<s>
A	O
head-mounted	B-General_Concept
display	I-General_Concept
(	O
HMD	O
)	O
is	O
a	O
display	O
device	O
,	O
worn	O
on	O
the	O
head	O
or	O
as	O
part	O
of	O
a	O
helmet	O
(	O
see	O
Helmet-mounted	B-Application
display	I-Application
for	O
aviation	O
applications	O
)	O
,	O
that	O
has	O
a	O
small	O
display	O
optic	O
in	O
front	O
of	O
one	O
(	O
monocular	O
HMD	O
)	O
or	O
each	O
eye	O
(	O
binocular	B-Algorithm
HMD	O
)	O
.	O
</s>
<s>
Virtual	B-Device
reality	I-Device
headsets	I-Device
are	O
HMDs	O
combined	O
with	O
IMUs	O
.	O
</s>
<s>
There	O
is	O
also	O
an	O
optical	B-General_Concept
head-mounted	I-General_Concept
display	I-General_Concept
(	O
OHMD	O
)	O
,	O
which	O
is	O
a	O
wearable	B-General_Concept
display	I-General_Concept
that	O
can	O
reflect	O
projected	O
images	O
and	O
allows	O
a	O
user	O
to	O
see	O
through	O
it	O
.	O
</s>
<s>
The	O
display	O
units	O
are	O
miniaturized	O
and	O
may	O
include	O
cathode	B-Device
ray	I-Device
tubes	I-Device
(	O
CRT	O
)	O
,	O
liquid-crystal	B-Device
displays	I-Device
(	O
LCDs	B-Device
)	O
,	O
liquid	B-Device
crystal	I-Device
on	I-Device
silicon	I-Device
(	O
LCos	B-Device
)	O
,	O
or	O
organic	B-Device
light-emitting	I-Device
diodes	I-Device
(	O
OLED	B-Device
)	O
.	O
</s>
<s>
This	O
is	O
sometimes	O
referred	O
to	O
as	O
augmented	B-General_Concept
reality	I-General_Concept
(	O
AR	O
)	O
or	O
mixed	B-General_Concept
reality	I-General_Concept
(	O
MR	O
)	O
.	O
</s>
<s>
This	O
method	O
is	O
often	O
called	O
optical	O
see-through	B-Device
.	O
</s>
<s>
An	O
optical	B-General_Concept
head-mounted	I-General_Concept
display	I-General_Concept
uses	O
an	O
optical	O
mixer	O
which	O
is	O
made	O
of	O
partly	O
silvered	O
mirrors	O
.	O
</s>
<s>
Various	O
methods	O
have	O
existed	O
for	O
see-through	B-Device
HMD	O
's	O
,	O
most	O
of	O
which	O
can	O
be	O
summarized	O
into	O
two	O
main	O
families	O
based	O
on	O
curved	O
mirrors	O
or	O
waveguides	O
.	O
</s>
<s>
Curved	O
mirrors	O
have	O
been	O
used	O
by	O
Laster	O
Technologies	O
,	O
and	O
by	O
Vuzix	B-Application
in	O
their	O
Star	O
1200	O
product	O
.	O
</s>
<s>
Recent	O
applications	O
have	O
included	O
the	O
use	O
of	O
HMD	O
for	O
paratroopers	B-Application
.	O
</s>
<s>
The	O
self-contained	O
color	O
monocular	O
organic	B-Device
light-emitting	I-Device
diode	I-Device
(	O
OLED	B-Device
)	O
display	O
replaces	O
the	O
NVG	O
tube	O
and	O
connects	O
to	O
a	O
mobile	O
computing	O
device	O
.	O
</s>
<s>
The	O
LE	O
has	O
see-through	B-Device
ability	O
and	O
can	O
be	O
used	O
as	O
a	O
standard	O
HMD	O
or	O
for	O
augmented	B-General_Concept
reality	I-General_Concept
applications	O
.	O
</s>
<s>
The	O
design	O
is	O
optimized	O
to	O
provide	O
high	O
definition	O
data	O
under	O
all	O
lighting	O
conditions	O
,	O
in	O
covered	O
or	O
see-through	B-Device
modes	O
of	O
operation	O
.	O
</s>
<s>
The	O
LE	O
has	O
a	O
low	O
power	O
consumption	O
,	O
operating	O
on	O
four	O
AA	O
batteries	O
for	O
35	O
hours	O
or	O
receiving	O
power	O
via	O
standard	O
Universal	B-Protocol
Serial	I-Protocol
Bus	I-Protocol
(	O
USB	B-Protocol
)	O
connection	O
.	O
</s>
<s>
The	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
continues	O
to	O
fund	O
research	O
in	O
augmented	B-General_Concept
reality	I-General_Concept
HMDs	O
as	O
part	O
of	O
the	O
Persistent	O
Close	O
Air	O
Support	O
(	O
PCAS	O
)	O
Program	O
.	O
</s>
<s>
Vuzix	B-Application
is	O
currently	O
working	O
on	O
a	O
system	O
for	O
PCAS	O
that	O
will	O
use	O
holographic	O
waveguides	O
to	O
produce	O
see-through	B-Device
augmented	B-General_Concept
reality	I-General_Concept
glasses	I-General_Concept
that	O
are	O
only	O
a	O
few	O
millimeters	O
thick	O
.	O
</s>
<s>
Engineers	O
and	O
scientists	O
use	O
HMDs	O
to	O
provide	O
stereoscopic	B-Algorithm
views	I-Algorithm
of	O
computer-aided	B-Application
design	I-Application
(	O
CAD	B-Application
)	O
schematics	O
.	O
</s>
<s>
Virtual	B-Application
reality	I-Application
,	O
when	O
applied	O
to	O
engineering	O
and	O
design	O
,	O
is	O
a	O
key	O
factor	O
in	O
integration	O
of	O
the	O
human	O
in	O
the	O
design	O
.	O
</s>
<s>
The	O
use	O
of	O
HMDs	O
for	O
VR	O
is	O
seen	O
as	O
supplemental	O
to	O
the	O
conventional	O
use	O
of	O
CAVE	B-General_Concept
for	O
VR	O
simulation	O
.	O
</s>
<s>
HMDs	O
are	O
predominantly	O
used	O
for	O
single-person	O
interaction	O
with	O
the	O
design	O
,	O
while	O
CAVEs	O
allow	O
for	O
more	O
collaborative	O
virtual	B-Application
reality	I-Application
sessions	O
.	O
</s>
<s>
Head	B-General_Concept
Mounted	I-General_Concept
Display	I-General_Concept
systems	O
are	O
also	O
used	O
in	O
the	O
maintenance	O
of	O
complex	O
systems	O
,	O
as	O
they	O
can	O
give	O
a	O
technician	O
a	O
simulated	O
x-ray	O
vision	O
by	O
combining	O
computer	O
graphics	O
such	O
as	O
system	O
diagrams	O
and	O
imagery	O
with	O
the	O
technician	O
's	O
natural	O
vision	O
(	O
augmented	O
or	O
modified	O
reality	O
)	O
.	O
</s>
<s>
In	O
visual	O
tracking	O
tests	O
,	O
a	O
HMD	O
unit	O
with	O
eye	B-General_Concept
tracking	I-General_Concept
ability	O
shows	O
an	O
object	O
moving	O
in	O
a	O
regular	O
pattern	O
.	O
</s>
<s>
People	O
without	O
brain	O
injury	O
are	O
able	O
to	O
track	O
the	O
moving	O
object	O
with	O
smooth	O
pursuit	O
eye	O
movements	O
and	O
correct	O
trajectory	B-Application
.	O
</s>
<s>
One	O
of	O
the	O
first	O
commercially	O
available	O
HMDs	O
was	O
the	O
Forte	B-Device
VFX1	I-Device
which	O
was	O
announced	O
at	O
Consumer	O
Electronics	O
Show	O
(	O
CES	O
)	O
in	O
1994	O
.	O
</s>
<s>
The	O
VFX-1	O
had	O
stereoscopic	B-Algorithm
displays	I-Algorithm
,	O
3-axis	O
head-tracking	O
,	O
and	O
stereo	O
headphones	O
.	O
</s>
<s>
Another	O
pioneer	O
in	O
this	O
field	O
was	O
Sony	O
,	O
which	O
released	O
the	O
Glasstron	B-Device
in	O
1997	O
.	O
</s>
<s>
One	O
novel	O
application	O
of	O
this	O
technology	O
was	O
in	O
the	O
game	O
MechWarrior	B-Application
2	I-Application
,	O
which	O
permitted	O
users	O
of	O
the	O
Sony	B-Device
Glasstron	I-Device
or	O
Virtual	O
I/O	O
'	O
s	O
iGlasses	O
to	O
adopt	O
a	O
new	O
visual	O
perspective	O
from	O
inside	O
the	O
cockpit	O
of	O
the	O
craft	O
,	O
using	O
their	O
own	O
eyes	O
as	O
visual	O
and	O
seeing	O
the	O
battlefield	O
through	O
their	O
craft	O
's	O
own	O
cockpit	O
.	O
</s>
<s>
Many	O
brands	O
of	O
video	B-General_Concept
glasses	I-General_Concept
can	O
be	O
connected	O
to	O
modern	O
video	O
and	O
DSLR	O
cameras	O
,	O
making	O
them	O
applicable	O
as	O
a	O
new	O
age	O
monitor	O
.	O
</s>
<s>
The	O
Oculus	B-Device
Rift	I-Device
is	O
a	O
virtual	B-Application
reality	I-Application
(	O
VR	O
)	O
head-mounted	B-General_Concept
display	I-General_Concept
created	O
by	O
Palmer	O
Luckey	O
that	O
the	O
company	O
Oculus	B-Application
VR	I-Application
developed	O
for	O
virtual	B-Application
reality	I-Application
simulations	O
and	O
video	O
games	O
.	O
</s>
<s>
The	O
HTC	B-Application
Vive	O
is	O
a	O
virtual	B-Device
reality	I-Device
head-mounted	I-Device
display	I-Device
.	O
</s>
<s>
The	O
headset	O
is	O
produced	O
by	O
a	O
collaboration	O
between	O
Valve	O
and	O
HTC	B-Application
,	O
with	O
its	O
defining	O
feature	O
being	O
precision	O
room-scale	O
tracking	O
,	O
and	O
high-precision	O
motion	O
controllers	O
.	O
</s>
<s>
The	O
PlayStation	B-Device
VR	I-Device
is	O
a	O
virtual	B-Device
reality	I-Device
headset	I-Device
for	O
gaming	O
consoles	O
,	O
dedicated	O
for	O
the	B-Operating_System
PlayStation	I-Operating_System
4	I-Operating_System
.	O
</s>
<s>
Windows	B-General_Concept
Mixed	I-General_Concept
Reality	I-General_Concept
is	O
a	O
platform	O
developed	O
by	O
Microsoft	O
which	O
includes	O
a	O
wide	O
range	O
of	O
headsets	O
produced	O
by	O
HP	O
,	O
Samsung	O
,	O
and	O
others	O
and	O
is	O
capable	O
of	O
playing	O
most	O
HTC	B-Application
Vive	O
games	O
.	O
</s>
<s>
Some	O
head-mounted	B-General_Concept
displays	I-General_Concept
are	O
designed	O
to	O
present	O
traditional	O
video	O
and	O
film	O
content	O
in	O
a	O
virtual	O
cinema	O
.	O
</s>
<s>
These	O
devices	O
typically	O
feature	O
a	O
relatively	O
narrow	O
field	O
of	O
view	O
(	O
FOV	O
)	O
of	O
50	O
–	O
60°	O
,	O
making	O
them	O
less	O
immersive	O
than	O
virtual-reality	B-Device
headsets	I-Device
,	O
but	O
they	O
offer	O
correspondingly	O
higher	O
resolution	O
in	O
terms	O
of	O
pixels	O
per	O
degree	O
.	O
</s>
<s>
Also	O
available	O
was	O
the	O
Avegant	O
Glyph	O
,	O
which	O
incorporated	O
720P	O
retinal	B-General_Concept
projection	I-General_Concept
per	O
eye	O
,	O
and	O
the	O
Cinera	O
Prime	O
,	O
which	O
featured	O
2560×1440	O
resolution	O
per	O
eye	O
as	O
well	O
as	O
a	O
66°	O
FOV	O
.	O
</s>
<s>
Other	O
products	O
available	O
in	O
2021	O
were	O
the	O
Cinemizer	O
OLED	B-Device
,	O
with	O
870×500	O
resolution	O
per	O
eye	O
,	O
the	O
VISIONHMD	O
Bigeyes	O
H1	O
,	O
with	O
1280x720	O
resolution	O
per	O
eye	O
,	O
and	O
the	O
Dream	O
Glass	O
4K	O
,	O
with	O
1920x1080	O
resolution	O
per	O
eye	O
.	O
</s>
<s>
First-person	O
view	O
(	O
FPV	O
)	O
drone	O
flying	O
uses	O
head-mounted	B-General_Concept
displays	I-General_Concept
which	O
are	O
commonly	O
called	O
"	O
FPV	O
goggles	O
"	O
.	O
</s>
<s>
But	O
digital	O
FPV	O
goggles	O
(	O
such	O
as	O
produced	O
by	O
DJI	B-Device
)	O
are	O
becoming	O
increasingly	O
popular	O
due	O
to	O
their	O
higher	O
resolution	O
video	O
.	O
</s>
<s>
A	O
HMD	O
system	O
has	O
been	O
developed	O
for	O
Formula	B-Application
One	I-Application
drivers	O
by	O
Kopin	O
Corp	O
.	O
and	O
the	O
BMW	O
Group	O
.	O
</s>
<s>
Recon	O
Instruments	O
released	O
on	O
3	O
November	O
2011	O
two	O
head-mounted	B-General_Concept
displays	I-General_Concept
for	O
ski	O
goggles	O
,	O
MOD	O
and	O
MOD	O
Live	O
,	O
the	O
latter	O
based	O
on	O
an	O
Android	O
operating	O
system	O
.	O
</s>
<s>
Training	O
with	O
HMDs	O
covers	O
a	O
wide	O
range	O
of	O
applications	O
from	O
driving	O
,	O
welding	O
and	O
spray	O
painting	O
,	O
flight	B-Application
and	O
vehicle	O
simulators	O
,	O
dismounted	O
soldier	O
training	O
,	O
medical	O
procedure	O
training	O
,	O
and	O
more	O
.	O
</s>
<s>
However	O
,	O
a	O
number	O
of	O
unwanted	O
symptoms	O
have	O
been	O
caused	O
by	O
prolonged	O
use	O
of	O
certain	O
types	O
of	O
head-mounted	B-General_Concept
displays	I-General_Concept
,	O
and	O
these	O
issues	O
must	O
be	O
resolved	O
before	O
optimal	O
training	O
and	O
simulation	O
is	O
feasible	O
.	O
</s>
<s>
Ability	O
to	O
show	O
stereoscopic	B-Algorithm
imagery	I-Algorithm
.	O
</s>
<s>
A	O
binocular	B-Algorithm
HMD	O
has	O
the	O
potential	O
to	O
display	O
a	O
different	O
image	O
to	O
each	O
eye	O
.	O
</s>
<s>
This	O
can	O
be	O
used	O
to	O
show	O
stereoscopic	B-Algorithm
images	I-Algorithm
.	O
</s>
<s>
It	O
should	O
be	O
borne	O
in	O
mind	O
that	O
so-called	O
'	O
Optical	O
Infinity	O
 '	O
is	O
generally	O
taken	O
by	O
flight	B-Application
surgeons	O
and	O
display	O
experts	O
as	O
about	O
9	O
meters	O
.	O
</s>
<s>
This	O
is	O
the	O
distance	O
between	O
the	O
two	O
eyes	O
,	O
measured	O
at	O
the	O
pupils	O
,	O
and	O
is	O
important	O
in	O
designing	O
head-mounted	B-General_Concept
displays	I-General_Concept
.	O
</s>
<s>
Binocular	B-Algorithm
overlap	O
–	O
measures	O
the	O
area	O
that	O
is	O
common	O
to	O
both	O
eyes	O
.	O
</s>
<s>
Binocular	B-Algorithm
overlap	O
is	O
the	O
basis	O
for	O
the	O
sense	O
of	O
depth	O
and	O
stereo	O
,	O
allowing	O
humans	O
to	O
sense	O
which	O
objects	O
are	O
near	O
and	O
which	O
objects	O
are	O
far	O
.	O
</s>
<s>
Humans	O
have	O
a	O
binocular	B-Algorithm
overlap	O
of	O
about	O
100°	O
(	O
50°	O
to	O
the	O
left	O
of	O
the	O
nose	O
and	O
50°	O
to	O
the	O
right	O
)	O
.	O
</s>
<s>
The	O
larger	O
the	O
binocular	B-Algorithm
overlap	O
offered	O
by	O
an	O
HMD	O
,	O
the	O
greater	O
the	O
sense	O
of	O
stereo	O
.	O
</s>
<s>
More	O
sophisticated	O
HMDs	O
incorporate	O
a	O
positioning	O
system	O
that	O
tracks	O
the	O
wearer	O
's	O
head	O
position	O
and	O
angle	O
,	O
so	O
that	O
the	O
picture	O
or	O
symbol	O
displayed	O
is	O
congruent	O
with	O
the	O
outside	O
world	O
using	O
see-through	B-Device
imagery	O
.	O
</s>
<s>
Head	B-Operating_System
tracking	I-Operating_System
–	O
Binding	O
the	O
imagery	O
.	O
</s>
<s>
Head-mounted	B-General_Concept
displays	I-General_Concept
may	O
also	O
be	O
used	O
with	O
tracking	O
sensors	O
that	O
detect	O
changes	O
of	O
angle	O
and	O
orientation	O
.	O
</s>
<s>
This	O
allows	O
the	O
user	O
to	O
look	O
around	O
a	O
virtual	B-Application
reality	I-Application
environment	O
simply	O
by	O
moving	O
the	O
head	O
without	O
the	O
need	O
for	O
a	O
separate	O
controller	O
to	O
change	O
the	O
angle	O
of	O
the	O
imagery	O
.	O
</s>
<s>
Eye	B-General_Concept
tracking	I-General_Concept
–	O
Eye	B-General_Concept
trackers	I-General_Concept
measure	O
the	O
point	O
of	O
gaze	O
,	O
allowing	O
a	O
computer	O
to	O
sense	O
where	O
the	O
user	O
is	O
looking	O
.	O
</s>
