Dynamic range expansion method for a CMD imager

Abstract

A dynamic range expansion method for charge modulation device (CMD) imagers has been proposed and an experimental circuit was evaluated. As active pixel sensors, CMD imagers offer significant advantages in terms of non-destructive readout and high-speed operation. Furthermore, the CMD imager used in this study has built-in dual vertical shift registers which can start their scanning independently. We applied our newly developed source reset method to the imager, which enables each pixel charge to be reset by external circuits. By using these functions, we have obtained our dynamic range expansion method. THe source reset method enables each pixel of a CMD imager to be reset selectively. We use it for a regional electrical shutter. For instance, if there is an overexposed pixel, its accumulated charge is reset from outside the imager, so that the pixel charge is prevented from becoming saturated. The non-destructive readout and high-speed operation enable the CMD imager to be read multiple times without destruction of accumulated charges. For instance, if the imager is driven twice in one field period, each pixel's accumulations can be checked and reset selectively from external of the device at the first scanning, then, all the signals are obtained at the second scanning and output through a small process circuit. This dynamic range expansion method for CMD imagers needs only a simple feedback circuit but does not need complex equipment externally nor adding reset circuits on a chip. An experimental circuit with this method was successfully evaluated here.