Camera Operation
Falcon 4 Camera User's Manual
•
39
Matching gain and offset values on multiple cameras
One way is of course to use flat field correction. All cameras would be set up under the same
conditions, including lighting, and then calibrated with FPN and PRNU. This process can be time-
consuming and complicated (especially the white target). Another way is to use global FPN (Sensor
Control > Black Level Selector > DigitalAll1):
1.
Starting from factory settings (factory flat field), take note what the highest dark offset is
among the set of cameras. If the highest dark offset is higher than about 20 DN you might
want to consider recalibrating the FPN correction. You can use the histogram feature
available in most image processing software applications (for example, Image-Pro), or in
CamExpert, to determine this value. Large differences in dark offset between the factory and
user are typically caused by differences in temperature from factory to user. Large dark
offsets will result in PRNU-correction-induced FPN and should therefore be avoided.
2.
Decrease global FPN (increase the offset in dark) on all cameras until they are the same and
reach at least 6 DN.
3.
Illuminate to about 80 % saturation and note the highest signal level among the set of
cameras.
4.
Increase the system gain (Sensor Control > Gain Selector > DigitalAll1) on the cameras
until they all reach the same output level (highest of all cameras).
5.
Place camera in the dark and repeat step 2 to 4 until both dark offset and 80 % sat signal
levels are equal on all cameras.
Defective Pixel Detection and Replacement
The camera has two methods of replacing pixels. Static pixel replacement uses the FFC coefficients
to mark pixels that will be replaced. Dynamic pixel replacement consists of a median filter that is
applied when the given pixel is above a threshold when compared to adjacent pixels.
Static Pixel Replacement
This is a technique for the elimination of dead or hot pixels. A pixel on the left edge (beginning of
the line) would be replaced with the pixel to its right, while a pixel on the right edge (end of the
line) is replaced with the pixel to its left. Any pixel within a line is replaced with the average of its
neighboring pixels (on the same line). For color sensors, the same algorithm is used except the
replacement pixel is of the same color. Note that three horizontally adjacent defective pixel cannot
be replaced.
The camera uses the FFC coefficients to indicate which pixels need to be replaced. If a pixel has a
Gain(PRNU) coefficient that is greater than the defectivePixelReplacementGainThreshold then the
pixel will be marked for replacement. Additionally, a pixel will be replaced if has an Base
Offset(FPN) coefficient that is greater than the offset pixel replacement threshold
(defectivePixelReplacementOffsetThreshold ). Lowering these thresholds will remove more pixels
with high gain and offset coefficients.
Most hot and dead pixels will be identified when a FPN or PRNU calibration is performed in camera.
The user can also manually mark a pixel for replacement by setting its Pixel Base Offset to 511.
Preliminary
