46
•
Camera Calibration
Background
Subtract
=
background subtract value
System Gain
=
digital gain value
The algorithm is performed in two steps. The fixed offset (FPN) is determined first by
performing a calibration without any light. This calibration determines exactly how much
offset to subtract per pixel in order to obtain flat output when the CCD is not exposed.
The white light calibration is performed next to determine the multiplication factors required
to bring each pixel to the required value (target) for flat, white output. Video output is set
slightly above the brightest pixel (depending on offset subtracted).
Flat Field Correction Restrictions
It is important to do the FPN correction first. Results of the FPN correction are used in the
PRNU procedure. We recommend that you repeat the correction when a temperature
change greater than 10°C occurs or if you change the analog gain, integration time, or line
rate.
PRNU correction requires a clean, white reference. The quality of this reference is important
for proper calibration. White paper is often not sufficient because the grain in the white
paper will distort the correction. White plastic or white ceramic will lead to better balancing.
For best results, ensure that:
•
50 or 60 Hz ambient light flicker is sufficiently low not to affect camera
performance and calibration results.
•
For best results, the analog gain should be adjusted for the expected
operating conditions and the ratio of the brightest to darkest pixel in a
tap should be less than 3 to 1 where:
•
The camera is capable of operating under a range of 8 to 1, but will
clip values larger than this ratio.
•
The brightest pixel should be slightly below the target output.
•
When 6.25% of pixels from a single row within the region of interest
are clipped, flat field correction results may be inaccurate.
•
Correction results are valid only for the current analog gain and offset
values. If you change these values, it is recommended that you
recalculate your coefficients.
3>
Darkest Pixel (per tap)
Brightest Pixel (per tap)
Note:
If your
illumination or
white reference
does not extend
the full field of view
of the camera, the
camera will send a
warning.
