Dalsa GiGE Vision Spyder3 Series User Manual page 46

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Analog Processing
Optimizing offset perform ance and gain in the analog d om ain lets you achieve a better signal-to-noise
ratio and d ynam ic range than you w ould achieve by trying to op tim ize the offset in the d igital d om ain
only. Therefore, you should perform all analog ad ju stments prior to any d igital ad justments.
1. Analog gain (the Gain (dB) or Calibrate Gain Target parameters in the Analog Controls set) is
multiplied by the analog signal to increase the signal strength before the A/ D conversion. It is used
to take ad vantage of the full d ynamic range of the A/ D converter. For example, in a low light
situ ation the brightest p art of the im age may be consistently coming in at only 50% of the DN . An
analog gain of 6 d B (2x) w ill ensure full u se of the d ynam ic range of the A/ D conv erter. Of course
the noise is also increased .
2. The analog offset or black level (Black Level (D N ) comm and ) is an ―artificial‖ offset introd uced into
the vid eo p ath to ensure that the A/ D is fu nctioning p roperly. The analog offset shou ld be set so that
it is at least 3 times the rms noise value at the current gain.
Digital Processing
To optimize camera performance, d igital signal processing should be completed after any analog
ad ju stments.
1. Fixed pattern noise (FPN ) calibration (calculated using the Calibrate FPN parameter) is used to
subtract aw ay ind ivid u al pixel d ark current.
2. The d igital offset (D igital Offset (D N ) parameter) enables the subtraction of the ―artificial‖ A/ D
offset (the analog offset) so that application of the PRN U coefficient d oes not resu lt in artifacts at low
light levels d ue to the offset value. You m ay w ant to set the D igital Offset (D N ) value if you are not
using FPN correction but w ant to perform PRN U correction.
3. Photo-Response N on-Uniformity (PRN U) coefficients (calculated u sing the PRN U Target and
Calibrate PRN U, or PRN U Calibration Algorithm Selector p arameters in the D ata Processing
family) are used to correct the d ifference in responsivity of ind ivid ual pixels (i.e. given the same
am ou nt of light d ifferen t pixels w ill charge up at d ifferent rates) and the change in light intensity
across the im age either because of the light source or d ue to optical aberrations (e.g. there m ay be
more light in the center of the im age). PRN U coefficients are m ultip liers and are d efined to be of a
value greater than or equ al to 1. This ensures that all p ixels w ill saturate together.
4. Background subtract (Background Subtract (D N ) p arameter) and system (d igital) gain (D igital
Gain (D N ) p arameter) are used to increase im age contrast after FPN and PRN U calibration. It is
useful for system s that process 8-bit d ata but w ant to take ad vantage of the camera's 12 bit d igital
processing chain. For exam ple, if you find that your im age is consistently betw een 128 and 255DN (8
bit), you can su btract off 128 (Background Subtract (D N )
(D N ) ) to get an outp ut range from 0 to 255.
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) and then multiply by 2 (D igital Gain
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