Saturation and blooming are phenomena that occur
in all CCDs and that affect both their quantitative
and qualitative imaging characteristics. If each
individual pixel can be thought of as a well of
electrons, then saturation refers to the condition
where the well becomes filled. The amount of charge
that can be accumulated in a single pixel is determined
largely by its area. However, due to the nature
of the potential well, which holds charge within
a pixel, there is less probability of trapping an
electron within a well that is approaching saturation.
Therefore, as a well approaches its limit, the linear
relationship between light intensity and signal
degrades. As a result, the apparent responsivity
of a saturated pixel drops.
At saturation, pixels lose their ability to accommodate
additional charge. This additional charge will then
spread into neighboring pixels, causing them to
either report erroneous values or also saturate.
This spread of charge to adjacent pixels is known
as blooming and appears as a white streak or blob
in the image. Because various CCDs contain different
architectures, saturation and blooming can be defined
and controlled in many ways.
Linear full well:
well starts to fill, the photometric response of
the pixel departs from linearity.
The point at which this deviation exceeds an acceptable
level is defined as linear full well. Cameras are
usually designed so that this signal level fills
the full (12-bit, 14-bit, or 16-bit) dynamic
range of the analog-to-digital converter.
One can see
signs of saturation even before the linear full
well condition is reached. As the wells fill, the
random noise (square root of the signal) starts
to be clipped at the top end. As a result, a condition
termed noise clipping can be observed where the
signal noise starts to decrease even though the
signal is still increasing.
Noise is clipped as signal nears saturation.
Output stage saturation:
many pixels on the CCD are saturated, or when extensive
parallel and serial binning
is being performed, the output stage may saturate.
Under extreme conditions (say, daylight illumination
of a scientific camera), the charge overload in
the output node can cause the output amplification
chain to collapse, resulting in a zero (completely
It should be noted that in all CCDs, wells can
hold more than they can transfer. When they begin
to fill, the saturation we observe is actually caused
by approaching this maximum charge-transfer condition.
Blooming can be controlled in a couple of ways.
For instance, a multiphase CCD can be partially
clocked during integration to eliminate image blooming.
During integration, two of the three clock-voltage
phases used to transfer charge between neighboring
pixels are alternately switched. When a pixel approaches
saturation, excess charge is forced into the barrier
between the Si and SiO2 layers, where
it recombines with holes. As the phases are switched,
excess charge in pixels approaching saturation is
lost, while the charge in non-saturated pixels is
preserved. As long as the switching period is fast
enough to keep up with overflow signal generation,
charge will not spread into neighboring pixels.
Known as clocked anti-blooming, this technique is
most appropriate for low-light applications.
Anti-blooming is traditionally controlled by
specific CCD architecture design. Some cameras utilize
CCDs that have charge drains running in a strip
between every other column. Excessive charge that
would normally cause blooming is siphoned off into
this drain. Although such an architecture causes
a reduction in the effective quantum
efficiency and modulation transfer function
(MTF), these devices are invaluable when light intensities
span many orders of magnitude within a single image.