Falcon 4M Camera Manual
03-032-20044-03
Teledyne DALSA
46
3.7 Flat Field Correction
This cam era has the ability to calcu late correction coefficients in ord er to rem ove non -
u niform ity in the im age. This vid eo correction op erates on a p ixel-by-p ixel basis and
im p lem ents a tw o p oint correction for each p ixel. This correction can red u ce or elim inate
im age d istortion cau sed by the follow ing factors:
Fixed Pattern N oise (FPN )
Photo Resp onse N on Uniform ity (PRN U)
Lens and light sou rce non -u niform ity
Correction is im p lem ented su ch that for each p ixel:
V
output
=[(V
input
- FPN( pixel ) - digital offset) * PRNU(pixel) – Background Subtract] x System Gain
w here
V
ou tp u t
=
d igital ou tp u t p ixel valu e
V
inp u t
=
d igital inp u t p ixel valu e from the sensor
PRN U( p ixel)
=
PRN U correction coefficient for this p ixel
FPN ( p ixel )
=
FPN correction coefficient for this p ixel
Backgrou nd Su btract
=
backgrou nd su btract valu e
System Gain
=
d igital gain valu e
The algorithm is p erform ed in tw o step s. The fixed offset (FPN ) is d eterm ined first by
p erform ing a calcu lation w ithou t any light. This calibration d eterm ines exactly how m u ch
offset to su btract p er p ixel in ord er to obtain flat ou tp u t w hen the sensor is not exp osed .
The w hite light (PRN U) calibration is p erform ed next to d eterm ine the m u ltip lication
factors requ ired to bring each p ixel to the requ ired valu e (target) for fla t, w hite ou tp u t.
Vid eo ou tp u t is set slightly above the brightest p ixel (d ep end ing on offset su btracted ).
It is im p ortant to d o the FPN correction first. Resu lts of the FPN correction are u sed in the
PRN U p roced u re. We recom m end that you rep eat the correct ion w hen a tem p eratu re
change greater than 10 °C occu rs (the factory tem p is abou t 37 °C,
vt
com m and ). In
snap shot m od e 1, FPN coefficients are not p articu larly sensitive to changes in fram e rate
or integration tim e. In snap shot m od es 0 and 2, FPN coefficients w ill be sensitive to
changes in fram e rate.
PRN U correction requ ires a clean, w hite reference. The qu ality of this reference is
im p ortant for p rop er calibration. White p ap er is often not su fficient becau se the grain in
the w hite p ap er w ill d istort the correction. White p lastic or w hite ceram ic w ill lead to
better balancing.
For best resu lts, ensu re that:
1)
60 H z am bient light flicker is su fficiently low not to affect cam era p erform ance and
calibration resu lts.
2)
The average p ixel shou ld be at least 25% below the target ou tp u t. If the target is too
close, then som e p ixels m ay not be able to reach fu ll sw ing (1023 DN ) d u e to
correction ap p lied by the cam era.
3)
When 6.25 % of p ixels from a single row w ithin the region of interest are clip p ed , flat
field correction resu lts m ay be inaccu rate.
Note: If your
illumination or white
reference does not
extend the full field of
view of the camera,
the camera will send a
warning.