Andor Technology Neo sCMOS, Руководство по оборудованию

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FEATURES & FUNCTIONALITY
Neo
SECTION 2
Page 29
2.3.1 - Rolling Shutter
Rolling shutter mode essentially means that adjacent rows of the array are exposed at slightly different times as
the readout ‘waves’ sweep through each half of the sensor. That is to say, each row will start and end its
exposure slightly offset in time from its neighbour. At the maximum readout rate of 560 MHz, this offset between
adjacent row exposures is 10 μ s. 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, and each column is read in parallel 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) of the extremes of the sensor halves would have started and ended its exposure 10
ms (1080 rows x 10 μ s/row) after the rows at the centre of the sensor.
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) 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.
The potential downside of rolling shutter, which is spatial distortion resulting from the above described exposure
mechanism, 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 will be used for the majority of scientific applications,
especially where the exposure time is equal to or greater than the sensor readout time, discussed later.
Figure 8: Rolling Shutter exposure & readout