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light sensitivity when compared to 5T
layouts or other designs.
Time-Delayed Integration
Time-delayed integration (TDI)
imaging is another clever way to
better capture moving objects. By
synchronizing pixel exposure with the
mo-tion of the camera or the object,
the effective exposure time can be
increased. TDI implementation in
CMOS has traditionally been difficult
because of the lack of a charge-
addition circuit. The application
requires the combination of a global
shutter and a low-noise readout
method. Recently TDI in the digital
domain, enabled by high frame rates,
has become more and more popular,
even though the improvement signal-
to noise ratio improvement is lower
than for the traditional TDI opera-tion.
High Dynamic Range
(HDR)
Another factor in improving global-
shutter CMOS sensors is applying
a specific method to achieve a high
dynamic range (HDR). HDR expands
the scale of the captured light and
dark areas of an image to depict
them in a satisfactory way. This is
appropriate when capturing an im-
age before a bright sky background
or against very bright light sources,
which tends to deliver overexposed
images with blurred-out white
areas, whereas the darkest shadows
appear underexposed and recede
into an unstructured black. Thus, the
exposure levels for both light and dark
areas have to be equalized across the
image.
The reason for the unbalanced
treatment of light and dark areas is
the linear response curve of CMOS
image sensors as opposed to the
Figure 7: Piece-wise linear forming of the sensor response curve
achieves a higher dynamic range
archi-tecture patented by CMOSIS
differentiates it from the traditional
4T rolling shutter or the 5T global
shutter concept. This eight-transistor
pixel design is now implemented in
all global-shutter sensor types of the
CMOSIS CMV Series sensor family.
The crucial point is that the 8T
architecture provides two storage ele-
ments inside the pixel, rather than
just one (as in the 5T structure).
They separately store an image taken
at the beginning of the exposure,
and another one at the end of the
exposure period. Deploying a clever
algorithm, both these images are
subtracted during readout to lower
the total noise account and increase
the shutter efficiency.
This way, noise levels below 10
electrons can be reached, and a
shutter efficiency of 99.999 percent
has been demonstrated. The
technique, called correlated double
sampling (CDS), enables the lowest
fixed-pattern noise and low parasitic-
Figure 6: Different exposure times for odd and even lines achieve a
higher dynamic range
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