Dalsa P3-80-**k40-R, User Manual

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Piranha 3 User Manual
DALSA 03-032-10216-02
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artifacts at low light levels due to the offset value. You may want to set the
sdo
value
if you are not using FPN correction but want to perform PRNU correction.
3. Photo-Response Non-Uniformity (PRNU) coefficients (calculated using the
ccp
or
cpa
commands) are used to correct the difference in responsivity of individual pixels
(i.e. given the same amount of light different pixels will charge up at different rates)
and the change in light intensity across the image either because of the light source or
due to optical aberrations (e.g. there may be more light in the center of the image).
PRNU coefficients are multipliers and are defined to be of a value greater than or
equal to 1. This ensures that all pixels will saturate together.
4. Background subtract (
ssb
command) and system (digital) gain (
ssg
command) are
used to increase image contrast after FPN and PRNU calibration. It is useful for
systems that process 8-bit data but want to take advantage of the camera’s 12 bit
digital processing chain. For example, if you find that your image is consistently
between 128 and 255DN(8 bit), you can subtract off 128 (
ssb 2048
) and then
multiply by 2 (
ssg 0 8192
) to get an output range from 0 to 255.
Calibrating the Camera to Remove Non-Uniformity (Flat
Field Correction)
Flat Field Correction Overview
This camera has the ability to calculate correction coefficients in order to remove non-
uniformity in the image. This video correction operates on a pixel-by-pixel basis and
implements a two point correction for each pixel. This correction can reduce or eliminate
image distortion caused by the following factors:
•
Fixed Pattern Noise (FPN)
•
Photo Response Non Uniformity (PRNU)
•
Lens and light source non-uniformity
Correction is implemented such that for each pixel:
V
output
=[(V
input
- FPN( pixel ) - digital offset) * PRNU(pixel) – Background Subtract] x System Gain
where V
output
= digital output pixel value
V
input
= digital input pixel value from the CCD
PRNU( pixel) = PRNU correction coefficient for this pixel
FPN( pixel ) = FPN correction coefficient for this pixel
Background Subtract = background subtract value
System Gain = digital gain value
The algorithm is performed in two steps. The fixed offset (FPN) is determined first by
performing a calibration without any light. This calibration determines exactly how much
offset to subtract per pixel in order to obtain flat output when the CCD is not exposed.
The white light calibration is performed next to determine the multiplication factors
required to bring each pixel to the required value (target) for flat, white output. Video
output is set slightly above the brightest pixel (depending on offset subtracted).