Photon Focus MV2-D1280-640, User Manual, Page: 33 / 85

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 4.15: 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. 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. 4.16). The peak in the histogram should be well below the hot pixel
threshold of 63 DN @ 8 bit.
• Camera settings such as exposure time, LinLog, skimming and digital gain 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 corrected image.
Figure 4.16: Histogram of a proper black reference image for offset correction
4.4.3 Gain Correction
The gain correction is based on a grey reference image, which is taken at uniform illumination
to give an image with a mid grey level.
4.4 Image Correction 31