(Introduction) Filmless radiation imaging tools have been available for many years, primarily through the use of real time fluoroscopic (radioscopy) systems with simple analog video outputs. These systems are usually employed in high volume inspection applications where inspection speed is critical or when inspection automation is required. Fluoroscopic systems overcame their inherently poor contrast and spatial resolution through long integration times and the use of microfocus x-ray tubes to achieve projection magnification. The evolution of such systems led to the development of intensified CCD cameras, where the analog video outputs could be digitized, allowing for further analysis and enhancement. However, the intrinsic image quality of radioscopy systems has never matched that of conventional film, making them a poor choice for film replacement in most radiography applications. Over the past decade, several new digital radiography methods have emerged which can achieve sufficient levels of image quality to allow for film replacement in some radiographic imaging applications. These methods include what can be referred to as "indirect" digital radiography technologies, such as photostimulable storage phosphors employed in computed radiography (CR), phosphor and scintillator coupled amorphous silicon ( a-Si) arrays. These systems are characterized by the use of turbid media that convert radiation into visible light, which is, in turn, electronically detected and digitized to form an image. Because of optical scattering within the media, spatial blurring and increased noise can be encountered which degrade image quality as compared to film. However, these systems offer superior performance relative to conventional radioscopy systems, while exhibiting faster overall image display times as compared to film. More recently, our group has investigated and introduced a "direct" method for acquiring digital radiographs which eliminates optical blurring and reduces noise in the x-ray conversion layer. This approach employs an amorphous selenium ( a-Se) photoconductor deposited on top of an a-Si thin film transistor (TFT) array. It has been demonstrated that this method produces image quality which exceeds indirect digital methods and is comparable to fine grain radiographic film. This paper will describe the particular structure and operating principle of this device, along with performance characteristics relevant to industrial imaging applications.
In summary, it has been found that though the pixel pitch of a-Se direct conversion flat panel arrays is limited to 139 microns, the contrast sensitivity is very high and comparable to fine grain NDT film. Furthermore, it is found that because of the systems high signal-to-noise ratio, the system can detect the presence of flaws smaller than the pixel pitch as long as sufficient object contrast is produced. Overall, the a-Se array offers significant performance improvements over indirect digital methods, making it useful for a wider range of film replacement applications.
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