Digital CCD Line Scan Camera SK 1024 XSD ( Rev.1.0. / 20.09.2005 ) - Manual Page
6
7. Anti Blooming
1
Oscilloscopical signal display of the CCD line scan
signal (barcode with incident light), SK 2048 XSD
Line scan signal with central enhanced illumination
and sharp rising signal edges. Integration time t
A
=
0,158 ms
Over exposure due to longer integrationtime (t
A
=
0,533 ms). The blooming effekt is raised in the sen-
sor by misadjusting the blooming control voltage
(low V
A
). The structures of the signal are distorted.
The blooming control voltage limits the output sig-
nal of the sensor to approx. 90% of the saturation
voltage V
SAT
. The anti blooming technique ist
active. Also, with even longer integration time (t
A
=
0,806 ms) the signal edge positions from Fig. 1 are
preserved.
1
Blooming
Extended illumination of saturated pixels, which are
not able to accumulate further charge due to long
exposure, leads to charge overflow into adjacent
pixels. This effect is called blooming. Blooming
causes a corruption of the geometrical allocation of
image and object in the line signal.
CCD line scan cameras with anti-blooming sensors
direct the abundant charge to a ”drain gate”. Charge
overflow into adjacent, less illuminated pixels is
prevented. Depending on pixel frequency and spec-
tral range, overexposure up to factor of 50 can thus
be handled.
The CCD line scan cameras of the XSD-series do con-
tain anti-bloooming sensors, i.e. they are prevented
from overexposure due to a special design. The Bloo-
ming Drain Gate has a limited capacity, though.
The less pixels are overexposed, the better is the anti
blooming effect of the drain gate. For single pixels a
charge excess of up to a factor of 50 above saturation
can be drained. With increasing quantities of oversatu-
rated pixels the charge drainage decreases.
The electronics of the XSD camera series support the
blooming control possibilities of the sensor. The satura-
tion load is regulated by the blooming control voltage
V
A
. The higher the voltage V
A
the lower the level of anti
blooming effect initiation. A high voltage V
A
increases
the saturation protection, but it limits the output voltage
of the video signal and reduces the dynamic range of
the camera. Tuning the voltage V
A
to a level too small,
the anti blooming effect is switched off completely. The
maximum output voltage of the sensor reaches the
saturation level V
SAT
.
Blooming Control Voltage V
A
The blooming control voltage V
A
is by default prepared
to a level where the output voltage of the camera rea-
ches approx. 90% of the saturation voltage V
SAT
at maxi-
mum. Thus, an optimum anti blooming effect is ensured.
The voltage V
A
schould only be altered in exceptio-
nal cases.
The anti blooming control voltage V
A
is adjusted with
the trimmer P3. Turning the trimmer P3 to the left, the
voltage V
A
rises. Turning it to the right the voltage V
A
decreases. Turning it to the right bedstop, the anti bloo-
ming is switched off.
The trimmer P3 is accessed from the front of the
2
3
camera with the lens taken off.
To prevent misadjustment of the line scan camera, the
effect of the P3 adjustment should be performed and
tracked with sufficient illumination observing an oscil-
loscopical display of the line scan signal on the PC
monitor.
2
3
Camera board,
sensor side
P3
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