Dissertation - FTP Directory Listing - University of Surrey

2.8. The Sensors2.8.1. LASThe Large Area Sensor (LAS) was designed for scientific applications such as X-ray diffraction studiesand direct X-ray medical imaging. The requirements for the sensor included a large sensing areawith a high dynamic range as X-ray imaging is unfocused and can vary over a large range ofintensities. To meet the large area requirement LAS was manufactured using a stitching techniquewhere the optical mask retical was split into 4 sections; one containing the pixels (A, contains270x270 pixels) a section containing top and bottom parts (B) left and right (C) and a corner piece(D). These blocks can then be repeated to build up larger sensors, this stitching technique allows forFigure 2.22 Showing stitching method [25]scalability in the design so different sized sensors can be built up from these 4 sections. For LAS thepixel section was repeated 5x5 times to create a sensor 54mm x 54mm containing 1350x1350 pixels.There are several methods available to achieve high dynamic range including logarithmic pixels [26-28]which directly measures the photogenerated current in a pixel. For LAS a method of multipleregions of reset (ROR) was used. This allows for up to three defined regions to have differentexposure times. The regions have to be inside each other with the shortest exposure time in thecentre but can be rectangles of any size. This method allows for a spatial high dynamic range acrossthe sensing area, suited for X-ray imaging.Figure 2.23 Showing region of reset configurations [29]31