Dalsa Linea SWIR GigE Series User Manual page 58

Flat Field Correction Overview
Flat Field Correction function (FFC) consists of using two coefficients per pixel to correct the gain
and offset of the corresponding pixel. These corrections compensate for Photo Response Non-
Uniformity (PRNU) and Fixed Pattern Noise (FPN), unique to each camera sensor.
It is imperative to perform FFC calibration under the same conditions the camera will be operated.
CMOS sensor variations (over temperature and exposure) will render the FFC calibration invalid.
Linea SWIR GigE cameras have multiple FFC user memory locations for storage for different
optimized exposure setups.
Flat Field Calibration Preparations
Before calibration, the Linea SWIR GigE should be powered on long enough to achieve its nominal
temperature (a minimum of 30 minutes).
When performing Flat Field (PRNU) Calibration, the camera should image a front illuminated white
target or rear bright field illumination source. The optical setup should be as per the inspection
system, including lens magnification, aperture and illumination intensity, spectral content, plus
illuminator beam structure.
When performing Flat Field Calibration, all pixels are adjusted to the same value as the peak pixel
value or target level selected.
If the Flat Field Calibration Target value is lower than the peak value and the system gain is set to
a low value, then it is possible that the sensor will maximize its output before the camera output
reaches 255 DN in 8-bit or 4095 DN in 12-bit output format. Visible when a portion of the output
stops increasing before reaching 255 DN with increasing illumination and the PRNU deteriorates.
This effect is resolved by decreasing the light level or exposure control time.
Following a Flat Field Calibration, all pixels will be at the target value. Changing sensor gain values
allow the user to make refinements to the operating responsivity level.
Note: The Linea camera has many different modes of operation. A Flat Field Calibration should
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be performed using the camera's intended operating mode.
Note: The best Flat Field Calibration is achieved when performed at mid-level DN of the
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working operating range. Any flat field error associated with residual pixel non-linearity will be
halved as compared to performing a calibration at the peak value of the operating range. A
simple method is to reduce the exposure time to half of what is used in typical operation in
order to get the mid DN level for Flat Field Calibration. Once complete, return the exposure
time to its original setting.
Note: Areas of the image where high luminance roll-off is present will show higher noise levels
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after Flat Field Calibration due to higher gain values of the correction coefficients. Flat Field
Calibration compensates up to an 8:1 variation. If the variation exceeds 8:1 the line profile
after calibration will include pixels that are below un-calibrated peak level.
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56 Operational Reference Linea SWIR GigE Series Camera