| Functionality | 31
can be used to flatten uneven illumination or to compensate shading effects of a lens. Both offset and gain correction
work on a pixel-per-pixel basis, i.e. every pixel is corrected separately. For the correction, a black reference and a
grey reference image are required. Then, the correction values are determined automatically in the camera.
The offset correction is based on a black reference image, which is taken at no illumination (e.g. lens aperture
completely closed). The black reference image contains the fixed-pattern noise of the sensor, which can be subtracted
from the live images in order to minimise the static noise.
Offset correction algorithm
After configuring the camera with a black reference image, the camera is ready to apply the offset correction:
1.
Determine the average value of the black reference image.
2.
Subtract the black reference image from the average value.
3.
Mark pixels that have a grey level higher than 1008 DN (@ 12 bit) as hot pixels.
4.
Store the result in the camera as the offset correction matrix.
5.
During image acquisition, subtract the correction matrix from the acquired image and interpolate the hot pixels.
How to Obtain a Black Reference Image
In order to improve the image quality, the black reference image must meet certain demands.
• The black reference image must be obtained at no illumination, e.g. with lens aperture closed or closed lens
opening.
• It may be necessary to adjust the black level offset of the camera. In the histogram of the black reference image,
ideally there are no grey levels at value 0 DN after adjustment of the black level offset. All pixels that are
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