Global Shutter

Global shutter mode, which can also be thought of as a ‘snapshot’ exposure mode, means that all pixels of the array are exposed simultaneously. In most respects, global shutter can be thought of as behaving like an Interline CCD sensor. Before the exposure begins, all pixels in the array will be held in a ‘keep clean state’, during which charge is drained into the anti-bloom structure of each pixel. At the start of the exposure each pixel simultaneously begins to collect charge and is allowed to do so for the duration of the exposure time. At the end of exposure each pixel transfers charge simultaneously to its readout node. Importantly, global shutter can be configured to operate in a continuous ‘overlap’ mode (analogous to Interline CCD), whereby an exposure can proceed while the previous exposure is being readout out from the readout nodes of each pixel. In this mode, the sensor has a 100% duty cycle, again resulting in optimal time resolution and photon collection efficiency. However, the mechanism of global shutter mode demands that a reference readout is performed ‘behind the scenes’, in addition to the actual readout of charge from each pixel. Due to this additional reference readout, global shutter mode carries the trade-off of halving the maximum frame rate that would otherwise have been achieved in rolling shutter mode. In addition, global shutter also increases the RMS read noise by a factor of 1.41 over rolling shutter readout. The figure below shows a simplified illustration showing sequence of events in global shutter mode:

Global Shutter Exposure and Readout