Dalsa Falcon 4M30, Руководство пользователя

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Falcon 4M Camera Manual
03-032-20044-01 DALSA
44
6) How can one match 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
ccf
and
cpa
. This can be
time-consuming and complicated (especially the white target). Another way is to use
analog offset and system gain (digital gain):
1) Starting from factory settings (
sao 0 0, ssg 0 4096, epc 1 1
), take note what
the highest dark offset is among the set of cameras. If the highest dark offset is higher
than about 16 DN (10 bit) you might want to consider recalibrating the FPN
correction (
ccf
). 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) Increase the offset (camera in dark) on all cameras (
sao
command) until they are the
same and reach at least 4 DN (10 bit).
3) Illuminate to about 80% saturation (820 DN, 10 bit) and note the highest signal level
among the set of cameras.
4) Increase the digital gain (
ssg
) 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.
An important note on window blemishes:
When flat field correction is performed, window cleanliness is paramount. The figure
below shows an example of what can happen if a blemish is present on the sensor
window when flat field correction is performed. The blemish will cast a shadow on the
wafer. FFC will compensate for this shadow by increasing the gain. Essentially FFC will
create a white spot to compensate for the dark spot (shadow). As long as the angle of the
incident light remains unchanged then FFC works well. However when the angle of
incidence changes significantly (i.e. when a lens is added) then the shadow will shift and
FFC will makes things worse by not correcting the new shadow (dark spot) and
overcorrecting where the shadow used to be (white spot). While the dark spot can be
potentially cleaned, the white spot is an FFC artifact that can only be corrected by another
FFC calibration.