<s>
Flicker	B-General_Concept
is	O
a	O
visible	O
change	O
in	O
brightness	O
between	O
cycles	O
displayed	O
on	O
video	O
displays	O
.	O
</s>
<s>
It	O
applies	O
to	O
the	O
refresh	O
interval	O
on	O
cathode	B-Device
ray	I-Device
tube	I-Device
(	O
CRT	B-Device
)	O
televisions	O
and	O
computer	B-Device
monitors	I-Device
,	O
as	O
well	O
as	O
plasma	O
computer	B-Device
displays	I-Device
and	O
televisions	O
.	O
</s>
<s>
Flicker	B-General_Concept
occurs	O
on	O
CRTs	O
when	O
they	O
are	O
driven	O
at	O
a	O
low	O
refresh	O
rate	O
,	O
allowing	O
the	O
brightness	O
to	O
drop	O
for	O
time	O
intervals	O
sufficiently	O
long	O
to	O
be	O
noticed	O
by	O
a	O
human	O
eye	O
–	O
see	O
persistence	O
of	O
vision	O
and	O
flicker	B-General_Concept
fusion	O
threshold	O
.	O
</s>
<s>
Most	O
people	O
find	O
that	O
refresh	O
rates	O
of	O
70	O
–	O
90Hz	O
and	O
above	O
enable	O
flicker-free	B-General_Concept
viewing	O
on	O
CRTs	O
.	O
</s>
<s>
Use	O
of	O
refresh	O
rates	O
above	O
120Hz	O
is	O
uncommon	O
,	O
as	O
they	O
provide	O
little	O
noticeable	O
flicker	B-General_Concept
reduction	O
and	O
limit	O
available	O
resolution	O
.	O
</s>
<s>
In	O
LCD	B-Device
screens	I-Device
,	O
the	O
LCD	B-Device
itself	O
does	O
not	O
flicker	B-General_Concept
,	O
it	O
preserves	O
its	O
opacity	O
unchanged	O
until	O
updated	O
for	O
the	O
next	O
frame	O
.	O
</s>
<s>
However	O
,	O
in	O
order	O
to	O
prevent	O
accumulated	O
damage	O
LCDs	B-Device
quickly	O
alternate	O
the	O
voltage	O
between	O
positive	O
and	O
negative	O
for	O
each	O
pixel	O
,	O
which	O
is	O
called	O
'	O
polarity	O
inversion	O
 '	O
.	O
</s>
<s>
In	O
practice	O
,	O
there	O
is	O
a	O
small	O
difference	O
,	O
which	O
means	O
that	O
every	O
pixel	O
flickers	B-General_Concept
at	O
about	O
30Hz	O
.	O
</s>
<s>
More	O
of	O
a	O
concern	O
is	O
the	O
LCD	B-Device
backlight	O
.	O
</s>
<s>
Earlier	O
LCDs	B-Device
used	O
fluorescent	O
lamps	O
which	O
flickered	O
at	O
100	O
–	O
120Hz	O
;	O
newer	O
fluorescently	O
backlit	O
LCDs	B-Device
use	O
an	O
electronic	O
ballast	O
that	O
flickers	B-General_Concept
at	O
25	O
–	O
60kHz	O
which	O
is	O
far	O
outside	O
the	O
human	O
perceptible	O
range	O
,	O
and	O
LED	O
backlights	O
have	O
no	O
inherent	O
need	O
to	O
flicker	B-General_Concept
at	O
all	O
.	O
</s>
<s>
On	O
top	O
of	O
any	O
inherent	O
backlight	O
flicker	B-General_Concept
,	O
most	O
fluorescent	O
and	O
LED	O
backlight	O
designs	O
use	O
digital	O
PWM	B-Algorithm
for	O
some	O
or	O
all	O
of	O
their	O
dimming	O
range	O
by	O
switching	O
on	O
and	O
off	O
at	O
rates	O
from	O
several	O
kHz	O
to	O
as	O
little	O
as	O
180Hz	O
,	O
though	O
some	O
flicker-free	B-General_Concept
designs	O
using	O
true	O
analog	O
DC	O
dimming	O
exist	O
.	O
</s>
<s>
Flicker	B-General_Concept
is	O
necessary	O
for	O
a	O
film-based	O
movie	B-Application
projector	I-Application
to	O
block	O
the	O
light	O
as	O
the	O
film	O
is	O
moved	O
from	O
one	O
frame	O
to	O
the	O
next	O
.	O
</s>
<s>
The	O
standard	O
framerate	O
of	O
24fps	O
produces	O
very	O
obvious	O
flicker	B-General_Concept
,	O
so	O
even	O
very	O
early	O
movie	B-Application
projectors	I-Application
added	O
additional	O
vanes	O
to	O
the	O
rotating	O
shutter	O
to	O
block	O
light	O
even	O
when	O
the	O
film	O
was	O
not	O
moving	O
.	O
</s>
<s>
Home	O
film	O
movie	B-Application
projectors	I-Application
(	O
and	O
early	O
theater	O
projectors	O
)	O
often	O
have	O
four	O
vanes	O
,	O
to	O
raise	O
the	O
18fps	O
used	O
by	O
silent	O
film	O
to	O
72Hz	O
.	O
</s>
<s>
Video	O
projectors	O
typically	O
use	O
either	O
LCDs	B-Device
which	O
operate	O
similarly	O
to	O
their	O
desktop	O
counterparts	O
,	O
or	O
DLP	B-General_Concept
mirrors	O
which	O
flicker	B-General_Concept
at	O
2.5	O
–	O
32kHz	O
,	O
though	O
"	O
single-chip	O
"	O
color	O
projectors	O
switch	O
between	O
displaying	O
a	O
frame	O
's	O
red	O
,	O
green	O
,	O
&	O
blue	O
channels	O
at	O
as	O
little	O
as	O
180Hz	O
using	O
a	O
color	O
wheel	O
or	O
RGB	O
lightsource	O
.	O
</s>
<s>
For	O
stereoscopic	B-Algorithm
3D	I-Algorithm
,	O
a	O
single-image	O
system	O
can	O
only	O
display	O
the	O
left-eye	O
or	O
right-eye	O
image	O
at	O
once	O
,	O
switching	O
between	O
them	O
at	O
90	O
–	O
144Hz	O
,	O
though	O
this	O
does	O
have	O
the	O
advantage	O
of	O
reduced	O
crosstalk	O
versus	O
two-image	O
3D	O
projection	O
.	O
</s>
<s>
Movie	B-Application
projectors	I-Application
typically	O
use	O
an	O
incandescent	O
lamp	O
or	O
arc	O
lamp	O
which	O
does	O
not	O
itself	O
flicker	B-General_Concept
noticeably	O
.	O
</s>
<s>
The	O
exact	O
refresh	O
rate	O
necessary	O
to	O
prevent	O
the	O
perception	O
of	O
flicker	B-General_Concept
varies	O
greatly	O
based	O
on	O
the	O
viewing	O
environment	O
.	O
</s>
<s>
In	O
a	O
completely	O
dark	O
room	O
,	O
a	O
sufficiently	O
dim	O
display	O
can	O
run	O
as	O
low	O
as	O
30Hz	O
without	O
visible	O
flicker	B-General_Concept
.	O
</s>
<s>
At	O
normal	O
room	O
and	O
TV	O
brightness	O
this	O
same	O
display	O
rate	O
would	O
produce	O
flicker	B-General_Concept
so	O
severe	O
as	O
to	O
be	O
unwatchable	O
.	O
</s>
<s>
The	O
human	O
eye	O
is	O
most	O
sensitive	O
to	O
flicker	B-General_Concept
at	O
the	O
edges	O
of	O
the	O
human	O
field	O
of	O
view	O
(	O
peripheral	O
vision	O
)	O
and	O
least	O
sensitive	O
at	O
the	O
center	O
of	O
gaze	O
(	O
the	O
area	O
being	O
focused	O
on	O
)	O
.	O
</s>
<s>
This	O
is	O
why	O
computer	B-Device
monitor	I-Device
CRTs	O
usually	O
run	O
at	O
70	O
to	O
90Hz	O
,	O
while	O
CRT	B-Device
TVs	O
,	O
which	O
are	O
viewed	O
from	O
further	O
away	O
,	O
are	O
seen	O
as	O
acceptable	O
at	O
60	O
or	O
50Hz	O
(	O
see	O
analog	O
television	O
standards	O
)	O
.	O
</s>
<s>
Chewing	O
something	O
crunchy	O
such	O
as	O
tortilla	O
chips	O
or	O
granola	O
can	O
induce	O
flicker	B-General_Concept
perception	O
due	O
to	O
the	O
vibrations	O
from	O
chewing	O
synchronizing	O
with	O
the	O
flicker	B-General_Concept
rate	O
of	O
the	O
display	O
.	O
</s>
<s>
Software	O
can	O
cause	O
flicker	B-General_Concept
effects	O
by	O
directly	O
displaying	O
an	O
unintended	O
intermediate	O
image	O
for	O
a	O
short	O
time	O
.	O
</s>
<s>
For	O
example	O
,	O
drawing	O
a	O
page	O
of	O
text	O
by	O
blanking	O
the	O
area	O
to	O
white	O
first	O
in	O
the	O
frame	B-Algorithm
buffer	I-Algorithm
,	O
then	O
drawing	O
'	O
on	O
top	O
 '	O
of	O
it	O
,	O
makes	O
it	O
possible	O
for	O
the	O
blank	O
region	O
to	O
appear	O
momentarily	O
onscreen	O
.	O
</s>
<s>
This	O
creates	O
an	O
off-screen	O
drawing	O
surface	O
,	O
drawing	O
to	O
it	O
(	O
with	O
as	O
much	O
flicker	B-General_Concept
as	O
you	O
want	O
)	O
,	O
and	O
then	O
copying	O
it	O
all	O
at	O
once	O
to	O
the	O
screen	O
.	O
</s>
<s>
While	O
this	O
technique	O
cuts	O
down	O
on	O
software	O
flicker	B-General_Concept
,	O
it	O
can	O
also	O
be	O
very	O
inefficient	O
.	O
</s>
<s>
Flicker	B-General_Concept
is	O
used	O
intentionally	O
by	O
developers	O
on	O
low-end	O
systems	O
to	O
create	O
the	O
illusion	O
of	O
more	O
objects	O
or	O
colors/shades	O
than	O
are	O
actually	O
possible	O
on	O
the	O
system	O
,	O
or	O
as	O
a	O
speedy	O
way	O
of	O
simulating	O
transparency	O
.	O
</s>
<s>
While	O
typically	O
thought	O
of	O
as	O
a	O
mark	O
of	O
older	O
systems	O
like	O
16-bit	O
game	O
consoles	O
,	O
such	O
flicker	B-General_Concept
techniques	O
continue	O
to	O
be	O
used	O
on	O
new	O
systems	O
,	O
as	O
in	O
the	O
temporal	B-General_Concept
dithering	I-General_Concept
used	O
to	O
fake	O
true	O
color	O
on	O
most	O
LCD	B-Device
monitors	I-Device
.	O
</s>
<s>
Video	O
hardware	O
outside	O
the	O
monitor	O
can	O
also	O
cause	O
flicker	B-General_Concept
through	O
many	O
different	O
timing	O
and	O
resolution-related	O
artifacts	O
such	O
as	O
screen	O
tearing	O
,	O
z-fighting	O
and	O
aliasing	B-Error_Name
.	O
</s>
<s>
The	O
flicker	B-General_Concept
of	O
a	O
CRT	B-Device
monitor	I-Device
can	O
cause	O
various	O
symptoms	O
in	O
those	O
sensitive	O
to	O
it	O
such	O
as	O
eye	O
strain	O
,	O
headaches	O
in	O
migraine	O
sufferers	O
,	O
and	O
seizures	O
in	O
epileptics	O
.	O
</s>
<s>
As	O
the	O
flicker	B-General_Concept
is	O
most	O
clearly	O
seen	O
at	O
the	O
edge	O
of	O
our	O
vision	O
there	O
is	O
no	O
obvious	O
risk	O
in	O
using	O
a	O
CRT	B-Device
,	O
but	O
prolonged	O
use	O
can	O
cause	O
a	O
sort	O
of	O
retinal	O
shock	O
where	O
the	O
flickering	O
is	O
seen	O
even	O
when	O
looking	O
away	O
from	O
the	O
monitor	O
.	O
</s>
<s>
The	O
symptoms	O
usually	O
disappear	O
in	O
less	O
than	O
a	O
week	O
without	O
CRT	B-Device
use	O
,	O
and	O
usually	O
only	O
last	O
a	O
few	O
hours	O
unless	O
the	O
exposure	O
has	O
been	O
over	O
a	O
long	O
period	O
.	O
</s>
