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4.6 Image Correction
4.6.1 Overview
The camera possesses image pre-processing features, that compensate for non-uniformities
caused by the sensor, the lens or the illumination. This method of improving the image quality
is generally known as ’Shading Correction’ or ’Flat Field Correction’ and consists of a
combination of offset correction, gain correction and pixel interpolation.
Since the correction is performed in hardware, there is no performance limita-
tion of the cameras for high frame rates.
The offset correction subtracts a configurable positive or negative value from the live image
and thus reduces the fixed pattern noise of the CMOS sensor. In addition, hot pixels can be
removed by interpolation. The gain correction 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.
Do not set any reference images when gain or LUT is enabled! Read the follow-
ing sections very carefully.
Correction values of both reference images can be saved into the internal flash memory, but
this overwrites the factory presets. Then the reference images that are delivered by factory
cannot be restored anymore.
4.6.2 Offset Correction (FPN, Hot Pixels)
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 (see Section 4.6.2).
4.6 Image Correction 59