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40
Calibrating the Camera to Remove Non-Uniformity (Flat Field
Correction)
Flat Field Correction Overview
This camera has the ability to calculate correction coefficients in order to remove non-uniformity in the
image when operating in TDI Mode. 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:
Fixed Pattern Noise (FPN)
•
•
Photo Response Non Uniformity (PRNU)
•
Lens and light source non-uniformity
Correction is implemented such that for each pixel:
V
=[(V
- FPN( pixel ) - digital offset) * PRNU(pixel) – Background Subtract] x System Gain
output
input
where
V
output
V
input
PRNU( pixel)
FPN( pixel )
Background
Subtract
System Gain
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 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, binning, or number of integration stages.
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:
1. 60 Hz ambient light flicker is sufficiently low not to affect camera performance and calibration
results.
2. 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:
Brightest Pixel (per tap)
3 >
Darkest Pixel (per tap)
03-032-20135-00
=
digital output pixel value
=
digital input pixel value from the CCD
=
PRNU correction coefficient for this pixel
=
FPN correction coefficient for this pixel
=
background subtract value
=
digital gain value
Piranha HN RoHS User Manual
Teledyne DALSA
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