Photon Focus MV2-D1280-640, User Manual, Page: 34 / 85

4 Functionality
Gain correction is not a straightforward feature. The quality of the grey refer-
ence image is crucial for proper gain correction.
Gain correction algorithm
After configuring the camera with a black and grey reference image, the camera is ready to
apply the gain correction:
1. Determine the average value of the grey reference image.
2. Subtract the offset correction matrix from the grey reference image.
3. Divide the average value by the offset corrected grey reference image.
4. Store the result in the camera as the gain correction matrix.
5. During image acquisition, multiply the gain correction matrix by the offset-corrected
acquired image.
:
7
1 0
9
7 9
7
8
7 9
4 3 2
3
4
1
1
9 6 8 4
6
1 0
1
3
4
g r e y r e f e r e n c e
p i c t u r e
a v e r a g e
o f g r e y
r e f e r e n c e
p i c t u r e
)
1
1 . 2
1
0 . 9 1
1 . 2 - 2
0 . 9 1
1 - 1 1
0 . 8
1
1
0
1 . 3
0 . 8
1
0
1 1 - 2 - 2 - 2
=
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
-
)
o f f s e t c o r r e c t i o n
m a t r i x
g a i n c o r r e c t i o n
m a t r i x
Figure 4.17: Gain Correction
Gain correction always needs an offset correction matrix, so the offset correction
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 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 (255 DN @ 8 bit). All pixels that are saturated white will not be properly
corrected (see Fig. 4.18).
• Camera settings such as exposure time, LinLog, skimming and digital gain may influence
the grey level. Therefore, the camera settings of the grey reference image must be
identical with the camera settings of the corrected image.
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