<s>
Fixed-pattern	B-Algorithm
noise	I-Algorithm
(	O
FPN	O
)	O
is	O
the	O
term	O
given	O
to	O
a	O
particular	O
noise	O
pattern	O
on	O
digital	B-Algorithm
imaging	I-Algorithm
sensors	O
often	O
noticeable	O
during	O
longer	O
exposure	O
shots	O
where	O
particular	O
pixels	B-Algorithm
are	O
susceptible	O
to	O
giving	O
brighter	O
intensities	O
above	O
the	O
average	O
intensity	O
.	O
</s>
<s>
FPN	O
is	O
a	O
general	O
term	O
that	O
identifies	O
a	O
temporally	O
constant	O
lateral	O
non-uniformity	O
(	O
forming	O
a	O
constant	O
pattern	O
)	O
in	O
an	O
imaging	O
system	O
with	O
multiple	O
detector	O
or	O
picture	B-Algorithm
elements	I-Algorithm
(	O
pixels	B-Algorithm
)	O
.	O
</s>
<s>
It	O
is	O
characterised	O
by	O
the	O
same	O
pattern	O
of	O
variation	O
in	O
pixel-brightness	O
occurring	O
in	O
images	O
taken	O
under	O
the	O
same	O
illumination	O
conditions	O
in	O
an	O
imaging	O
array	O
.	O
</s>
<s>
This	O
problem	O
arises	O
from	O
small	O
differences	O
in	O
the	O
individual	O
responsitivity	O
of	O
the	O
sensor	O
array	O
(	O
including	O
any	O
local	O
postamplification	O
stages	O
)	O
that	O
might	O
be	O
caused	O
by	O
variations	O
in	O
the	O
pixel	B-Algorithm
size	O
,	O
material	O
or	O
interference	O
with	O
the	O
local	O
circuitry	O
.	O
</s>
<s>
The	O
term	O
"	O
fixed	B-Algorithm
pattern	I-Algorithm
noise	I-Algorithm
"	O
usually	O
refers	O
to	O
two	O
parameters	O
.	O
</s>
<s>
One	O
is	O
the	O
dark	O
signal	O
non-uniformity	O
(	O
DSNU	O
)	O
,	O
which	O
is	O
the	O
offset	O
from	O
the	O
average	O
across	O
the	O
imaging	O
array	O
at	O
a	O
particular	O
setting	O
(	O
temperature	O
,	O
integration	O
time	O
)	O
but	O
no	O
external	O
illumination	O
and	O
the	O
photo	B-Algorithm
response	I-Algorithm
non-uniformity	I-Algorithm
(	O
PRNU	B-Algorithm
)	O
,	O
which	O
describes	O
the	O
gain	O
or	O
ratio	O
between	O
optical	O
power	O
on	O
a	O
pixel	B-Algorithm
versus	O
the	O
electrical	O
signal	O
output	O
.	O
</s>
<s>
Often	O
PRNU	B-Algorithm
as	O
defined	O
above	O
is	O
subdivided	O
in	O
pure	O
"	O
(	O
offset	O
)	O
FPN	O
"	O
which	O
is	O
the	O
part	O
not	O
dependent	O
on	O
temperature	O
and	O
integration	O
time	O
,	O
and	O
the	O
integration	O
time	O
and	O
temperature	O
dependent	O
"	O
DSNU	O
"	O
.	O
</s>
<s>
Sometimes	O
pixel	B-Algorithm
noise	O
as	O
the	O
average	O
deviation	O
from	O
the	O
array	O
average	O
under	O
different	O
illumination	O
and	O
temperature	O
conditions	O
is	O
specified	O
.	O
</s>
<s>
Pixel	B-Algorithm
noise	O
therefore	O
gives	O
a	O
number	O
(	O
commonly	O
expressed	O
in	O
rms	B-General_Concept
)	O
that	O
identifies	O
FPN	O
in	O
all	O
permitted	O
imaging	O
conditions	O
,	O
which	O
might	O
strongly	O
deteriorate	O
if	O
additional	O
electrical	O
gain	O
(	O
and	O
noise	O
)	O
is	O
included	O
.	O
</s>
<s>
Recent	O
uses	O
for	O
PRNU	B-Algorithm
include	O
measures	O
to	O
fighting	O
motion	O
picture	O
piracy	O
.	O
</s>
<s>
In	O
practice	O
,	O
a	O
long	O
exposure	O
(	O
integration	O
time	O
)	O
emphasizes	O
the	O
inherent	O
differences	O
in	O
pixel	B-Algorithm
response	O
so	O
they	O
may	O
become	O
a	O
visible	O
defect	O
,	O
degrading	O
the	O
image	O
.	O
</s>
<s>
Although	O
FPN	O
does	O
not	O
change	O
appreciably	O
across	O
a	O
series	O
of	O
captures	O
,	O
it	O
may	O
vary	O
with	O
integration	O
time	O
,	O
imager	O
temperature	O
,	O
imager	O
gain	O
and	O
incident	O
illumination	O
,	O
it	O
is	O
not	O
expressed	O
in	O
a	O
random	O
(	O
uncorrelated	O
or	O
changing	O
)	O
spatial	O
distribution	O
,	O
occurring	O
only	O
at	O
certain	O
,	O
fixed	O
pixel	B-Algorithm
locations	O
.	O
</s>
<s>
FPN	O
is	O
commonly	O
suppressed	O
by	O
flat-field	O
correction	O
(	O
FFC	O
)	O
that	O
uses	O
DSNU	O
and	O
PRNU	B-Algorithm
to	O
linearly	O
interpolate	O
and	O
reduce	O
the	O
local	O
photo	O
response	O
(	O
non-uniform	O
PRNL	O
)	O
to	O
the	O
array	O
average	O
.	O
</s>
<s>
The	O
main	O
challenge	O
is	O
to	O
generate	O
a	O
flat	O
field	O
illumination	O
for	O
short	O
time	O
exposures	O
and	O
wavelengths	O
,	O
to	O
avoid	O
speckle	B-Algorithm
(	O
in	O
monochromatic	O
light	O
conditions	O
)	O
and	O
statistical	O
fluctuations	O
of	O
the	O
light	O
stream	O
that	O
become	O
most	O
obvious	O
at	O
short	O
integration	O
times	O
.	O
</s>
<s>
Many	O
patents	O
and	O
methods	O
exist	O
to	O
reduce	O
or	O
eliminate	O
fixed	B-Algorithm
pattern	I-Algorithm
noise	I-Algorithm
in	O
digital	O
imagers	O
.	O
</s>
