<s>
In	O
digital	B-Application
photography	I-Application
,	O
dark-frame	B-Algorithm
subtraction	I-Algorithm
is	O
a	O
way	O
to	O
reduce	O
image	B-Algorithm
noise	I-Algorithm
in	O
photographs	O
shot	O
with	O
long	O
exposure	O
times	O
,	O
at	O
high	O
ISO	O
sensor	O
sensitivity	O
or	O
at	O
high	O
temperatures	O
.	O
</s>
<s>
It	O
takes	O
advantage	O
of	O
two	O
components	O
of	O
image	B-Algorithm
noise	I-Algorithm
that	O
remain	O
the	O
same	O
from	O
one	O
shot	O
to	O
the	O
next	O
,	O
dark	O
current	O
and	O
fixed-pattern	B-Algorithm
noise	I-Algorithm
.	O
</s>
<s>
Noise	O
from	O
the	O
image	B-Algorithm
sensor	I-Algorithm
include	O
hot	O
pixels	O
,	O
which	O
light	O
up	O
more	O
brightly	O
than	O
surrounding	O
pixels	O
.	O
</s>
<s>
A	O
dark	B-Algorithm
frame	I-Algorithm
is	O
an	O
image	O
captured	O
with	O
the	O
sensor	O
in	O
complete	O
darkness	O
(	O
i.e.	O
</s>
<s>
Such	O
a	O
dark	B-Algorithm
frame	I-Algorithm
is	O
essentially	O
an	O
image	O
of	O
noise	O
produced	O
by	O
the	O
sensor	O
.	O
</s>
<s>
A	O
dark	B-Algorithm
frame	I-Algorithm
,	O
or	O
an	O
average	O
of	O
several	O
dark	B-Algorithm
frames	I-Algorithm
,	O
can	O
then	O
be	O
subtracted	O
from	O
subsequent	O
images	O
to	O
correct	O
for	O
fixed-pattern	B-Algorithm
noise	I-Algorithm
.	O
</s>
<s>
It	O
is	O
important	O
that	O
dark	B-Algorithm
frames	I-Algorithm
are	O
taken	O
at	O
the	O
same	O
ISO	O
sensitivity	O
and	O
exposure	O
time	O
as	O
the	O
original	O
photo	O
,	O
because	O
the	O
brightness	O
of	O
fixed	O
pixel	O
noise	O
is	O
dependent	O
on	O
both	O
.	O
</s>
<s>
Under	O
these	O
circumstances	O
,	O
a	O
single	O
dark	B-Algorithm
frame	I-Algorithm
can	O
be	O
subtracted	O
from	O
multiple	O
photos	O
taken	O
at	O
these	O
parameters	O
,	O
amounting	O
to	O
a	O
time	O
saving	O
and	O
allowing	O
noise	O
reduction	O
for	O
stacked	O
star	O
trail	O
images	O
,	O
which	O
do	O
not	O
allow	O
interruption	O
.	O
</s>
<s>
Dark-frame	B-Algorithm
subtraction	I-Algorithm
is	O
also	O
used	O
in	O
digital	O
photogrammetry	B-Application
,	O
to	O
improve	O
the	O
contrast	O
of	O
satellite	O
and	O
air	O
photograms	O
,	O
and	O
is	O
considered	O
a	O
best	O
practice	O
,	O
along	O
with	O
flat-field	O
correction	O
,	O
for	O
astrophotography	O
.	O
</s>
