracted from the peaks corresponding to the artificial defects: peak height (difference between maximal intensity and background level), full width at half maximum (FWHM) and area. The values obtained are shown in
Fig. 6, as well as the background intensity under the peak. Repeated scans (nine-plus) were performed in order to evaluate the dispersion of the results. The FWHM and peak height appear to be the most reproducible parameters. Although the defects have the same depth (2 mm) and thus, for a sufficiently narrow X-ray beam, should yield the same change in transmitted photon flux, the defect diameter actually influences the peak height. Indeed the measurement probes an area of 1 mm diameter, defined by the detector aperture (cf. discussion in Section 2.1 of the resolution for MOw1), and this area is greater than or equal to the defect diameter for two of the defects. The peak height varies little when the defect diameter is bigger or equal to the pencil beam. However, for the 0.5 mm-diameter defect, which has an area four times smaller than the probing beam, the peak height is significantly smaller. The choice of the diameter of the detector aperture is obviously a compromise between a sufficient number of photons arriving on the detector, and the spatial resolution of the measurement.
Fig. 4. Stability of the X-ray tube (Andrex 140 kV): 91 kV, 1 mA, through 60 mm of aluminium.
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Fig. 5. Line-scans across artificial defects (2 mm deep holes through 62 mm of aluminium with diameters of 1.5, 1.0 and 0.5 mm): X-ray intensity versus X translation. Left-side plots: raw data; right-side plots: normalised intensity. Dotted vertical lines mark discarded points (tolerance 1%).
Taking Mw1, the expected line shape of the peaks corresponding to the artificial defects was calculated according to Ref. [10] as the convolution of a 1 mm circle (detector aperture) with circles (artificial defects) of 1.5, 1.0 and 0.5 mm diameters, and calculated FWHM were extracted. The observed and calculated FWHM values are respectively of 1.38 and 1.38 mm for the 1.5 mm-diameter defect, 0.88 and 0.81 mm for the 1.0 mm-diameter defect, and 0.86 and 0.96 mm for the 0.5 mm-diameter defect. The observed FWHM value is in rather good correspondence with
the calculated values. Note that, unlike for the convolution of squares, in the case of circles the FWHM is not simply equal to the larger aperture. The peak area is a parameter involving both the contrast of the defect, and its size. This parameter may be interesting for particular defects, i.e. scattered porosities. Indeed, in case of scattered porosities, the peak would not be high in intensity, but usually this type of defect is extended on a rather big zone so that the peak area would increase and become a sensitive parameter. We should recall, however, that under real production inspection circumstances
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Fig. 6. Full width at half maximum (FWHM), peak height and area of the peaks corresponding to the artificial defects and background after normalisation of the X-ray intensity. The background average and standard deviation (SD) have been calculated for each scan for the points measured before and after the peaks corresponding to the defect, on 62 mm of aluminium. Each artificial defect was characterised by repeated scans, each represented by one point.
the photon-counting system will not to be used in a scanning mode, due to lack of time. 2.3.3. Sensitivity in thickness The thickness sensitivity was determined from the measurements of peak heights of the 1.5 mm-diameter artificial defect, corresponding to a 2 mm thickness variation (from 62 to 60 mm) (see Fig. 6). Indeed, the 1.5 mm-diameter defect is wider than the 1 mm probing
beam, so the spatial resolution will not affect the determination of sensitivity in thickness. The 2 mm thickness variation corresponds to a variation in normalised intensity DInormZInorm_defectKInorm_backgroundZ0.083. For each energy of the spectrum, the number of transmitted photons follows the attenuation law, i.e. an exponential variation of the thickness, related to the attenuation coefficient of the material. Here, the intensity level measured corresponds to the sum of all the transmitted
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Fig. 7. Simulation of the number of transmitted photons over the whole energy spectrum (for a X-ray voltage of 100 kV), for a range of aluminium thicknesses of 60–64 mm. The error bars correspond to three times the standard deviation in number of counts.
photons over the whole energy spectrum. On a limited range of thickness, it is possible to assume a linear variation of the transmitted intensity with the crossed thickness. This is visible on the Fig. 7, where the number of photons was simulated for an energy spectrum corresponding to a 100 kV high voltage. Setting the limit of detection to a peak height equal to three times the noise level (standard deviation (SD) of the background intensity level), i.e. to DI norm_limitZ 3SDbackgroundZ0.014, we can conclude that a thickness variation deZ0.3 mm can be reliably detected. This gives a sensitivity in thickness de/eZ0.5% for an aluminium thickness eZ62 mm. It should be noticed that this value is obtained from the mean value of the repeated scans. For on-line measurements, assessing the dispersion of the results, including other possible noise sources, would be necessary.
i.e. real-time radioscopic systems without image integration. Compared to the photon-counting system, the signal delivered here is proportional to the total energy absorbed by the scintillating layer of the detector (CsI). This means that the number of photons absorbed by the CsI, each multiplied by its energy, constitutes the signal. Only a part of the photons transmitted by the object is absorbed by the scintillator, which is a very thin layer. In a first step, images of three aluminium step-wedges 2–20, 22–40 and 42–60 mm with steps of 2 mm were acquired with different X-ray tube high-voltages. The acquisition was done separately for each step-wedge in order to determine the best high-voltage for each range of thickness. In a second step, the sensitivity in thickness of the radioscopic system has been determined in a similar way as for the photon-counting system using the same artificial defects and the optimum acquisition conditions determined previously.
3. Comparison with radioscopy 3.1. Acquisition conditions The radioscopy system involves the same X-ray tube (Andrex 140 kV), an X-ray image intensifier and a manipulation unit with a three-axis translation and one-axis rotation table. The image intensifier is coupled to a CCD camera (Thomson radiological unit TH59464 6 00 dual-field). The useful entrance field size is 116!87 mm2 and the limiting resolution 30 lp/cm. Images are standard video format 768!576 pixels, 8 bits. The acquisition time for an image is 40 ms. A recursive filter, i.e. real-time averaging of 16 consecutive frames, is applied. The distance sourcedetector was set to 1 m, and the distance sample-detector was on average (various thicknesses imaged) 30 mm, resulting in magnifications MZ1.002–1.064. According to the standard EN 13068-3, the radioscopic system described above belongs to the system class SC3,
The choice of the acquisition conditions lies in a compromise between a low value of the high-voltage, in order to obtain a high contrast between the different thicknesses, and a sufficient intensity transmitted through the aluminium thickness in order to obtain a good signal to noise ratio (the limitations in current of the X-ray tube have to be compensated by an increase in the high-voltage). The range of thickness 2–20 mm was inspected with high-voltages of 50, 60 and 70 kV, and the ranges of thickness 22–40 and 42–60 mm were inspected, respectively, with 60, 70 and 80 kV and 70, 80 and 90 kV. Fig. 8 shows the mean grey-level value (GLV) and SD in grey-level (error bars) measured for each step of the stepwedges as a function of the aluminium thickness.
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Fig. 8. Mean grey-level value determined on aluminium step-wedges as a function of thickness for different high-voltage conditions. The error bars correspond to the standard deviation in pixel GLV measured for each thickness of the step-wedge. The image acquisition has been done independently for the ranges of thickness 2–20, 20–40 and 40–60 mm.
For each range of thickness, the current of the X-ray tube was adapted in such a way to prevent saturating the lowest thickness of the range. Thus, for the different high voltages and thickness ranges, the first point has a similar GLV y200. Each curve indicates both the evolution of signal (i.e. mean GLV measured on a wide zone) with the aluminium thickness, and the noise or uncertainty of the measurement (SD of the grey-levels indicated by the error bars). The signal difference between two consecutive steps indicates their contrast, which is the information needed to detect a potential defect. However, in the frame of an automatic detection, based on the statistical variation of the grey-levels, the noise level is important for differentiating between two thicknesses. In order to choose the optimal acquisition conditions, we selected the contrast to noise ratio (CNR) as an indicator of
a statistical presence of a defect: the CNR is defined by the difference between the mean grey-levels of two consecutive steps, divided by the mean quadratic noise of the two steps. This parameter is both related to the contrast itself, which is determined by the energy of the incident X-rays, and the signal to noise ratio, which is determined by the number of photons absorbed by the scintillator, multiplied by their energy. Fig. 9 shows the evolution of CNR for the smallest range of thicknesses, where the signal measurement is most relevant. The 50 kV–1.5 mA conditions appear to be best adapted to the smallest thicknesses (!6 mm), while the 70 kV–0.4 mA exhibits better values for the range 10–20 mm. As a general remark, we can observe that when a component with variable thicknesses is inspected, the same image quality cannot be obtained for the whole range of thicknesses. This is a well-known difficulty in X-ray inspection, which is even more crucial in film
Fig. 9. Evolution of the contrast to noise ratio (CNR) for a 2 mm step in thickness, depending on the acquisition conditions, for the 2–20 mm range of thicknesses.
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Fig. 10. (a) Radioscopic image of artificial defects (2 mm deep holes) through 17 mm of aluminium. The defects with a diameter of 1.5 and 1.0 mm are clearly visible (white dots). The black arrow indicates the position of the 0.5 mm diameter defect, which can hardly be distinguished from the noise. (b) Line profiles on radioscopic images across the 2 mm-deep and 1.5 mm-diameter artificial defect through different thicknesses of aluminium: 17, 42 and 62 mm.
radiography where several films are often needed for the inspection of one sample. Here, the detector performance is highly responsible for this image quality, and the values presented shows that the image intensifier used is well adapted to the inspection of small thicknesses, at low voltages. The highest thickness range (40–60 mm) presents a very bad signal evolution (Fig. 8), as several consecutive steps have the same GLV. Clearly, no information can be extracted from such a signal variation.
3.2. Results on artificial defects The same artificial defects as detailed in Section 2 were used to assess the sensitivity in thickness of the radioscopic system. The three holes (0.5, 1.0 and 1.5 mm of diameters) drilled in a 2 mm thick aluminium plate, were superimposed successively on a 15-, 40-, and 60 mm-thick plate. In that way, the sensitivity of the radioscopic system was measured for different ranges of thickness. The acquisition conditions were chosen from Section 3.1 study, for each thickness (15, 40 and 60 mm). A background correction procedure was applied beforehand to compensate for irregularities due to uneven lighting and non-uniform detector response. Fig. 10a shows the image obtained after background correction, where the artificial defects are superimposed on 15 mm of aluminium. The two biggest defects are clearly visible. The 0.5 mm-diameter defect is visible but can hardly be distinguished from the noise. None of the artificial defects are visible on the images through 42 and 60 mm of aluminium.
3.3. Sensitivity in thickness compared for radioscopy and photon-counting system To assess the thickness sensitivity in a similar way as for the counting system, line profiles were plot on the radioscopic images across the 1.5 mm-diameter defect in order to determine the signal variation related to the 2 mm thickness variation due to the defect (see Fig. 10b). The peak corresponding to the defect is clearly visible through 17 mm aluminium, can hardly be seen through 42 mm of aluminium and there is no detectable peak through 62 mm of aluminium. To arrive at a quantitative comparison between sensitivities of counting and radioscopic systems, the same criterion was applied as that used for the counting system, i.e. setting the limit of detection of a peak to a peak height equal to three times the noise level and from that, deriving the sensitivity in thickness. Here, the peak height corresponds to a grey-level variation between defect and background. The background and noise levels were measured on the line profiles respectively as the mean GLV and SD in GLVs before and after the peak corresponding to the defect. The sensitivity in thickness determined from line profiles for the radioscopic system is shown Table 1. It is also possible to determine the sensitivity in thickness by computing the smallest detectable thickness from the curves obtained previously on step wedges. The advantage is that the measurements are statistically better because performed on an area. Results, as shown in Table 2, are not better for 18 mm of aluminium but similar, and significantly better for the 40 mm thickness. Nevertheless, it is still far from the value obtained by the counting system, and moreover, no value could be obtained on 60 mm.
Table 1 Determination of sensitivity in thickness for radioscopic system, using a 2 mm-deep and 1.5 mm-diameter defect through different thicknesses of aluminium Al thickness e (mm)
Background (mean GLV)
Noise (GLV)
Peak height (GLV)
Minimum peak height detectable (GLV)
de (mm)
de/e (%)
17 42 62
104.9 130.9 144.8
1.52 1.77 2.38
8.3 3.3 !
4.6 5.3 7.1
1.1 3.2 !
6.5 7.7 !
GLV, grey-level value; peak height, corresponding to a thickness variation of 2 mm; de, minimum detectable variation in thickness; de/e, sensitivity in thickness; !, no value as no peak could be detected for the defect through 62 mm of aluminium.
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Table 2 Comparison between minimum detectable variation in thickness de and sensitivity in thickness de/e of photon-counting and radioscopic systems for a given thickness e
Radioscopy Radioscopy Radioscopy Photon-counting system
e (mm)
de (mm)
de/e (%)
18 40 60 62
1.4 1.7 ! 0.3
7.8 4.3 ! 0.5
The values obtained for the radioscopic system were measured on the stepwedges curves (optimum value). e, aluminium thickness; de, minimum detectable variation in thickness; de/e, sensitivity in thickness; !, no value was relevant for exploitation in the curves.
Comparing the radioscopic and photon-counting system in terms of acquisition time and detector resolution explains partly the better results obtained with the photon-counting system. There is a factor f1Z47 between the acquisition time of the photon-counting and radioscopic systems with respectively 30 s and 640 ms (40 ms per frame and recursive filter of 16 frames) and a factor f2Z28 between the integration area of the photon-counting and radioscopic systems with respectively a pinhole of 1 mm in diameter and a pixel size of 166 mm. This means that the signal to noise ratio in the photon-counting system should be improved by pffiffiffiffiffiffiffiffiffi a factor f1 $f2 ; i.e. 36. In first approximation, an improvement of the sensitivity in thickness of the same factor could be expected from the photon-counting system. Table 2 shows that the sensitivity in thickness is greatly improved, although by a less important factor. This result was expected for high thicknesses. Indeed a sensitivity in thickness of about 5% is typical for real-time radioscopic systems without image integration (system class SC3 according to standard EN 13068). However, the bad sensitivity obtained by radioscopy in the low thickness range is very disappointing, and very far from the performance of radiography. Another study [11] was conducted with the new generation of digital detectors (amorphous silicon flat panels), having a pixel size of 0.127 mm and a digital dynamic range of 14 bits. The sensitivity was measured using wire-type image quality
indicators (IQI), which is another method to determine the thickness sensitivity, and a value of 1.1% was obtained on 30 mm of aluminium. This value is much closer to radiography performance. However, high thickness inspection was also not possible with this type of detector.
4. Inspection of cast aluminium component with real defects A cast aluminium component (Fig. 11a) was inspected using both the radioscopic and photon-counting systems. The cast component had been previously carefully checked using classical radiography. Radiography showed several gas pores in a 15 mm-thick part and a gas pore of 2–3 mm in diameter in the 60 mm-thick part of the sample. After X-ray characterisation, the sample was cut through the thicker part (Fig. 11b) and Fig. 11c shows an optical image of the porosity. On the Fig. 11b, it appears that the gas pore is very close to the boundary of the sample, which has a difficult shape. Indeed, the thicker part of the component is not exactly cylindrical, the outer surface consisting of two cones. Thus the pore is placed in a zone where the total crossed thickness varies steeply with position. Gas pores were detected by radioscopy in the 15 mmthick part (Fig. 12), but not in the 60 mm-thick part. The 60 mm-thick part was scanned using the photon-counting system in the area of the gas pore. The scanning area was 6 mm!12 mm (area underlined in black in Fig. 11a) with 0.5 mm steps. The data were treated as described before for the artificial defects. The area scan was performed three times, ensuring that even after discarding points due to glitches in the X-ray tube, there would be a valid observation of each point and the normalised intensity of the valid points was averaged. Fig. 13a shows the data of the photon-counting scan. The axes are defined in Fig. 13b, which is a zoom of the zone underlined in Fig. 11a. The two-dimensional scan illustrated in Fig. 13a represents the lines of same intensity in the sample. The ‘C’ sign means that the value was discarded because it was
Fig. 11. (a) Photograph of a cast aluminium component: the white square indicates the 15 mm-thick part with gas pores detected by radioscopy, the black rectangle indicates the area scanned with the photon-counting system and containing a few-mm diameter gas pore in the nearly cylindrical 60 mm thick part; (b) After non-destructive examination, the cast component was cut in the area of the gas pore and polished; (c) Optical microscopy picture of the gas pore.
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Fig. 12. Radioscopic image of the cast aluminium component: the gas pores in the 15 mm-thick part appear as clearer spots and are indicated by the arrows.
too high, due to the fact that the point was outside the sample (left-side point), or in a thin zone (right-side points). Assuming a sample of homogeneous density and chemical composition, such a scan reveals the lines of same crossed thickness. Lines very close together indicate a steep gradient of thickness, such as altitude changes in a geographical map. This is the case at the left-side boundary, where the intensity decreases steeply along the sx direction, as the beam enters the component. The round shaped lines in the central part of the sample correspond to a zone where the thickness is rather stable with low intensity changes, as witnessed by the fact that the lines are far from each other. In the right-side part of the scan, closely spaced lines occur again, due a steep
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change of thickness when the probing beam exits the component. However, in the area with coordinates (SX, SZ)Z(113G0.5 mm, 99.9G1.0 mm), underlined by the black rectangle in Fig. 13a, the lines are curved in a concave shape compared to the real shape of the sample, indicating the area of porosity. If we compare the measured line level to what it should be if the lines followed the sample shape, this area corresponds to a local difference of normalised intensity higher by DInormZ0.08, equivalent to a difference of thickness of about 2 mm, referring to the same calibration as for the artificial defects. This is the signature of the porosity, which after cutting of the specimen (Fig. 11), was found to be located precisely at this position and has a size of 2–3 mm. However, the porosity is placed in a zone of the sample where the thickness varies rapidly with position, and therefore it does not yield a peak of intensity in a uniform background, such as the artificial defect. Thus, in the case of a real sample like this cast component, it is crucial to know the real thickness expected for the sample at each position of the counting scan. If the CAD file of the sample is available, it is possible to calculate the map of the counting scan in order to get a reference. Otherwise, a reference sample without defect is needed. This experiment on a real sample shows that the presence of a defect might not always appear as a peak in intensity, but also in the shape of the intensity lines. This yields another possibility for an automatic defect detection procedure, either based on peak detection or on shape
Fig. 13. (a) Averaged normalised intensity versus SX and SZ. Contour levels up to InormZ2.1. Only valid points were allowed to contribute to the average. There was at least one valid observation for each point. The ‘C’ indicate invalid points outside of the cast component, where the X-ray intensity was higher than 2!104 cps. The rectangle underlines an area of additional intensity which may correspond to a gas pore; (b) The rectangle on the photograph represents the scan area of 6 mm!12 mm in the cast aluminium component.
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recognition, although it might be difficult and restricted to a particular series of samples.
5. Conclusion In this study, a photon-counting system combined with radioscopy was evaluated with regard to inspection of cast aluminium components. Photon-counting appears as a method extremely sensitive to thickness variations. This method can be used in particular for high thickness areas where the performance of radioscopy is rather low. However, due to the inspection time, it must be limited to particular zones in the component, known to be critical. As photon-counting is not an imaging tool and as any variation of thickness is detected (within the sensitivity limit), one must be able to differentiate between a geometrical external thickness variation and internal defects submitted to acceptance criteria (like porosities and microshrinkages). CAD file of the sample could be a solution, or a reference sample without defect. Finally, it must be noticed that the sensitivity values were obtained assuming that the SD of the photon flux was the only noise source, and after normalisation of the intensity. In order for this method to be applied as an industrial system, the inspection time must be carefully determined taking into account the time needed for carrying out reference measurements. Moreover, the production dispersion of the measurements must be determined on the industrial inspection machine in order to assess other noise sources. Although the radioscopic system is not well adapted to high thicknesses, defects are detected in thin parts of the cast component. The image intensifier did not show very good sensitivities, but better values are now obtained with amorphous silicon flat panels. However, radioscopy remains poorly adapted to the inspection of high thicknesses, making the combination with the photon-counting system very promising for global inspection of cast aluminium components.
Acknowledgements This work was performed as part of the project Qume G1RD-2000-00444, funded by the European Community. The authors would like to gratefully acknowledge Prof.-Dr Dierk Hartmann (IfG, GbmH) for supplying the aluminium cast components, Dr Jørgen Rheinla¨nder (Innospexion, Denmark) for the loan of the X-ray tube and the entire Consortium for their interest in the project.
References [1] Roye W. Nondestructive inspection methods in the automotive industry. In: Proceedings of the eighth ECNDT, Barcelona, 17–21 June 2002. [2] Kazantsev IG, Lemahieu I, Salov GI, Denys R. Statistical detection of defects in radiographic images in non-destructive testing. Signal Process 2002;82:791–801. [3] Mery D, Filbert D. Classification of potential defects in the automatic inspection of aluminium castings using statistical pattern recognition. In: Proceedings of the eighth ECNDT, Barcelona, 17–21 June 2002. [4] Theis G. Fully automatic X-ray inspection of aluminium wheels. In: Proceedings of the eighth ECNDT, Barcelona, 17–21 June 2002. [5] Herold F, Bavendiek K, Grigat R-R. A new analysis and classification method for automatic defect recognition in X-ray images of castings. In: Proceedings of the eighth ECNDT, Barcelona, 17–21 June 2002. [6] Vincent AG, Rebuffel V, Guillemaud R, Gerfault L. Defect detection in industrial casting components using digital X-ray radiography. In: Proceedings of the eighth ECNDT, Barcelona, 17–21 June 2002. [7] Kaftandjian V, Zhu YM, Peix G, Babot D. Automatic recognition of defects inside aluminium ingots by X-ray imaging. INSIGHT 1996; 38(9):618–25. [8] Matscheko G, Ribberfors R. A Compton scattering spectrometer for determining X-ray photon energy spectra. Phys Med Biol 1987;32(5): 577–94. [9] Knoll G. Radiation detection and measurement, 3rd ed. New York: Wiley; 2000. p. 802. [10] Braun MA, Pajares C. Implications of color-string percolation on multiplicities, correlations, and the transverse momentum. Eur Phys J C 2000;16:349–59. [11] Kaftandjian V, Duvauchelle P. Comparaison d’un de´tecteur au silicium amorphe avec le film radiographique: application en fonderie. In: Proceedings of the Journe´e de la Radiologie Industrielle JRI 0 2002, Lyon, 21–22 November 2002. p. 3.32–3.43.
Complementarity of a photon-counting system and radioscopy for inspection of cast aluminium components Emmanuelle Cendrea,*, Vale´rie Kaftandjianb, Gwenae¨le Lecomteb, Kristian Kjaera b
a RISØ National Laboratory, Materials Research Department, P.O. Box 49, 4000 Roskilde, Denmark INSA-CNDRI Laboratory for Non Destructive Testing by Ionising Radiations, Bat. St Exupe´ry, 20 av. A. Einstein, 69621 Villeurbanne, France
Received 24 September 2003; revised 8 July 2004; accepted 26 July 2004
Abstract This study aims at establishing X-ray methods for inspection of cast aluminium components by combining two approaches, a radioscopic inspection, and a photon-counting system. Indeed, radioscopy is widely used in castings inspection for automatic defect detection and is an efficient method for characterising rather small thicknesses, typically less than 40 mm. However, cast components often show a high range of thicknesses and imaging the whole range is difficult. We propose to use photon-counting measurements to complement the radioscopic image for high thicknesses. Photon-counting is not an imaging tool, but the thickness sensitivity obtained from photon-counting measurements (number of X-ray photons transmitted through the object) is much better than what can be obtained by classical radioscopy for up to 60 mm of aluminium. For high thicknesses of aluminium, within small volumes where defects are believed to be critical, the photon-counting system allows getting information additional to that obtained from classical radioscopy images. q 2004 Elsevier Ltd. All rights reserved. Keywords: Radioscopy; Photon-counting; Thickness sensitivity; Aluminium casting inspection; X-ray characterisation
1. Introduction In the field of X-ray inspection, radiography is the usual reference in terms of image quality. However, in a number of industrial application areas [1], radioscopic devices are now preferred over film radiography [2]. Castings inspection is a typical field where a real-time imaging system is of great interest, especially because the optimal projection can be chosen while moving the object in front of the detector. Another interest is obviously the possibility of using pattern recognition methods for automatic defect detection and characterisation, which is reported by a number of recent communications [3–7]. However, these studies do not mention the difficulty of optimising the acquisition conditions in such a way to get an image of sufficient quality. In particular, cast components usually feature a complex shape with varying thicknesses, making it difficult to inspect. At present, thicknesses over * Corresponding author. Tel.: C45-46775822; fax: C45-46775758. E-mail address: [email protected] (E. Cendre). 0963-8695/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2004.07.008
40 mm are difficult to inspect by radioscopy and defect detection performance is low in this range. In this paper, we investigate the potential of a photoncounting system for inspecting high thicknesses of cast components, and we compare the sensitivity performance obtained with that of a classical radioscopic system using an image intensifier. This study is carried out in the frame of the EU-funded project QUME1. The overall objective of the QUME project is to implement an inspection technology to enable fast correction for manufacturing irregularities, causing structural imperfections, by integrating quantitative non-destructive techniques and intelligent data processing, to the manufacturing control. Since the photon-counting system is not an imaging tool, only a limited number of measurements are carried out, 1
QUME project: On-line process and quality optimisation for the manufacturing of cast metallic parts. European project G1RD-2000-00444. Partners: InnospeXion ApS (DK), Carl Bro A/S (DK), RISOE (DK), University Liverpool (UK), Monition Ltd (UK), ISQ (PT), Stampal Spa (IT), IFG (DE), INSA-CNDRI (FR), CSIRO (AU).
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within small volumes where defects are believed to be critical, and where a quantitative assessment of structural integrity is of high importance. Due to the sensitivity expected for the measurements, the photon-counting system requires a rigorous calibration, which is detailed in Section 2. In the same section, the sensitivity obtained on artificial defects show the potential of the method. In Section 3, the results obtained by radioscopy on the same artificial defects illustrate the difficulties of defect detection for large thicknesses. The first results obtained on a cast component with real defects are shown in Section 4 before concluding.
aperture. The slit system, composed of four tungsten plates (2 mm thick), is adjustable in size and stepper motors control the position of each plate (Fig. 1). Based on a Euro Card Bus (ECB), the ECB real-time computer, developed at Risø, generates steps for steppermotors, monitors limit switches and performs other real-time tasks. The ECB computer is supervised via a GPIB interface by a linux host computer. The acquisition software (running on the host computer) is the command-driven experiment control program TASCOM2 developed at Risø. 2.2. Calibration of the measurement chain
2. Photon-counting system 2.1. Experimental set-up The photon-counting system consists of an X-ray tube (Andrex 140 kVp, source size: 1.5!1.5 mm2), appropriate lead and tungsten shielding and collimators, a photoncounting detector, a multi-channel analyser (MCA) and computers. The counting detector is a NaI(Tl) Crismatec scintillation detector (crystal size 5 cm in diameter, 1 cm thick, with a 0.5 mm thick Al window) mounted on a Canberra 2007B preamplifier. The pre-amplified pulses from the NaI are fed into an amplifier (Canberra type 2012) whose output goes both to a single channel analyser (SCA), detecting the total counts without energy discrimination, and a MCA. The MCA was set to an ADC gain of 1k, a range of 2k, and the counts were read into an array of 1200 channels of 0.15 keV each. The photon-counting chain requires counting rates of less than 2!104 counts per second (cps) to avoid pulse pile-up. Appropriate count rates are obtained by using a sourcedetector distance of about 1 m and a detector aperture of 1 mm diameter. The sample, mounted on a three-axis translation and oneaxis rotation table, is 10 mm from the detector aperture, giving magnifications MZ1.01–1.07 for defects inside a 60 mm thick sample. The detector aperture consists of a hole of 1 mm in diameter drilled in a 4 mm thick tungsten circular plate, embedded in a lead plate (Fig. 1). The resolution at the level of the object is determined by the total unsharpness Wtotal related to the source size WS and the detector aperture WD as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðWS ðM K 1ÞÞ2 C WD2 =M Wtotal Z (1) Since MOw1, the detector aperture WD dominates. To limit the background signal due to air scattering, tight shielding and collimation is employed. The solid angle illuminated by the source is limited first by a slit system with an opening adjusted to 3!3 mm2, positioned between the X-ray tube and a 600!600 mm2 lead shield with a 10 mm
The photon-counting method is potentially very sensitive to small thickness or density variations, by detecting the small changes they would cause in the intensity of a fine pencil beam. Therefore, characterising the stability of the whole photon-counting chain, i.e. the detector intensity and energy stability and the X-ray tube stability, is essential for determining the capability of the system. 2.2.1. Detector efficiency and energy stability The stability of the efficiency of the detection system (detector intensity stability) and the stability of the energy calibration of the detection system (detector energy stability) were checked using a radioactive isotope of Co-57. Spectra of Co-57 were collected over 60 h using a counting time of 12,000 s. Fig. 2 shows an example of a Co-57 spectrum. The detector intensity stability is found to be very good, with a variation of intensity of less than G0.5% over 60 h (Fig. 3a). The peak position of the Co-57 isotope shifts slightly in energy, apparently systematically with time (Fig. 3b). If the photons were discriminated in energy, which is not the case in our study, an energy calibration would have to be repeated regularly. 2.2.2. Inherent energy resolution of the NaI(Tl) detector Unlike for example germanium detectors [8], scintillation detectors are low-resolution devices. In the case of our study, photons were counted without energy discrimination. The expectation from the counting system lies in its high intensity efficiency compared to the radioscopic system. The energy resolution of the NaI(Tl) detector was however evaluated as an indication. The 59.6 and 122.1 keV lines of respectively, Am-241 and Co-57 isotopes were used to determine an energy resolution of the detector of 13.2 keV (FWHM) at 60 keV and of 19.6 keV at 122.1 keV. This performance corresponds well to that reported for scintillating devices [9]. 2.2.3. X-ray source stability The stability of the X-ray tube (Andrex 140 kV) was evaluated over 18 h through 60 mm of aluminium at 91 kV 2
http://www.risoe.dk/afm/external/pska/Tascom_manuals.htm.
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Fig. 1. Schema of the photon-counting experimental set-up.
and 1 mA. Fig. 4 shows that the X-ray tube is rather unstable with intensity (number of photons) variations greater than 18% over time. The stability of the X-ray tube is essential for the photon-counting measurements. Indeed, the photon-counting technique in our study is expected to have a better sensitivity in thickness than radioscopy, in the order of a few percent, in order to be able to inspect high thicknesses of aluminium. The sensitivity in thickness is directly related to the detection of variations of intensity of the X-ray beam, due to thickness (or density) variations, assuming the incident X-ray intensity to be stable. In order to compensate the instability of the incident beam, the fluctuations of the X-ray tube were monitored and normalised out, by performing a reference measurement on known aluminium thickness, in-between each measurement performed. Moreover, the counting measurements, including reference measurements were repeated several times in order to assess the dispersion of the results.
has occurred. To guard against this, a relative tolerance level of tZ1% was chosen such that if the two reference counts ref Iiref and IiC1 differ by more than the tolerance t, the point is discarded. The 1% value corresponds to a variation of approximately four times the relative photonic noise. The criterion for discarding measurement points is represented by the following equation ref IiC1 K Iiref sðIÞ 4 Z pffiffi R t z4 ref ref I ðIiC1 C Ii Þ=2 I
(2)
with s(I) the photonic noise for an intensity I Due to time considerations for the inspection control and photon statistics, the total number of photons transmitted through the object and counted without discrimination in energy was used. Indeed, selecting an energy window might increase the sensitivity compared to using the whole energy spectrum, but reduces the photon statistics and increases the photon noise at the same time. Fig. 5 (left-side plots) shows the line scans crossing the three artificial defects. The reference counts, all at the same X position, cluster in the left side of the plots. The artificial
2.3. Results obtained on artificial defects 2.3.1. Acquisition conditions and data handling Artificial defects were used to assess the sensitivity in thickness of the photon-counting system. They consist of three holes of 1.5, 1.0 and 0.5 mm in diameter, drilled in a 2 mm thick aluminium plate stacked on a 60 mm thick aluminium block. Line-scans were carried out across the artificial defects, with measurement points every 0.25 mm. A counting time of 30 s and an X-ray tube high-voltage of 90 kV were used, and the measurements were repeated several times. During the scans, between each single counting measurement, a reference measurement of 30 s was done at a reference position. The intensity (number of photons) of the counting measurement is normalised by the average intensity of the two reference intensities measured before and after. The resulting normalised intensity may still be invalid if, at some undetermined time in-between the reference measurements, a large variation of the X-ray incident flux
Fig. 2. Co-57 spectrum measured with the photon-counting system: Co-57 peak at 122.06 keV; FWHM: full width at half maximum; MIDP: mid-point of the Co57 peak (one channel is equivalent to 0.15 keV).
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Fig. 3. Stability of the detection system assessed from Co-57 peaks measured over 60 h: (a) Intensity stability (total number of counts), (b) Energy stability (peak position: ‘MIDP’).
defects all correspond to a reduction of 2 mm in aluminium thickness (from 62 to 60 mm) and have diameters of 1.5, 1.0 and 0.5 mm. They appear quite clearly as peaks of higher intensity. However, variations of the incident X-ray flux, as witnessed by the reference counts, are sometimes as large as the intensity variations caused by the defects. The points to be discarded according to the criterion (2) are marked by vertical dotted lines. Fig. 5 (right-side plots) shows the resulting normalised intensity, with the discarded points indicated by vertical dotted lines. The peaks appear more clearly after normalisation, on a rather constant background. 2.3.2. Peak parameters and influence of defect size From the normalised profiles, several parameters were extracted from the peaks corresponding to the artificial defects: peak height (difference between maximal intensity and background level), full width at half maximum (FWHM) and area. The values obtained are shown in
Fig. 6, as well as the background intensity under the peak. Repeated scans (nine-plus) were performed in order to evaluate the dispersion of the results. The FWHM and peak height appear to be the most reproducible parameters. Although the defects have the same depth (2 mm) and thus, for a sufficiently narrow X-ray beam, should yield the same change in transmitted photon flux, the defect diameter actually influences the peak height. Indeed the measurement probes an area of 1 mm diameter, defined by the detector aperture (cf. discussion in Section 2.1 of the resolution for MOw1), and this area is greater than or equal to the defect diameter for two of the defects. The peak height varies little when the defect diameter is bigger or equal to the pencil beam. However, for the 0.5 mm-diameter defect, which has an area four times smaller than the probing beam, the peak height is significantly smaller. The choice of the diameter of the detector aperture is obviously a compromise between a sufficient number of photons arriving on the detector, and the spatial resolution of the measurement.
Fig. 4. Stability of the X-ray tube (Andrex 140 kV): 91 kV, 1 mA, through 60 mm of aluminium.
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Fig. 5. Line-scans across artificial defects (2 mm deep holes through 62 mm of aluminium with diameters of 1.5, 1.0 and 0.5 mm): X-ray intensity versus X translation. Left-side plots: raw data; right-side plots: normalised intensity. Dotted vertical lines mark discarded points (tolerance 1%).
Taking Mw1, the expected line shape of the peaks corresponding to the artificial defects was calculated according to Ref. [10] as the convolution of a 1 mm circle (detector aperture) with circles (artificial defects) of 1.5, 1.0 and 0.5 mm diameters, and calculated FWHM were extracted. The observed and calculated FWHM values are respectively of 1.38 and 1.38 mm for the 1.5 mm-diameter defect, 0.88 and 0.81 mm for the 1.0 mm-diameter defect, and 0.86 and 0.96 mm for the 0.5 mm-diameter defect. The observed FWHM value is in rather good correspondence with
the calculated values. Note that, unlike for the convolution of squares, in the case of circles the FWHM is not simply equal to the larger aperture. The peak area is a parameter involving both the contrast of the defect, and its size. This parameter may be interesting for particular defects, i.e. scattered porosities. Indeed, in case of scattered porosities, the peak would not be high in intensity, but usually this type of defect is extended on a rather big zone so that the peak area would increase and become a sensitive parameter. We should recall, however, that under real production inspection circumstances
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Fig. 6. Full width at half maximum (FWHM), peak height and area of the peaks corresponding to the artificial defects and background after normalisation of the X-ray intensity. The background average and standard deviation (SD) have been calculated for each scan for the points measured before and after the peaks corresponding to the defect, on 62 mm of aluminium. Each artificial defect was characterised by repeated scans, each represented by one point.
the photon-counting system will not to be used in a scanning mode, due to lack of time. 2.3.3. Sensitivity in thickness The thickness sensitivity was determined from the measurements of peak heights of the 1.5 mm-diameter artificial defect, corresponding to a 2 mm thickness variation (from 62 to 60 mm) (see Fig. 6). Indeed, the 1.5 mm-diameter defect is wider than the 1 mm probing
beam, so the spatial resolution will not affect the determination of sensitivity in thickness. The 2 mm thickness variation corresponds to a variation in normalised intensity DInormZInorm_defectKInorm_backgroundZ0.083. For each energy of the spectrum, the number of transmitted photons follows the attenuation law, i.e. an exponential variation of the thickness, related to the attenuation coefficient of the material. Here, the intensity level measured corresponds to the sum of all the transmitted
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Fig. 7. Simulation of the number of transmitted photons over the whole energy spectrum (for a X-ray voltage of 100 kV), for a range of aluminium thicknesses of 60–64 mm. The error bars correspond to three times the standard deviation in number of counts.
photons over the whole energy spectrum. On a limited range of thickness, it is possible to assume a linear variation of the transmitted intensity with the crossed thickness. This is visible on the Fig. 7, where the number of photons was simulated for an energy spectrum corresponding to a 100 kV high voltage. Setting the limit of detection to a peak height equal to three times the noise level (standard deviation (SD) of the background intensity level), i.e. to DI norm_limitZ 3SDbackgroundZ0.014, we can conclude that a thickness variation deZ0.3 mm can be reliably detected. This gives a sensitivity in thickness de/eZ0.5% for an aluminium thickness eZ62 mm. It should be noticed that this value is obtained from the mean value of the repeated scans. For on-line measurements, assessing the dispersion of the results, including other possible noise sources, would be necessary.
i.e. real-time radioscopic systems without image integration. Compared to the photon-counting system, the signal delivered here is proportional to the total energy absorbed by the scintillating layer of the detector (CsI). This means that the number of photons absorbed by the CsI, each multiplied by its energy, constitutes the signal. Only a part of the photons transmitted by the object is absorbed by the scintillator, which is a very thin layer. In a first step, images of three aluminium step-wedges 2–20, 22–40 and 42–60 mm with steps of 2 mm were acquired with different X-ray tube high-voltages. The acquisition was done separately for each step-wedge in order to determine the best high-voltage for each range of thickness. In a second step, the sensitivity in thickness of the radioscopic system has been determined in a similar way as for the photon-counting system using the same artificial defects and the optimum acquisition conditions determined previously.
3. Comparison with radioscopy 3.1. Acquisition conditions The radioscopy system involves the same X-ray tube (Andrex 140 kV), an X-ray image intensifier and a manipulation unit with a three-axis translation and one-axis rotation table. The image intensifier is coupled to a CCD camera (Thomson radiological unit TH59464 6 00 dual-field). The useful entrance field size is 116!87 mm2 and the limiting resolution 30 lp/cm. Images are standard video format 768!576 pixels, 8 bits. The acquisition time for an image is 40 ms. A recursive filter, i.e. real-time averaging of 16 consecutive frames, is applied. The distance sourcedetector was set to 1 m, and the distance sample-detector was on average (various thicknesses imaged) 30 mm, resulting in magnifications MZ1.002–1.064. According to the standard EN 13068-3, the radioscopic system described above belongs to the system class SC3,
The choice of the acquisition conditions lies in a compromise between a low value of the high-voltage, in order to obtain a high contrast between the different thicknesses, and a sufficient intensity transmitted through the aluminium thickness in order to obtain a good signal to noise ratio (the limitations in current of the X-ray tube have to be compensated by an increase in the high-voltage). The range of thickness 2–20 mm was inspected with high-voltages of 50, 60 and 70 kV, and the ranges of thickness 22–40 and 42–60 mm were inspected, respectively, with 60, 70 and 80 kV and 70, 80 and 90 kV. Fig. 8 shows the mean grey-level value (GLV) and SD in grey-level (error bars) measured for each step of the stepwedges as a function of the aluminium thickness.
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Fig. 8. Mean grey-level value determined on aluminium step-wedges as a function of thickness for different high-voltage conditions. The error bars correspond to the standard deviation in pixel GLV measured for each thickness of the step-wedge. The image acquisition has been done independently for the ranges of thickness 2–20, 20–40 and 40–60 mm.
For each range of thickness, the current of the X-ray tube was adapted in such a way to prevent saturating the lowest thickness of the range. Thus, for the different high voltages and thickness ranges, the first point has a similar GLV y200. Each curve indicates both the evolution of signal (i.e. mean GLV measured on a wide zone) with the aluminium thickness, and the noise or uncertainty of the measurement (SD of the grey-levels indicated by the error bars). The signal difference between two consecutive steps indicates their contrast, which is the information needed to detect a potential defect. However, in the frame of an automatic detection, based on the statistical variation of the grey-levels, the noise level is important for differentiating between two thicknesses. In order to choose the optimal acquisition conditions, we selected the contrast to noise ratio (CNR) as an indicator of
a statistical presence of a defect: the CNR is defined by the difference between the mean grey-levels of two consecutive steps, divided by the mean quadratic noise of the two steps. This parameter is both related to the contrast itself, which is determined by the energy of the incident X-rays, and the signal to noise ratio, which is determined by the number of photons absorbed by the scintillator, multiplied by their energy. Fig. 9 shows the evolution of CNR for the smallest range of thicknesses, where the signal measurement is most relevant. The 50 kV–1.5 mA conditions appear to be best adapted to the smallest thicknesses (!6 mm), while the 70 kV–0.4 mA exhibits better values for the range 10–20 mm. As a general remark, we can observe that when a component with variable thicknesses is inspected, the same image quality cannot be obtained for the whole range of thicknesses. This is a well-known difficulty in X-ray inspection, which is even more crucial in film
Fig. 9. Evolution of the contrast to noise ratio (CNR) for a 2 mm step in thickness, depending on the acquisition conditions, for the 2–20 mm range of thicknesses.
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Fig. 10. (a) Radioscopic image of artificial defects (2 mm deep holes) through 17 mm of aluminium. The defects with a diameter of 1.5 and 1.0 mm are clearly visible (white dots). The black arrow indicates the position of the 0.5 mm diameter defect, which can hardly be distinguished from the noise. (b) Line profiles on radioscopic images across the 2 mm-deep and 1.5 mm-diameter artificial defect through different thicknesses of aluminium: 17, 42 and 62 mm.
radiography where several films are often needed for the inspection of one sample. Here, the detector performance is highly responsible for this image quality, and the values presented shows that the image intensifier used is well adapted to the inspection of small thicknesses, at low voltages. The highest thickness range (40–60 mm) presents a very bad signal evolution (Fig. 8), as several consecutive steps have the same GLV. Clearly, no information can be extracted from such a signal variation.
3.2. Results on artificial defects The same artificial defects as detailed in Section 2 were used to assess the sensitivity in thickness of the radioscopic system. The three holes (0.5, 1.0 and 1.5 mm of diameters) drilled in a 2 mm thick aluminium plate, were superimposed successively on a 15-, 40-, and 60 mm-thick plate. In that way, the sensitivity of the radioscopic system was measured for different ranges of thickness. The acquisition conditions were chosen from Section 3.1 study, for each thickness (15, 40 and 60 mm). A background correction procedure was applied beforehand to compensate for irregularities due to uneven lighting and non-uniform detector response. Fig. 10a shows the image obtained after background correction, where the artificial defects are superimposed on 15 mm of aluminium. The two biggest defects are clearly visible. The 0.5 mm-diameter defect is visible but can hardly be distinguished from the noise. None of the artificial defects are visible on the images through 42 and 60 mm of aluminium.
3.3. Sensitivity in thickness compared for radioscopy and photon-counting system To assess the thickness sensitivity in a similar way as for the counting system, line profiles were plot on the radioscopic images across the 1.5 mm-diameter defect in order to determine the signal variation related to the 2 mm thickness variation due to the defect (see Fig. 10b). The peak corresponding to the defect is clearly visible through 17 mm aluminium, can hardly be seen through 42 mm of aluminium and there is no detectable peak through 62 mm of aluminium. To arrive at a quantitative comparison between sensitivities of counting and radioscopic systems, the same criterion was applied as that used for the counting system, i.e. setting the limit of detection of a peak to a peak height equal to three times the noise level and from that, deriving the sensitivity in thickness. Here, the peak height corresponds to a grey-level variation between defect and background. The background and noise levels were measured on the line profiles respectively as the mean GLV and SD in GLVs before and after the peak corresponding to the defect. The sensitivity in thickness determined from line profiles for the radioscopic system is shown Table 1. It is also possible to determine the sensitivity in thickness by computing the smallest detectable thickness from the curves obtained previously on step wedges. The advantage is that the measurements are statistically better because performed on an area. Results, as shown in Table 2, are not better for 18 mm of aluminium but similar, and significantly better for the 40 mm thickness. Nevertheless, it is still far from the value obtained by the counting system, and moreover, no value could be obtained on 60 mm.
Table 1 Determination of sensitivity in thickness for radioscopic system, using a 2 mm-deep and 1.5 mm-diameter defect through different thicknesses of aluminium Al thickness e (mm)
Background (mean GLV)
Noise (GLV)
Peak height (GLV)
Minimum peak height detectable (GLV)
de (mm)
de/e (%)
17 42 62
104.9 130.9 144.8
1.52 1.77 2.38
8.3 3.3 !
4.6 5.3 7.1
1.1 3.2 !
6.5 7.7 !
GLV, grey-level value; peak height, corresponding to a thickness variation of 2 mm; de, minimum detectable variation in thickness; de/e, sensitivity in thickness; !, no value as no peak could be detected for the defect through 62 mm of aluminium.
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Table 2 Comparison between minimum detectable variation in thickness de and sensitivity in thickness de/e of photon-counting and radioscopic systems for a given thickness e
Radioscopy Radioscopy Radioscopy Photon-counting system
e (mm)
de (mm)
de/e (%)
18 40 60 62
1.4 1.7 ! 0.3
7.8 4.3 ! 0.5
The values obtained for the radioscopic system were measured on the stepwedges curves (optimum value). e, aluminium thickness; de, minimum detectable variation in thickness; de/e, sensitivity in thickness; !, no value was relevant for exploitation in the curves.
Comparing the radioscopic and photon-counting system in terms of acquisition time and detector resolution explains partly the better results obtained with the photon-counting system. There is a factor f1Z47 between the acquisition time of the photon-counting and radioscopic systems with respectively 30 s and 640 ms (40 ms per frame and recursive filter of 16 frames) and a factor f2Z28 between the integration area of the photon-counting and radioscopic systems with respectively a pinhole of 1 mm in diameter and a pixel size of 166 mm. This means that the signal to noise ratio in the photon-counting system should be improved by pffiffiffiffiffiffiffiffiffi a factor f1 $f2 ; i.e. 36. In first approximation, an improvement of the sensitivity in thickness of the same factor could be expected from the photon-counting system. Table 2 shows that the sensitivity in thickness is greatly improved, although by a less important factor. This result was expected for high thicknesses. Indeed a sensitivity in thickness of about 5% is typical for real-time radioscopic systems without image integration (system class SC3 according to standard EN 13068). However, the bad sensitivity obtained by radioscopy in the low thickness range is very disappointing, and very far from the performance of radiography. Another study [11] was conducted with the new generation of digital detectors (amorphous silicon flat panels), having a pixel size of 0.127 mm and a digital dynamic range of 14 bits. The sensitivity was measured using wire-type image quality
indicators (IQI), which is another method to determine the thickness sensitivity, and a value of 1.1% was obtained on 30 mm of aluminium. This value is much closer to radiography performance. However, high thickness inspection was also not possible with this type of detector.
4. Inspection of cast aluminium component with real defects A cast aluminium component (Fig. 11a) was inspected using both the radioscopic and photon-counting systems. The cast component had been previously carefully checked using classical radiography. Radiography showed several gas pores in a 15 mm-thick part and a gas pore of 2–3 mm in diameter in the 60 mm-thick part of the sample. After X-ray characterisation, the sample was cut through the thicker part (Fig. 11b) and Fig. 11c shows an optical image of the porosity. On the Fig. 11b, it appears that the gas pore is very close to the boundary of the sample, which has a difficult shape. Indeed, the thicker part of the component is not exactly cylindrical, the outer surface consisting of two cones. Thus the pore is placed in a zone where the total crossed thickness varies steeply with position. Gas pores were detected by radioscopy in the 15 mmthick part (Fig. 12), but not in the 60 mm-thick part. The 60 mm-thick part was scanned using the photon-counting system in the area of the gas pore. The scanning area was 6 mm!12 mm (area underlined in black in Fig. 11a) with 0.5 mm steps. The data were treated as described before for the artificial defects. The area scan was performed three times, ensuring that even after discarding points due to glitches in the X-ray tube, there would be a valid observation of each point and the normalised intensity of the valid points was averaged. Fig. 13a shows the data of the photon-counting scan. The axes are defined in Fig. 13b, which is a zoom of the zone underlined in Fig. 11a. The two-dimensional scan illustrated in Fig. 13a represents the lines of same intensity in the sample. The ‘C’ sign means that the value was discarded because it was
Fig. 11. (a) Photograph of a cast aluminium component: the white square indicates the 15 mm-thick part with gas pores detected by radioscopy, the black rectangle indicates the area scanned with the photon-counting system and containing a few-mm diameter gas pore in the nearly cylindrical 60 mm thick part; (b) After non-destructive examination, the cast component was cut in the area of the gas pore and polished; (c) Optical microscopy picture of the gas pore.
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Fig. 12. Radioscopic image of the cast aluminium component: the gas pores in the 15 mm-thick part appear as clearer spots and are indicated by the arrows.
too high, due to the fact that the point was outside the sample (left-side point), or in a thin zone (right-side points). Assuming a sample of homogeneous density and chemical composition, such a scan reveals the lines of same crossed thickness. Lines very close together indicate a steep gradient of thickness, such as altitude changes in a geographical map. This is the case at the left-side boundary, where the intensity decreases steeply along the sx direction, as the beam enters the component. The round shaped lines in the central part of the sample correspond to a zone where the thickness is rather stable with low intensity changes, as witnessed by the fact that the lines are far from each other. In the right-side part of the scan, closely spaced lines occur again, due a steep
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change of thickness when the probing beam exits the component. However, in the area with coordinates (SX, SZ)Z(113G0.5 mm, 99.9G1.0 mm), underlined by the black rectangle in Fig. 13a, the lines are curved in a concave shape compared to the real shape of the sample, indicating the area of porosity. If we compare the measured line level to what it should be if the lines followed the sample shape, this area corresponds to a local difference of normalised intensity higher by DInormZ0.08, equivalent to a difference of thickness of about 2 mm, referring to the same calibration as for the artificial defects. This is the signature of the porosity, which after cutting of the specimen (Fig. 11), was found to be located precisely at this position and has a size of 2–3 mm. However, the porosity is placed in a zone of the sample where the thickness varies rapidly with position, and therefore it does not yield a peak of intensity in a uniform background, such as the artificial defect. Thus, in the case of a real sample like this cast component, it is crucial to know the real thickness expected for the sample at each position of the counting scan. If the CAD file of the sample is available, it is possible to calculate the map of the counting scan in order to get a reference. Otherwise, a reference sample without defect is needed. This experiment on a real sample shows that the presence of a defect might not always appear as a peak in intensity, but also in the shape of the intensity lines. This yields another possibility for an automatic defect detection procedure, either based on peak detection or on shape
Fig. 13. (a) Averaged normalised intensity versus SX and SZ. Contour levels up to InormZ2.1. Only valid points were allowed to contribute to the average. There was at least one valid observation for each point. The ‘C’ indicate invalid points outside of the cast component, where the X-ray intensity was higher than 2!104 cps. The rectangle underlines an area of additional intensity which may correspond to a gas pore; (b) The rectangle on the photograph represents the scan area of 6 mm!12 mm in the cast aluminium component.
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recognition, although it might be difficult and restricted to a particular series of samples.
5. Conclusion In this study, a photon-counting system combined with radioscopy was evaluated with regard to inspection of cast aluminium components. Photon-counting appears as a method extremely sensitive to thickness variations. This method can be used in particular for high thickness areas where the performance of radioscopy is rather low. However, due to the inspection time, it must be limited to particular zones in the component, known to be critical. As photon-counting is not an imaging tool and as any variation of thickness is detected (within the sensitivity limit), one must be able to differentiate between a geometrical external thickness variation and internal defects submitted to acceptance criteria (like porosities and microshrinkages). CAD file of the sample could be a solution, or a reference sample without defect. Finally, it must be noticed that the sensitivity values were obtained assuming that the SD of the photon flux was the only noise source, and after normalisation of the intensity. In order for this method to be applied as an industrial system, the inspection time must be carefully determined taking into account the time needed for carrying out reference measurements. Moreover, the production dispersion of the measurements must be determined on the industrial inspection machine in order to assess other noise sources. Although the radioscopic system is not well adapted to high thicknesses, defects are detected in thin parts of the cast component. The image intensifier did not show very good sensitivities, but better values are now obtained with amorphous silicon flat panels. However, radioscopy remains poorly adapted to the inspection of high thicknesses, making the combination with the photon-counting system very promising for global inspection of cast aluminium components.
Acknowledgements This work was performed as part of the project Qume G1RD-2000-00444, funded by the European Community. The authors would like to gratefully acknowledge Prof.-Dr Dierk Hartmann (IfG, GbmH) for supplying the aluminium cast components, Dr Jørgen Rheinla¨nder (Innospexion, Denmark) for the loan of the X-ray tube and the entire Consortium for their interest in the project.
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