Rolling Shutter

In Rolling Shutter mode, adjacent rows of the array are exposed at slightly different times as the readout ‘waves’ sweep through each half of the sensor. Therefore, each row will start and end its exposure slightly offset in time from its neighbour. In the case of the Zyla 5.5, at the maximum readout rate of 560 MHz (as each half of the sensor is at 280 MHz), this offset between adjacent row exposures is ~10 μs. The rolling shutter readout mechanism is illustrated in Figure 8 below. From the point of view of readout, the sensor is split in half horizontally. Rows are read out from the centre outwards, row after row. At the start of an exposure, the wave sweeps through each half of the sensor, switching each row in turn from a ‘keep clean state’, in which all charge is drained from the pixels, to an ‘exposing state’, in which light induced charge is collected in each pixel. At the end of the exposure, the readout wave again sweeps through the sensor, transferring the charge from each row into the readout node of each pixel. The important point is that each row will have been subject to exactly the same exposure time, but the row at the top (or bottom) edge of the sensor would have started and ended its exposure ~10 ms (1080 rows x 10 μs/row) after the rows at the centre of the sensor (when using 560 MHz readout rate).

Rolling Shutter Exposure and Readout

Rolling shutter can be operated in a ‘continuous’ mode when capturing a kinetic series of images, whereby after each row has been read out it immediately enters its next exposure. This ensures a 100% duty cycle, meaning that no time is wasted between exposures and, perhaps more importantly, no photons are wasted. At the maximum frame rate for a given readout speed (e.g. 100 fps at 560 MHz for the Zyla 5.5) the sensor is continuously reading out, i.e. as soon as the readout fronts reach the top and bottom of the sensor, they immediately return to the centre to readout the next exposure.

A potential downside of rolling shutter is spatial distortion resulting from the above described exposure mechanism. This has historically been more apparent in devices such as CMOS camcorders, where the entire image field could be moved (for example by the user rapidly panning the camera) at a rate that the image readout could not match; thus, objects could appear at an angle compared to their actual orientation. In reality, despite the time-offset readout pattern, rolling shutter mode is appropriate for the majority of scientific applications, especially where the exposure time is equal to or greater than the sensor readout time, discussed later.