<s>
A	O
peripheral	B-Device
head-mounted	I-Device
display	I-Device
(	O
PHMD	O
)	O
is	O
avisual	O
display	B-Device
(	O
monocular	O
or	O
binocular	B-Algorithm
)	O
mounted	O
to	O
the	O
user	O
's	O
head	O
that	O
is	O
in	O
the	O
peripheral	O
of	O
the	O
user	O
's	O
field	O
of	O
view	O
(	O
FOV	O
)	O
/	O
peripheral	O
vision	O
.	O
</s>
<s>
Whereby	O
the	O
actual	O
position	O
of	O
the	O
mounting	O
(	O
as	O
the	O
display	B-Device
technology	I-Device
)	O
is	O
considered	O
to	O
be	O
irrelevant	O
as	O
long	O
as	O
it	O
does	O
not	O
cover	O
the	O
entire	O
FOV	O
.	O
</s>
<s>
The	O
term	O
PHMD	O
includes	O
devices	O
such	O
as	O
Google	B-Application
Glass	I-Application
,	O
which	O
are	O
often	O
misclassified	O
as	O
a	O
Head-up	B-Application
display	I-Application
(	O
HUD	B-Application
)	O
if	O
following	O
the	O
original	O
definition	O
by	O
NASA	O
.	O
</s>
<s>
While	O
NASA	O
defined	O
this	O
term	O
over	O
centuries	O
of	O
space	O
flight	O
research	O
,	O
it	O
actually	O
describes	O
a	O
display	B-Device
that	O
addresses	O
the	O
eyes-free	O
problem	O
,	O
by	O
absolving	O
the	O
user	O
from	O
the	O
need	O
to	O
angle	O
down	O
their	O
head	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
Head-Down	B-Application
Display	I-Application
(	O
HDD	O
)	O
is	O
located	O
at	O
the	O
instrument	O
control	O
panel	O
.	O
</s>
<s>
Also	O
,	O
a	O
HUD	B-Application
is	O
mainly	O
used	O
to	O
augment	B-General_Concept
additional	I-General_Concept
information	I-General_Concept
into	I-General_Concept
reality	I-General_Concept
,	O
which	O
is	O
technically	O
not	O
feasible	O
yet	O
for	O
products	O
such	O
as	O
Google	B-Application
Glass	I-Application
(	O
lens	O
focus	O
on	O
the	O
display	B-Device
causes	O
a	O
blurred	O
environment	O
–	O
see	O
figure	O
below	O
)	O
.	O
</s>
<s>
This	O
taxonomy	O
for	O
head-mounted	B-General_Concept
displays	I-General_Concept
is	O
based	O
on	O
the	O
property	O
of	O
its	O
functionality	O
and	O
the	O
ability	O
of	O
the	O
human	O
eye	O
to	O
perceive	O
peripheral	O
information	O
,	O
instead	O
of	O
being	O
technology-dependent	O
.	O
</s>
<s>
In	O
this	O
article	O
Human	O
Factors	O
for	O
visual	O
perception	O
are	O
being	O
summarized	O
,	O
which	O
are	O
important	O
to	O
be	O
taken	O
into	O
consideration	O
when	O
designing	O
visual	B-Application
interfaces	I-Application
for	O
PHMDs	O
.	O
</s>
<s>
The	O
most	O
important	O
uniqueness	O
is	O
that	O
the	O
user	O
's	O
FOV	O
is	O
not	O
being	O
fully	O
covered	O
,	O
allowing	O
the	O
user	O
to	O
perform	O
real	O
world	O
tasks	O
without	O
limitations	O
,	O
while	O
not	O
having	O
the	O
pretension	O
to	O
raise	O
or	O
create	O
immersion	O
,	O
such	O
as	O
HMDs	B-General_Concept
often	O
aim	O
for	O
.	O
</s>
<s>
For	O
current	O
display	B-Device
technologies	I-Device
,	O
while	O
projecting	O
image	O
onto	O
the	O
eye	O
,	O
the	O
screen	O
needs	O
to	O
be	O
focused	O
by	O
the	O
pupil	O
to	O
enable	O
a	O
clear	O
reading	O
of	O
the	O
screen	O
,	O
thus	O
the	O
environment	O
becomes	O
blurred	O
and	O
out-of-focus	O
.	O
</s>
<s>
So	O
a	O
PHMD	O
such	O
as	O
Google	B-Application
Glass	I-Application
is	O
capable	O
of	O
displaying	O
detailed	O
information	O
,	O
when	O
the	O
pupil	O
is	O
focusing	O
the	O
display	B-Device
itself	O
,	O
as	O
it	O
also	O
allows	O
for	O
peripheral	O
information	O
when	O
the	O
eye	O
focuses	O
on	O
the	O
real	O
world	O
.	O
</s>
<s>
Still	O
,	O
simple	O
information	O
such	O
as	O
notifications	O
are	O
perceivable	O
when	O
focusing	O
on	O
the	O
real	O
world	O
instead	O
of	O
the	O
display	B-Device
.	O
</s>
<s>
Research	O
shows	O
that	O
designing	O
an	O
optimal	O
visual	O
output	O
for	O
Head-Mounted	B-General_Concept
Displays	I-General_Concept
is	O
a	O
complex	O
issue	O
,	O
since	O
there	O
are	O
several	O
physiological	O
factors	O
that	O
significantly	O
impact	O
users’	O
perception	O
.	O
</s>
<s>
A	O
display	B-Device
mounted	O
somehow	O
to	O
user	O
's	O
eye	O
has	O
fixed	O
focal	O
distance	O
.	O
</s>
<s>
Focusing	O
information	O
such	O
as	O
presented	O
on	O
a	O
screen	O
leads	O
to	O
a	O
change	O
in	O
the	O
depth	B-Algorithm
of	I-Algorithm
focus	I-Algorithm
.	O
</s>
<s>
However	O
,	O
when	O
wearing	O
an	O
HMD	B-General_Concept
with	O
eye-movements	O
that	O
exceed	O
this	O
angle	O
,	O
since	O
head	O
movements	O
do	O
not	O
have	O
any	O
effect	O
on	O
the	O
interface	O
,	O
a	O
drop	O
in	O
comfort	O
might	O
occur	O
due	O
to	O
tired	O
eye	O
muscle	O
.	O
</s>
<s>
HMDs	B-General_Concept
often	O
do	O
not	O
cover	O
the	O
whole	O
FOV	O
,	O
which	O
is	O
also	O
a	O
reason	O
for	O
increased	O
cybersickness	O
.	O
</s>
<s>
This	O
effect	O
often	O
occurs	O
when	O
wearing	O
a	O
monocular	O
HMD	B-General_Concept
.	O
</s>
<s>
While	O
one	O
eye	O
is	O
closed	O
or	O
being	O
obstructed	O
by	O
a	O
display	B-Device
,	O
phoria	O
can	O
occur	O
,	O
which	O
has	O
the	O
potential	O
to	O
cause	O
Vertigo	B-Application
and	O
Nausea	O
as	O
well	O
.	O
</s>
<s>
It	O
is	O
recommended	O
to	O
wear	O
a	O
monocular	O
HMD	B-General_Concept
over	O
the	O
dominant	O
eye	O
.	O
</s>
<s>
While	O
most	O
of	O
these	O
factors	O
mentioned	O
above	O
become	O
problematic	O
when	O
both	O
eyes	O
are	O
covered	O
with	O
displays	O
,	O
a	O
single	O
display	B-Device
resting	O
in	O
the	O
peripheral	O
vision	O
can	O
be	O
considered	O
to	O
be	O
unproblematic	O
,	O
since	O
it	O
does	O
not	O
permanently	O
influence	O
the	O
perceived	O
picture	O
of	O
the	O
real	O
world	O
.	O
</s>
<s>
As	O
mentioned	O
earlier	O
,	O
there	O
are	O
two	O
types	O
of	O
information	O
being	O
perceivable	O
with	O
a	O
peripheral	O
head	B-General_Concept
mounted	I-General_Concept
display	I-General_Concept
:	O
(	O
1	O
)	O
detailed	O
information	O
:	O
when	O
consciously	O
focusing	O
on	O
the	O
display	B-Device
and	O
(	O
2	O
)	O
peripheral	O
information	O
:	O
through	O
the	O
human	O
's	O
visual	O
perception	O
,	O
when	O
focusing	O
at	O
the	O
'	O
real	O
world	O
 '	O
.	O
</s>
<s>
Furthermore	O
,	O
researchers	O
proposed	O
to	O
additionally	O
utilize	O
an	O
eye	O
tracker	O
for	O
a	O
peripheral	B-Device
head-mounted	I-Device
display	I-Device
,	O
in	O
order	O
to	O
improve	O
user	O
experience	O
.	O
</s>
<s>
There	O
has	O
also	O
been	O
investigations	O
on	O
which	O
display	B-Device
positions	O
are	O
most	O
suitable	O
.	O
</s>
<s>
While	O
most	O
HMDs	B-General_Concept
suffer	O
badly	O
of	O
the	O
effects	O
of	O
binocular	B-Algorithm
rivalry	O
,	O
Depth	O
of	O
Field	O
and	O
Phoria	O
it	O
is	O
different	O
for	O
the	O
PHMD	O
.	O
</s>
<s>
Since	O
the	O
PHMD	O
is	O
not	O
totally	O
covering	O
the	O
FOV	O
and	O
also	O
not	O
augmenting	O
information	O
on	O
real	O
objects	O
,	O
it	O
is	O
not	O
affected	O
by	O
known	O
problems	O
monocular	O
HMDs	B-General_Concept
usually	O
suffer	O
from	O
,	O
such	O
as	O
the	O
effect	O
of	O
attention	O
switching	O
between	O
reality	O
and	O
projection	O
.	O
</s>
<s>
Such	O
problems	O
have	O
been	O
figured	O
out	O
over	O
centuries	O
of	O
airspace	O
research	O
and	O
usually	O
occur	O
when	O
trying	O
to	O
augment	B-General_Concept
reality	I-General_Concept
.	O
</s>
<s>
Furthermore	O
,	O
significant	O
changes	O
-	O
depending	O
on	O
the	O
stimuli	O
-	O
of	O
the	O
screen	O
content	O
is	O
still	O
perceivable	O
without	O
focusing	O
the	O
display	B-Device
.	O
</s>
<s>
An	O
efficient	O
response	O
to	O
such	O
perceived	O
information	O
could	O
be	O
accomplished	O
in	O
quick	O
peripheral	O
input	B-Device
,	O
such	O
as	O
a	O
quick	O
hand	O
movement	O
.	O
</s>
<s>
Notwithstanding	O
,	O
suitable	O
input	B-Device
modalities	O
for	O
PHMDs	O
that	O
are	O
not	O
socially	O
awkward	O
remain	O
to	O
be	O
discovered	O
.	O
</s>
<s>
In	O
addition	O
,	O
taking	O
part	O
in	O
traffic	O
while	O
focusing	O
on	O
a	O
visual	O
input	B-Device
modality	O
can	O
lead	O
to	O
a	O
considerable	O
decrease	O
of	O
attention	O
to	O
the	O
road	O
(	O
see	O
also	O
Semantic	O
memory	O
&	O
Multimodal	B-Application
Interaction	I-Application
)	O
.	O
</s>
<s>
However	O
,	O
compared	O
to	O
smartphone	B-Application
interaction	O
,	O
a	O
quick	O
switch	O
to	O
real	O
world	O
tasks	O
is	O
attainable	O
,	O
because	O
there	O
is	O
no	O
need	O
for	O
getting	O
the	O
device	O
out	O
of	O
a	O
pocket	O
or	O
bag	O
.	O
</s>
<s>
Since	O
it	O
is	O
always	O
available	O
,	O
it	O
can	O
provide	O
peripheral	O
visual	O
information	O
at	O
any	O
time	O
,	O
whereas	O
peripheral	O
information	O
on	O
smartphone	B-Application
in	O
a	O
pocket	O
is	O
not	O
at	O
all	O
or	O
barely	O
perceivable	O
(	O
e.g.	O
</s>
