Photon Focus MV3-D640I-M01-144-CL, User Manual, Page: 57 / 121

9.2 Image Correction
4
4
4
3 1
2 1
3 1
4 3 2
3
4
1
1
2 4 1 4
4
3
1
3
4
b l a c k r e f e r e n c e
i m a g e
1
1
1
2 - 1
2 - 2
- 1 0
1 - 1 1
- 1
0
2
0
- 1
0
- 2
0
1
1
- 2 - 2 - 2
a v e r a g e
o f b l a c k
r e f e r e n c e
p i c t u r e
=
-
o f f s e t c o r r e c t i o n
m a t r i x
Figure 9.2: Schematic presentation of the offset correction algorithm
How to Obtain a Black Reference Image
In order to improve the image quality, the black reference image must meet certain demands.
The detailed procedure to set the black reference image is described in Section
?? .
• 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 saturated black (0 DN) will not be properly
corrected (see Fig. 9.3). The peak in the histogram should be well below the
hot-pixel-threshold.
• Camera settings may influence the grey level. Therefore, for best results the camera
settings of the black reference image must be identical with the camera settings of the
image to be corrected.
0 200 400 600 800 1000 1200 1400 1600
0
0.2
0.4
0.6
0.8
1
Histogram of the uncorrected black reference image
Grey level, 12 Bit [DN]
Relative number of pixels [−]
black level offset ok
black level offset too low
Figure 9.3: Histogram of a proper black reference image for offset correction
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