Dynamic range extension of ccd imagers

Abstract

Dynamic range extension (DRE) is achieved in CCD imagers with charge overflow structures by varying the potential V,on the gate electrodes of the pixel signal charge wells during charge integration. The wells are formed to function with the charge overflow structures, e.g., vertical overflow drains, VODs, a) to control the signal charge spreading due to illumination overload and/or b) as a means for the control of signal integration time. Two different V,levels are applied to the gate electrodes, the lower of the two levels V,-LOW is applied for a long duration and the higher level V,-HIGH is applied for a shorter duration during charge integration. At low levels of incident illumination on the CCD surface, charge signals are integrated without saturation, but, at levels of very intense illumination during the period with voltage V,-LOW on the gates, the pixel wells will saturate before the end of charge integration, resulting in the vertical overflow drain (VOD) operating to dump excess charge to the substrate. With the subsequent imposition of V,-HIGH for a short time period near the end of the integration interval, pixel well saturation is delayed and signals corresponding to high intensity are not lost. Consequently, imposing V,-HIGH for a short time period near the end of the integration interval (a) increases pixel charge storage capacity and (b) shortens the exposure time, which combination allows high intensity signal information to be integrated without loss by saturation.