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a v e r a g e
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o f g r a y
r e f e r e n c e
p i c t u r e
Figure 9.4: Schematic presentation of the gain correction algorithm
Gain correction always needs an offset correction matrix. Thus, the offset correc-
tion always has to be performed before the gain correction.
How to Obtain a Grey Reference Image
In order to improve the image quality, the grey reference image must meet certain demands.
The detailed procedure to set the grey reference image is described in Section
??.
•
The grey reference image must be obtained at uniform illumination.
Use a high quality light source that delivers uniform illumination. Standard illu-
mination will not be appropriate.
•
When looking at the histogram of the grey reference image, ideally there are no grey
levels at full scale (4095 DN @ 12 bit). All pixels that are saturated white will not be
properly corrected (see Fig. 9.5).
•
Camera settings may influence the grey level. Therefore, the camera settings of the grey
reference image must be identical with the camera settings of the image to be corrected.
9.2.4 Hot and cold pixel correction
Calibration
The positions of the hot and cold pixels are determined in the Calculate step.
Hot (white) defect pixels are identified using the black reference image: every pixel that
exceeds the hot-pixel-threshold in the black reference image is marked as a defect pixel.
Cold (black) defect pixels are identified using the grey reference image: every pixel with an
intensity value below the cold-pixel-threshold in the grey reference image is marked as a
defect pixel.
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