LT-400CL
18
To correct for PRNU, the camera’s internal correction circuit captures one or several
lines of data under non-saturated illuminated conditions which are not more than 80% of
maximum (recommend level is half of maximum), and the average across the line is
calculated. Based on this average, coefficients are then generated for each individual
pixel. The coefficient has the function of multiplying the pixel output with a factor
greater or less than 1. These coefficients are stored in a non-volatile memory, and are
therefore maintained after power down.
Fig.17 Conceptual drawing for PRNU correction (2)
6.3.9 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.18 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.19 Conceptual drawing of DSNU 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
