Image Processing Reference
In-Depth Information
V dd
RST
Tx
V LG
V ss
FPN cancellation circuit (CDS)
Horizontal access circuit
FIGURE 5.80
Schematic diagram of CMOS image sensor with vertical access circuit for electronic shutter.
Since the exposure timing of each row shifts in series, as shown in Figure 5.79b, cap-
tured images of objects moving at high speed appear quite different from the real shape
because of exposure zone shift with time.
An example of a captured still image, of a rotary fan, is shown in Figure 5.81. Pictures
of the still fan and rotating fan captured by a CMOS image sensor in electronic shutter
mode are shown in Figure 5.81a and b, respectively. In CMOS sensors, exposure timing
shifts from the first row to the final row. Except for the speed, this behavior is the same
with focal plane shutters used in single lens reflex cameras. As shown in Figure 5.81c, the
exposure zone runs from the top to the bottom. This fan turns in a clockwise direction. It
is hoped that readers can visualize how the exposure zone shifts with time on the rotat-
ing fan.
In the right half of the fan, the exposure zone and fan blades move in the same direc-
tion: downward. This shows that five blades got ahead of the exposure zone during the
exposure period. On the other hand, in the left half of the fan, as the exposure zone and
fan blades move in opposite directions, many more blades can be seen passing through
the exposure zone. Blades seen in both the right and left sides moved from one side to the
other during exposure.
It is clear that a shorter time is necessary for the exposure zone to run from the top to
the bottom to realize less distortion. This is one of the reasons for the focus on the devel-
opment of high-speed readout CMOS sensors. The cameras that have a certain level of
performance with CMOS sensors are equipped with mechanical shutter systems to avoid
these phenomena by controlling the exposure period of whole images simultaneously.
In the case of CCDs, these phenomena do not occur because exposure timings of all
pixels are the same, however it causes blur. The electronic shutter mode of CCDs is called
global shutter mode while that of CMOS sensors is called rolling shutter mode.
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