Prnu Shading Correction - Basler A400K User Manual

3.6.3 PRNU Shading Correction

In theory, when an area scan camera with a digital sensor captures a frame with the camera
viewing a uniform white target in bright light, all of the pixel values in the frame should be near
their maximum gray value and they should be equal. In practice, slight variations in the
performance of the pixels in the sensor, variations in the optics, and variations in the lighting will
cause some variation the pixel values output from the camera. This variation is know as Photo
Response Non-uniformity (PRNU). The PRNU shading correction feature on A400k cameras can
correct for the variations caused by PRNU. In the color version, PRNU shading correction is
executed for each color separately.
Shading correction values will only be generated for the pixels inside of the current area of
interest. No changes will be made to the pixels outside of the area of interest.
The camera can have only one set of shading correction values but you can have special shading
correction values for each area of interest in the same set if the areas of interest do not overlap.
Creating different shading correction values for each area of interest will be necessary if you have
two or more areas of interest to be captured after the other under different illumination. For
example, in order to create special shading correction values for two areas of interest within the
same set, you would set the first area of interest and create correction values under the
illumination for the first area of interest so the values go into the set and then, you would set the
second area of interest and create correction values under the illumination for the second area of
interest so these values also go into the set. The set would then contain shading correction values
for the two areas of interest.
Generating a Set of PRNU Shading Values
Before you can use PRNU shading correction, you must generate a set of PRNU shading
correction values. If you have two or more areas of interest to be captured under different
illumination, repeat the below procedure for each area of interest. Make sure that the areas do not
overlap. To generate a set of values, perform the following steps:
1. Make sure that a set of column FPN shading correction values and a set of DSNU shading
correction values has been created before. Doing PRNU shading correction before column
FPN shading correction or before DSNU shading correction can result in significant image
quality degradation.
2. Make sure that the area of interest is set to the area where you want to generate values.
3. Place a uniform white or light colored target in the field of view of the camera. Adjust your
lighting and optics as you would for normal system operation.
4. Set the gain on the camera to default.
5. Make sure that no part of the area of interest has reached saturation, that is, all gray values
are lower than 255 (8-bit) or 1023 (10-bit).
6. Capture several frames and examine the pixel values returned from the camera. The pixel
values should be about 80% of maximum.
a) If the pixel values are not at 80% of maximum adjust your lighting and/or lens aperture
setting to achieve 80%.
b) If you can not achieve 80% output by adjusting the lighting, then adjust the gain setting to
achieve the correct output.
7. Capture several frames and examine the pixel values returned from the camera. In each
frame, the values for the darkest pixels must not be less 1/2 of the values for the lightest
pixels in the line. (If the values for the darkest pixels are less than 1/2 of the value for the
lightest pixels, the camera will not be able to fully correct for shading variations.)
a) If the values for the darkest pixels are not less than 1/2 of the value for the lightest pixels,
go on to step 8.
Basler A400k
DRAFT
Basic Operation and Features
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