Flat Field Correction - Dalsa 4M30 User Manual

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3.7 Flat Field Correction

This camera has the ability to calculate correction coefficients in order to remove non-
uniformity in the image. This video correction operates on a pixel-by-pixel basis and
implements a two point correction for each pixel. This correction can reduce or eliminate
image distortion caused by the following factors:
•
•
•
Correction is implemented such that for each pixel:
V
output
where
The algorithm is performed in two steps. The fixed offset (FPN) is determined first by
performing a calculation without any light. This calibration determines exactly how much
offset to subtract per pixel in order to obtain flat output when the sensor is not exposed.
The white light (PRNU) 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).
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 (the factory temp is about 37°C, vt command). In
snapshot mode 1, FPN coefficients are not particularly sensitive to changes in frame rate
or integration time. In snapshot modes 0 and 2, FPN coefficients will be sensitive to
changes in frame rate.
Note: If your
PRNU correction requires a clean, white reference. The quality of this reference is
illumination or white
important for proper calibration. White paper is often not sufficient because the grain in
reference does not
the white paper will distort the correction. White plastic or white ceramic will lead to
extend the full field of
view of the camera, better balancing.
the camera will send a
For best results, ensure that:
warning.
1)
2)
03-032-20044-01
Fixed Pattern Noise (FPN)
Photo Response Non Uniformity (PRNU)
Lens and light source non-uniformity
=[(V - FPN( pixel ) - digital offset) * PRNU(pixel) – Background Subtract] x System Gain
input
V
output
V
input
PRNU( pixel)
FPN( pixel )
Background Subtract
System Gain
60 Hz ambient light flicker is sufficiently low not to affect camera performance and
calibration results.
The average pixel should be at least 25% below the target output. If the target is too
close, then some pixels may not be able to reach full swing (1023 DN) due to
correction applied by the camera.
= digital output pixel value
= digital input pixel value from the sensor
= PRNU correction coefficient for this pixel
= FPN correction coefficient for this pixel
= background subtract value
= digital gain value
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