Photon Focus OEM-D1024E, User Manual

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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 4.27: Offset correction
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 modules. 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. 4.28). The peak in the histogram should be well below
the hot pixel threshold of 1008 DN @ 12 bit.
• Camera module settings such as exposure time, LinLog, skimming and digital gain may
influence the grey level. Therefore, for best results the camera module settings of the
black reference image must be identical with the camera module 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 4.28: Histogram of a proper black reference image for offset correction
Hot pixel correction
Every pixel that exceeds a certain threshold in the black reference image is marked as a hot
pixel. If the hot pixel correction is switched on, the camera module replaces the value of a hot
pixel by an average of its neighbour pixels (see Fig. 4.29).
4.5 Image Correction 35