The 223-lunar month Saros Dial.

Red text is traced from data; blue reconstructed from context; green is uncertain. Reproduced with permission from [6].</p

Performance of a novel wafer scale CMOS active pixel sensor for bio-medical imaging

Recently CMOS Active Pixels Sensors (APSs) have become a valuable alternative to amorphous Silicon and Selenium Flat Panel Imagers (FPIs) in bio-medical imaging applications. CMOS APSs can now be scaled up to the standard 20 cm diameter wafer size by means of a reticle stitching block process. However despite wafer scale CMOS APS being monolithic, sources of non-uniformity of response and regional variations can persist representing a significant challenge for wafer scale sensor response. Non-uniformity of stitched sensors can arise from a number of factors related to the manufacturing process, including variation of amplification, variation between readout components, wafer defects and process variations across the wafer due to manufacturing processes. This paper reports on an investigation into the spatial non-uniformity and regional variations of a wafer scale stitched CMOS APS. For the first time a per-pixel analysis of the electro-optical performance of a wafer CMOS APS is presented, to address inhomogeneity issues arising from the stitching techniques used to manufacture wafer scale sensors. A complete model of the signal generation in the pixel array has been provided and proved capable of accounting for noise and gain variations across the pixel array. This novel analysis leads to readout noise and conversion gain being evaluated at pixel level, stitching block level and in regions of interest, resulting in a coefficient of variation ? 1.9%. The uniformity of the image quality performance has been further investigated in a typical X-ray application, i.e. mammography, showing a uniformity in terms of CNR among the highest when compared with mammography detectors commonly used in clinical practise. Finally, in order to compare the detection capability of this novel APS with the currently used technology (i.e. FPIs), theoretical evaluation of the Detection Quantum Efficiency (DQE) at zero-frequency has been performed, resulting in a higher DQE for this detector compared to FPIs. Optical characterization, X-ray contrast measurements and theoretical DQE evaluation suggest that a trade off can be found between the need of a large imaging area and the requirement of a uniform imaging performance, making the DynAMITe large area CMOS APS suitable for a range of bio-medical applications.</p

A magnified image of part of fragment A showing gear wheel b3, reconstructed using four methods to compensate for the missing projections.

Fig 4A does not correct for missing projections; Fig 4B erroneously assumes 27 missing projections as had been previously assumed; Fig 4C correctly takes 25 missing projections and replaces the missing projections with grey images with a value of 64,000; Fig 4D uses a weighed mean of the projections on either side of the missing projection.</p

Percentage of pixels that exceed a threshold of mean + 5.3 standard deviations.

A cluster of outliers can be seen around projection 950.</p

Deciphering eclipse times for Glyphs 13 and 125 on the Saros Dial.

The X-ray CT images were reconstructed assuming 27 and 25 missing projections. Annotations shown in orange were traced from the data; those in blue (highlighted with a box) were inferred from the data or context and those in green (with a box) are of uncertain interpretation. The diagonal lines in some of the X-ray CT images are the result of dead pixels in the detector.</p

Characterisation of regional variations in a stitched CMOS active pixel sensor

Stitched, large area, complementary metal-oxide-semiconductor (CMOS), active pixel sensors (APS) show promises for X-ray imaging applications. In this paper we present an investigation of the effects of stitching on uniformity of sensor response for an experimental APS. The sensor, known as LAS (large area sensor), was made by reticular stitching onto a single silicon wafer of a 5x5 array of regions consisting of 270x270 pixels with 40 ?m pixel pitch, to yield 1350x1350 pixels and an imaging area of 54x54 mm. Data acquired from two different sensors of the same type were filtered to remove spiking pixels and electromagnetic interference (EMI). The non-linear compensation (NLC) technique for CMOS sensor analysis was used to determine the variation in gain, read noise, full well capacity and dynamic range between stitched regions. Variations across stitched regions were analysed using profiles, analysis of pixel variations at stitch boundaries and using a measurement of non-uniformity within a stitched region. The results showed that non-uniformity variations were present, which increased with signal (1.5-3.5% at dark signal, rising to 3-8%). However, these were found to be smaller than variations caused by differences in readout electronics, particularly at low signal levels. The results suggest these variations should be correctable using standard calibration methods. © 2010 Elsevier B.V.</p

Characterisation of regional variations in a stitched CMOS active pixel sensor

Stitched, large area, complementary metal-oxide-semiconductor (CMOS), active pixel sensors (APS) show promises for X-ray imaging applications. In this paper we present an investigation of the effects of stitching on uniformity of sensor response for an experimental APS. The sensor, known as LAS (large area sensor), was made by reticular stitching onto a single silicon wafer of a 5x5 array of regions consisting of 270x270 pixels with 40 ?m pixel pitch, to yield 1350x1350 pixels and an imaging area of 54x54 mm. Data acquired from two different sensors of the same type were filtered to remove spiking pixels and electromagnetic interference (EMI). The non-linear compensation (NLC) technique for CMOS sensor analysis was used to determine the variation in gain, read noise, full well capacity and dynamic range between stitched regions. Variations across stitched regions were analysed using profiles, analysis of pixel variations at stitch boundaries and using a measurement of non-uniformity within a stitched region. The results showed that non-uniformity variations were present, which increased with signal (1.5-3.5% at dark signal, rising to 3-8%). However, these were found to be smaller than variations caused by differences in readout electronics, particularly at low signal levels. The results suggest these variations should be correctable using standard calibration methods. © 2010 Elsevier B.V.</p