LQ-401CL
21
Fig.16 Conceptual drawing for PRNU correction (2)
6.2.8 DSNU (Dark Signal Non-Uniformity) correction
DSNU (Dark Signal Non-Uniformity) is, as the name implies, a non-uniformity of offset level of each
pixel, which is
not
dependent on the incoming light.
Fig.17 Conceptual drawing of DSNU correction
To correct for DSNU, the camera internal correction circuit captures one or several lines of data
under dark conditions (the lens
must
be covered by a lens cap), and the average across the line is
calculated. Based on the average, coefficients are then generated for each individual pixel. The
coefficient has the function of adding or subtracting a value to the pixel output. These coefficients
are stored in a non-volatile memory, and are therefore maintained after power down.
As the dark signal is highly dependent on the exposure time, this correction must be performed
under the operating conditions (exposure time and line rate) that will be used by the application.
Fig.18 Conceptual drawing of DSNU correction
6.2.10 Shading correction
Shading is caused either by illumination with uneven distribution of light across the surface, or by
reductions in the light transmission ratio towards the edges of a lens.
The shading correction incorporated in the camera will compensate for this effect by as much as
20% of the brightest signal.
Shading is not compensated for each individual pixel. The signal is averaged across groups of 8
pixels in relation to the whole line. The pixel response non-uniformity will be superimposed on the
output also after shading correction has been performed. Therefore, it is recommended to perform
PRNU correction before shading correction.
Average
After correction: Flat dark signal response from pixel to pixel
Max
Before correction: dark signal non-uniformity from pixel to pixel
Subtract factor
Add factor
Min
Average
After correction: flat response from pixel to pixel
Average
