36
•
Falcon 4 Camera User's Manual
Camera Operation
Flat Field Correction Algorithm Description
Flat Field Correction Algorithm–Method1 (feature: flatfieldCorrectionAlgorithm) applies the following
FFC formula for correcting pixel values:
newPixelValue
x,y
= (sensorPixelValue
x,y
– FFCOffsetBase
x,y – normalized
FFCOffsetDelta
x,y
)
* FFCGain
x,y
Where:
•
x & y are the Flat Field Correction Pixel coordinates. (See the
flatfieldCorrectionPixelXCoordinate and flatfieldCorrectionPixelYCoordinate features.)
•
newPixelValue is the pixel value after Flat Field Correction is applied.
•
sensorPixelValue is the pixel value before Flat Field correction is applied.
•
FFCOffsetBase is one offset coefficient value to subtract from the sensorPixelValue, this
value is measured at minimal exposure time.
•
FFCOffsetDelta is another offset coefficient value to subtract from the sensorPixelValue.
This value is measured at current exposure time, and is the deviation from
FFCOffsetSetBase. It is measured at the same time as FFCOffsetBase.
•
FFCGain is the gain coefficient value that is multiplied with the sensorPixelValue.
The implementation of this formula requires that both the FPN and PRNU coefficient are stored in
32 bits. Internally in Falcon4, we reserve 9 bits for the FFCOffsetBase, 9 bits for FFCOffsetDelta
(FPN) coefficient and 14 bits for the FFCGain (PRNU) coefficient.
General Notes on FFC calibration
The camera comes calibrated with two factory sets, one for each shutter mode. In addition to the
factory calibrations, the camera provides three user configurable FFC sets. These can be calibrated
and saved in the camera. For more information on this, see “How to do an FFC Setup in the
Camera”.
Another option is to perform the flat field correction in the frame grabber.
In either case, we recommend that you repeat the correction when a temperature change of
greater than 10 °C occurs.
For best results, ensure that:
•
Gain (PRNU) calibration has 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.
•
Ambient light flicker (e.g. fluorescent lights) is sufficiently low not to affect camera
performance and calibration results.
•
The average pixel should be at least 20 % below the target output. If the target is too close,
then some pixels may not be able to reach full swing due to correction applied by the
camera.
•
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 black offset values. If you change this value,
it is recommended that you recalculate your coefficients.
Preliminary
