|NDT.net - December 2002, Vol. 7 No.12|
Large-area flat panel detectors based on the amorphous silicon flat panel technology have recently been developed for X-ray imaging applications. This paper introduces a high resolution digital X-ray detector. The size of the X-ray image is 28 cm x 40 cm and the pixel size is 127 µm. The output signal is digital, 14 bit coded. The paper demonstrates several NDT applications in which such a detector brings significant advantages compared to the radiographic film technique. It compares image quality obtained with both the radiographic film and the flat panel detector in several applications.
Film screen radiography has been used for years to acquire and record X-ray NDT images in a number of different modalities. The general trend is towards replacement of the radiographic film by digital acquisition systems. It is a considerable technological challenge to get an image quality at least equivalent, and if possible better than that obtained with a screen film system. Several technologies compete to fulfil these requirements. Among them, large area flat panel image sensors based on amorphous silicon have recently been developed for X-ray imaging applications. Providing both high resolution and high dynamic range, these image sensors are well suited to most NDT applications.
This paper introduces a new high resolution digital X-ray subsystem based on a phosphor screen coupled to an array of amorphous silicon photodiodes and a-Si thin-film transistors (TFT) connecting the diodes to the readout electronics. The a-Si array is processed on a monolithic glass substrate. The TFT and photodiodes are deposited using large area thin film semiconductor processes.
The large size of this X-ray image sensor (30 x 40 cm), combined to a 127 µm square pixel, allows to use it in NDT filmless applications, where both high resolution and large field of view are needed.
The paper describes the general characteristics of this imager, and gives the main detection performances, such as dynamic range, linearity, contrast ratio and Modulation Transfer Function, up to 450 kV. Finally, the paper lists the NDT techniques and NDT fields concerned by this X-ray image sensor along with the major applications involving this flat panel.
Geometrical and physical characteristics
|Overall dimensions ( mm )||365||500||40|
|Active area ( mm )||292.6||406.4|
|Pixel size ( µm )||127||127|
|Number of lines for the Data correction region||2240||3200|
|Image area ( mm )||284.5||406.4|
|Total number of pixels||7 168 000|
|X-ray window material||Carbon fiber|
|X-ray conversion screen||Gd2O2S : Tb|
|Conversion screen density||34 or 133 mg/cm2|
|Analog to digital converter||14 bits|
|Image readout time||1.4 s|
|Number of electrons per LSB||300|
|RMS noise||4 LSB|
|Dynamic range||> 3500 : 1|
|Non uniformity after correction||< 1 %|
|Frame time including data correction||2.6 s|
The classic procedure for image correction uses at least two calibration images : a dark image acquired without emission of X-ray photons, for offset calibration, and a so-called light image taken at the X-ray conditions close to saturation, for gain calibration.
Furthermore in order to avoid any non-linear phenomenon and to increase the correction quality, a third calibration image is acquired at X-ray conditions close to half the saturation dose.
Thanks to this bilinear correction achieved pixel per pixel, the non uniformity in the corrected image is reduced to less than 1 %.
The imager can be equipped with Gd2O2S:Tb phosphor screens of optional thickness in order to optimize either for absorption or spatial resolution. A Lanex Fine screen from Kodak is installed for the measurements reported in this paper. Its thickness is 80 µm of a Gd2O2S:Tb powder phosphor (34 mg/cm2). Throughout all measurements x-ray radiation from a 450kV X-ray generator with a tungsten anode was used.
The saturation dose (ion dose in air at the entrance of the detector) varies with the spectral characteristics of the radiation. Different filters were used depending on the high voltage (see Fig 1). This is why, for the same high voltage, we can get several saturation doses, as the energy transmitted by the filter is different.
|Fig 1: Saturation dose from 50 to 450 kV.|
The linearity as a function of intensity has been measured at 40 kV and 400 kV using different thicknesses of Al absorbers. The exposure time has been kept constant. The signal response versus the absorbed dose in the phosphor screen of the detector is shown in Fig. 2. The linear range extends over the whole grey-level range. The signal is shown with and without gain correction, which shows a good linearity even without correction.
Fig 2: Linearity of the grey-levels at 40 kV (a) and 400 kV (b).
For comparison, the signal response curve of a typical x-ray film is added (fig. 3).
|Fig 3: Characteristic curve of the Kodak M 100.|
The detector was compared to the radiographic film in terms of the contrast between two different thicknesses of aluminium. In order to be shown on the same scale, the values of grey-levels and optical density were normalised by the maximal value.
|Fig 4: Grey-level or optical density difference between two consecutive thicknesses, at 60 kV (normalised with max. value). The thickness difference is 2 mm. The dashed lines correspond to the common specifications of use for radiographic films (optical density between 1.5 and 4.5).|
A test target with different diameters of Al wires has been used (6 ISO 12) to determine the image contrast ratio (IQI sensitivity). Contrast is defined here as the diameter of the smallest Al wire which is still visible in the image, divided by the thickness of an Al block placed in front of the detector. The test target was placed between the focus and the Al. The contrast ratio is very similar to that obtained on fine grain film (fig.5).
|Fig 5: IQI sensitivity or contrast ratio obtained in the range 0-160 kV. Comparison is done with a fine grain film (Kodak M100) at 100 kV.|
A bar test pattern of 50 µm in lead thickness was used to obtain the contrast transfer function (CTF) of the detector.
The different points obtained at the same frequency are issued from the processing of various grey-level profiles of the pattern. Thus, the dispersion observed around 4 lp/mm clearly indicates that the resolution limit is attained (this also corresponds to the Nyquist frequency).
|Fig 6: Contrast transfer function obtained by the bar/space pattern method, at 70 kV.|
It is obvious that X-ray digital radiography appears to be the major technique in which the flat panel detector is involved. Due to its light weight and compact shape, this image sensor can be used in portable equipment as well as settled installation.
Nevertheless, other NDT techniques such as 3D tomography, or voludensitometry, are potential users of large area flat panel detectors, which are distortion free and not subject to magnetic fields due to moving metal parts. For instance, the acquisition time for 360 projections of 7 million pixels should be approximately 20 minutes. However, the time for the 3D reconstruction may be much longer.
The high resolution offered by this flat panel is particularly favorable to the inspection of printed circuit boards, electronic devices and connection techniques. Furthermore, using a microfocus X-ray tube, with typical 3-10 µm focal spot size and magnification ratios from 12 to 25 in order to optimize the contributions of the X-ray focal spot and the detector pixel sizes to the object sampling, it is possible to detect and measure 25 µm diameter bonding wires.
Thanks to its wide dynamic range, this detector is able to detect tiny voids in µBGA and lack of solder wetting.
The 300 x 400 mm dimensions of this flat panel allows the inspection of large PCB’s.
Weld and Casting inspection
This flat panel detector is particularly well suited to the automotive industry, which is a major supplier of this domain of applications.
Detecting cracks, voids, inclusions, porosities in weld or casting parts of various sizes, such as engines or brake components, requires a good spatial resolution together with a wide dynamic range.
We may also cite the application of weld inspection on pipeline in oil industry.
The detection performances described above allow this detector to be an alternative to the radiographic film.
Aeronautics and space products examination
Among the various parts involved in aeronautics and space industry, the turbine blade is may be the most inspected component, as it is a critical device of an aeronautics assembly. Some images show the detection of tiny cracks in a blade, as well as the conformity control of its internal structure.
Composite material evaluation
The detection of delamination, internal flaw or other microstructural anomalies in complex composite materials is a well known NDT application of X-ray imaging.
Its large dynamic range associated with a good Contrast Transfer Function allows this flat panel detector to address a large part of applications in that NDT field.
Compared to radiographic film, this image sensor offers on-line imaging.
The detector is linear over a wider dynamic range than film and this feature is particularly important for quantitative measurements and X-ray Computed Tomography.
The large number of pixels, more than 7 million, associated with a large geometrical size allows high resolution images and a large field of view, as required in many NDT fields.
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