Dsnu Shading Correction - Basler A400K User Manual

3.6.2 DSNU Shading Correction

In theory, when an area scan camera with a digital sensor captures a frame in complete darkness,
all of the pixel values in the frame should be near zero and they should be equal. In practice, slight
variations in the performance of the pixels in the sensor will cause some variation the pixel values
output from the camera when the camera is capturing frames in darkness. This variation is known
as Dark Signal Non-uniformity (DSNU). The DSNU shading correction feature on A400k cameras
can correct for the variations caused by DSNU.
Generating a Set of DSNU Shading Correction Values
Before you can use DSNU shading correction, you must generate a set of DSNU shading
correction values. To generate a set of values, perform the following steps:
1. For optimum performance, make sure that a set of column FPN shading correction values
has been created before. Doing DSNU shading correction before column FPN shading
correction can result in image quality degradation.
2. As DSNU varies depending on the temperature, make sure that the camera has reached its
operating temperature.
3. Make sure that the area of interest is set to the area where you want to generate values.
4. Cover the camera lens, close the iris in the camera lens, or darken the room so that the
camera will be capturing frames in complete darkness.
5. Set the gain as you would for normal system operation.
6. Make sure that the offset is set so all gray values including the noise are around 16 (8-bit
mode) or 64 (10-bit mode) or lower.
7. Signal the camera to generate a set of DSNU shading values:
c) You can start the generation of a set of DSNU shading values by using the Camera
Configuration Tool Plus (see Section 4.1). With the CCT+, you set the Shading Value
Generate parameter in the DSNU & PRNU Shading Correction parameters group to start
the generation of a set of DSNU shading values.
d) You can also start the generation of the DSNU shading table by using a binary write
command (see Section 4.3) to write a value to the Generate field of the DSNU or PRNU
Shading Value Generate CSR (see page 4-24).
8. The camera must capture at least eight frames to create a set of DSNU shading correction
values. If your camera is set to control exposure with an ExSync signal, you must generate at
least eight ExSync signal cycles after you signal the camera to begin generating the values.
If you are running the camera in a free-run exposure mode, you must wait long enough for
the camera to capture at least eight frames.
9. Once eight frames have been captured, the camera calculates the DSNU shading correction
values:
a) The camera uses the data from the eight captured frames to calculate an average gray
value for each pixel in the frame.
b) The camera finds the pixel with the highest average gray value in the frame.
c) For each of the other pixels in the frame, the camera determines the offset that would be
needed to make the pixel's average value equal to the average value for the highest
pixel.
d) The camera creates a set of DSNU shading values that contains the calculated offsets.
The set of DSNU values will be placed in the camera's volatile memory. This set of values
will overwrite any shading values that are already in the memory. The current set of values
in the volatile memory is used whenever DSNU is enabled.
Basler A400k
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Basic Operation and Features
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