<s>
An	O
overhead	B-Device
projector	I-Device
(	O
often	O
abbreviated	O
to	O
OHP	O
)	O
,	O
like	O
a	O
film	O
or	O
slide	B-Device
projector	I-Device
,	O
uses	O
light	O
to	O
project	B-Application
an	O
enlarged	O
image	O
on	O
a	O
screen	O
,	O
allowing	O
the	O
view	O
of	O
a	O
small	O
document	O
or	O
picture	O
to	O
be	O
shared	O
with	O
a	O
large	O
audience	O
.	O
</s>
<s>
In	O
the	O
overhead	B-Device
projector	I-Device
,	O
the	O
source	O
of	O
the	O
image	O
is	O
a	O
page-sized	O
sheet	O
of	O
transparent	O
plastic	O
film	O
(	O
also	O
known	O
as	O
"	O
foils	O
"	O
or	O
"	O
transparencies	B-Algorithm
"	O
)	O
with	O
the	O
image	O
to	O
be	O
projected	O
either	O
printed	O
or	O
hand-written/drawn	O
.	O
</s>
<s>
These	O
are	O
placed	O
on	O
the	O
glass	O
platen	O
of	O
the	O
projector	B-Application
,	O
which	O
has	O
a	O
light	O
source	O
below	O
it	O
and	O
a	O
projecting	O
mirror	O
and	O
lens	O
assembly	O
above	O
it	O
(	O
hence	O
,	O
"	O
overhead	O
"	O
)	O
.	O
</s>
<s>
They	O
were	O
widely	O
used	O
in	O
education	O
and	O
business	O
before	O
the	O
advent	O
of	O
video	B-Device
projectors	I-Device
.	O
</s>
<s>
An	O
overhead	B-Device
projector	I-Device
works	O
on	O
the	O
same	O
principle	O
as	O
a	O
slide	B-Device
projector	I-Device
,	O
in	O
which	O
a	O
focusing	O
lens	O
projects	O
light	O
from	O
an	O
illuminated	O
slide	O
onto	O
a	O
projection	O
screen	O
where	O
a	O
real	O
image	O
is	O
formed	O
.	O
</s>
<s>
However	O
some	O
differences	O
are	O
necessitated	O
by	O
the	O
much	O
larger	O
size	O
of	O
the	O
transparencies	B-Algorithm
used	O
(	O
generally	O
the	O
size	O
of	O
a	O
printed	O
page	O
)	O
,	O
and	O
the	O
requirement	O
that	O
the	O
transparency	B-Algorithm
be	O
placed	O
face	O
up	O
(	O
and	O
readable	O
to	O
the	O
presenter	O
)	O
.	O
</s>
<s>
For	O
the	O
latter	O
purpose	O
,	O
the	O
projector	B-Application
includes	O
a	O
mirror	O
just	O
before	O
or	O
after	O
the	O
focusing	O
lens	O
to	O
fold	O
the	O
optical	O
system	O
toward	O
the	O
horizontal	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
transparency	B-Algorithm
is	O
placed	O
face	O
up	O
(	O
toward	O
the	O
mirror	O
and	O
focusing	O
lens	O
)	O
,	O
in	O
contrast	O
with	O
a	O
35mm	O
slide	B-Device
projector	I-Device
or	O
film	B-Application
projector	I-Application
(	O
which	O
lack	O
such	O
a	O
mirror	O
)	O
where	O
the	O
slide	O
's	O
image	O
is	O
non-reversed	O
on	O
the	O
side	O
opposite	O
the	O
focusing	O
lens	O
.	O
</s>
<s>
The	O
opaque	B-General_Concept
projector	I-General_Concept
,	O
or	O
episcope	O
is	O
a	O
device	O
which	O
displays	O
opaque	O
materials	O
by	O
shining	O
a	O
bright	O
lamp	O
onto	O
the	O
object	O
from	O
above	O
.	O
</s>
<s>
The	O
episcope	O
must	O
be	O
distinguished	O
from	O
the	O
diascope	O
,	O
which	O
is	O
a	O
projector	B-Application
used	O
for	O
projecting	O
images	O
of	O
transparent	O
objects	O
(	O
such	O
as	O
films	O
or	O
slides	O
)	O
,	O
and	O
from	O
the	O
epidiascope	B-General_Concept
,	O
which	O
is	O
capable	O
of	O
projecting	O
images	O
of	O
both	O
opaque	O
and	O
transparent	O
objects	O
.	O
</s>
<s>
Because	O
the	O
focusing	O
lens	O
(	O
typically	O
less	O
than	O
in	O
diameter	O
)	O
is	O
much	O
smaller	O
than	O
the	O
transparency	B-Algorithm
,	O
a	O
crucial	O
role	O
is	O
played	O
by	O
the	O
optical	O
condenser	O
which	O
illuminates	O
the	O
transparency	B-Algorithm
.	O
</s>
<s>
Since	O
this	O
requires	O
a	O
large	O
optical	O
lens	O
(	O
at	O
least	O
the	O
size	O
of	O
the	O
transparency	B-Algorithm
)	O
but	O
may	O
be	O
of	O
poor	O
optical	O
quality	O
(	O
since	O
the	O
sharpness	O
of	O
the	O
image	O
does	O
not	O
depend	O
on	O
it	O
)	O
,	O
a	O
Fresnel	O
lens	O
is	O
employed	O
.	O
</s>
<s>
The	O
Fresnel	O
lens	O
is	O
located	O
at	O
(	O
or	O
is	O
part	O
of	O
)	O
the	O
glass	O
plate	O
on	O
which	O
the	O
transparency	B-Algorithm
is	O
placed	O
,	O
and	O
serves	O
to	O
redirect	O
most	O
of	O
the	O
light	O
hitting	O
it	O
into	O
a	O
converging	O
cone	O
toward	O
the	O
focusing	O
lens	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
normally	O
include	O
a	O
manual	O
focusing	O
mechanism	O
which	O
raises	O
and	O
lowers	O
the	O
position	O
of	O
the	O
focusing	O
lens	O
(	O
including	O
the	O
folding	O
mirror	O
)	O
in	O
order	O
to	O
adjust	O
the	O
object	O
distance	O
(	O
optical	O
distance	O
between	O
the	O
slide	O
and	O
the	O
lens	O
)	O
to	O
focus	O
at	O
the	O
chosen	O
image	O
distance	O
(	O
distance	O
to	O
the	O
projection	O
screen	O
)	O
given	O
the	O
fixed	O
focal	B-Algorithm
length	I-Algorithm
of	O
the	O
focusing	O
lens	O
.	O
</s>
<s>
Using	O
the	O
projector	B-Application
near	O
its	O
recommended	O
projection	O
distance	O
allows	O
a	O
focusing	O
position	O
where	O
this	O
is	O
avoided	O
and	O
the	O
intensity	O
across	O
the	O
screen	O
is	O
approximately	O
uniform	O
.	O
</s>
<s>
The	O
lamp	O
technology	O
of	O
an	O
overhead	B-Device
projector	I-Device
is	O
typically	O
very	O
simple	O
compared	O
to	O
a	O
modern	O
LCD	B-Device
or	O
DLP	B-General_Concept
video	B-Device
projector	I-Device
.	O
</s>
<s>
Further	O
,	O
the	O
intense	O
heat	O
accelerates	O
failure	O
of	O
the	O
high	O
intensity	O
lamp	O
,	O
often	O
burning	O
out	O
in	O
less	O
than	O
100	O
hours	O
,	O
requiring	O
replacement	O
,	O
which	O
is	O
often	O
the	O
most	O
expensive	O
part	O
of	O
owning	O
a	O
projector	B-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
modern	O
LCD	B-Device
or	O
DLP	B-General_Concept
projector	I-General_Concept
often	O
uses	O
an	O
Ultra-high-performance	O
lamp	O
which	O
has	O
a	O
higher	O
luminous	O
efficacy	O
and	O
lasts	O
for	O
thousands	O
of	O
hours	O
.	O
</s>
<s>
Older	O
overhead	B-Device
projectors	I-Device
used	O
a	O
tubular	O
quartz	O
bulb	O
which	O
was	O
mounted	O
above	O
a	O
bowl-shaped	O
polished	O
reflector	O
.	O
</s>
<s>
However	O
,	O
because	O
the	O
lamp	O
was	O
suspended	O
above	O
and	O
outside	O
the	O
reflector	O
,	O
a	O
large	O
amount	O
of	O
light	O
was	O
cast	O
to	O
the	O
sides	O
inside	O
the	O
projector	B-Application
body	O
that	O
was	O
wasted	O
,	O
thus	O
requiring	O
a	O
higher	O
power	O
lamp	O
for	O
sufficient	O
screen	O
illumination	O
.	O
</s>
<s>
More	O
modern	O
overhead	B-Device
projectors	I-Device
use	O
an	O
integrated	O
lamp	O
and	O
conical	O
reflector	O
assembly	O
,	O
allowing	O
the	O
lamp	O
to	O
be	O
located	O
deep	O
within	O
the	O
reflector	O
and	O
sending	O
a	O
greater	O
portion	O
of	O
its	O
light	O
towards	O
the	O
Fresnel	O
lens	O
;	O
this	O
permits	O
using	O
a	O
lower	O
power	O
lamp	O
for	O
the	O
same	O
screen	O
illumination	O
.	O
</s>
<s>
A	O
useful	O
innovation	O
for	O
overhead	B-Device
projectors	I-Device
with	O
integrated	O
lamps/reflectors	O
is	O
the	O
quick-swap	O
dual-lamp	O
control	O
,	O
allowing	O
two	O
lamps	O
to	O
be	O
installed	O
in	O
the	O
projector	B-Application
in	O
movable	O
sockets	O
.	O
</s>
<s>
Some	O
ancient	O
projectors	B-Application
like	O
the	O
magic	B-Device
lantern	I-Device
can	O
be	O
regarded	O
as	O
predecessors	O
of	O
the	O
overhead	B-Device
projector	I-Device
.	O
</s>
<s>
The	O
steganographic	O
mirror	O
possibly	O
came	O
closest	O
to	O
how	O
the	O
overhead	B-Device
projector	I-Device
was	O
used	O
.	O
</s>
<s>
An	O
overhead	B-Device
projector	I-Device
designed	O
by	O
American	O
scientist	O
Henry	O
Morton	O
was	O
marketed	O
around	O
1880	O
as	O
a	O
"	O
vertical	O
lantern	O
"	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
were	O
introduced	O
into	O
U.S.	O
military	O
training	O
during	O
World	O
War	O
II	O
as	O
early	O
as	O
1940	O
and	O
were	O
quickly	O
being	O
taken	O
up	O
by	O
tertiary	O
educators	O
,	O
and	O
within	O
the	O
decade	O
they	O
were	O
being	O
used	O
in	O
corporations	O
.	O
</s>
<s>
Allied	O
to	O
the	O
US	O
Navy	O
development	O
of	O
the	O
improved	O
lightweight	O
overhead	B-Device
projector	I-Device
was	O
its	O
adaptation	O
of	O
the	O
Ozalid	O
dry	O
printing	O
process	O
,	O
developed	O
in	O
Germany	O
in	O
1923	O
,	O
to	O
copy	O
training	O
documents	O
and	O
illustrations	O
on	O
projection	O
transparencies	B-Algorithm
,	O
a	O
process	O
simple	O
enough	O
to	O
be	O
carried	O
out	O
in	O
the	O
field	O
and	O
which	O
ensured	O
uniformity	O
of	O
instructional	O
material	O
used	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
were	O
used	O
early	O
on	O
for	O
police	O
work	O
with	O
a	O
cellophane	O
roll	O
over	O
a	O
9-inch	O
stage	O
,	O
allowing	O
facial	O
characteristics	O
to	O
be	O
rolled	O
across	O
the	O
stage	O
.	O
</s>
<s>
As	O
the	O
demand	O
for	O
projectors	B-Application
grew	O
,	O
Buhl	O
Industries	O
was	O
founded	O
in	O
1953	O
,	O
and	O
became	O
the	O
leading	O
US	O
contributor	O
for	O
several	O
optical	O
refinements	O
for	O
the	O
overhead	B-Device
projector	I-Device
and	O
its	O
projection	O
lens	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
began	O
to	O
be	O
widely	O
used	O
in	O
schools	O
and	O
businesses	O
in	O
the	O
late	O
1950s	O
and	O
early	O
1960s	O
,	O
beside	O
the	O
contemporaneously-developed	O
carousel	O
slide	B-Device
projectors	I-Device
with	O
a	O
horizontally	O
mounted	O
tray	O
manufactured	O
by	O
Kodak	O
.	O
</s>
<s>
In	O
the	O
late	O
1950s	O
Roger	O
Appeldorn	O
was	O
challenged	O
by	O
his	O
boss	O
at	O
3M	O
to	O
find	O
a	O
use	O
for	O
the	O
transparencies	B-Algorithm
that	O
were	O
the	O
waste	O
of	O
their	O
color	O
copy	O
process	O
.	O
</s>
<s>
Appeldorn	O
developed	O
a	O
process	O
for	O
the	O
projection	O
of	O
transparent	O
sheets	O
that	O
led	O
to	O
3M	O
's	O
first	O
marketable	O
transparency	B-Algorithm
film	O
.	O
</s>
<s>
3M	O
then	O
decided	O
to	O
develop	O
their	O
own	O
overhead	B-Device
projector	I-Device
instead	O
of	O
the	O
one	O
they	O
had	O
been	O
selling	O
until	O
then	O
,	O
which	O
was	O
produced	O
by	O
an	O
outside	O
manufacturer	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
were	O
widely	O
used	O
in	O
education	O
and	O
business	O
before	O
the	O
advent	O
of	O
computer-based	O
projection	O
.	O
</s>
<s>
The	O
overhead	B-Device
projector	I-Device
facilitates	O
an	O
easy	O
low-cost	O
interactive	O
environment	O
for	O
educators	O
.	O
</s>
<s>
This	O
saves	O
time	O
,	O
since	O
the	O
transparency	B-Algorithm
can	O
be	O
pre-printed	O
and	O
used	O
repetitively	O
,	O
rather	O
than	O
having	O
materials	O
written	O
manually	O
before	O
each	O
class	O
.	O
</s>
<s>
The	O
enlarging	O
features	O
of	O
the	O
projector	B-Application
allow	O
the	O
educator	O
to	O
write	O
in	O
a	O
comfortable	O
small	O
script	O
in	O
a	O
natural	O
writing	O
position	O
rather	O
than	O
writing	O
in	O
an	O
overly	O
large	O
script	O
on	O
a	O
blackboard	O
and	O
having	O
to	O
constantly	O
hold	O
their	O
arm	O
out	O
in	O
midair	O
to	O
write	O
on	O
the	O
blackboard	O
.	O
</s>
<s>
When	O
the	O
transparency	B-Algorithm
sheet	O
is	O
full	O
of	O
written	O
or	O
drawn	O
material	O
,	O
it	O
can	O
simply	O
be	O
replaced	O
with	O
a	O
new	O
,	O
fresh	O
sheet	O
with	O
more	O
pre-printed	O
material	O
,	O
again	O
saving	O
class	O
time	O
vs	O
a	O
blackboard	O
that	O
would	O
need	O
to	O
be	O
erased	O
and	O
teaching	O
materials	O
rewritten	O
by	O
the	O
educator	O
.	O
</s>
<s>
Following	O
the	O
class	O
period	O
,	O
the	O
transparencies	B-Algorithm
are	O
easily	O
restored	O
to	O
their	O
original	O
unused	O
state	O
by	O
washing	O
off	O
with	O
soap	O
and	O
water	O
.	O
</s>
<s>
In	O
the	O
early	O
1980s	O
–	O
1990s	O
,	O
overhead	B-Device
projectors	I-Device
were	O
used	O
as	O
part	O
of	O
a	O
classroom	O
computer	O
display/projection	O
system	O
.	O
</s>
<s>
A	O
liquid-crystal	O
panel	O
mounted	O
in	O
a	O
plastic	O
frame	O
was	O
placed	O
on	O
top	O
of	O
the	O
overhead	B-Device
projector	I-Device
and	O
connected	O
to	O
the	O
video	O
output	O
of	O
the	O
computer	O
,	O
often	O
splitting	O
off	O
the	O
normal	O
monitor	O
output	O
.	O
</s>
<s>
A	O
cooling	O
fan	O
in	O
the	O
frame	O
of	O
the	O
LCD	B-Device
panel	O
would	O
blow	O
cooling	O
air	O
across	O
the	O
LCD	B-Device
to	O
prevent	O
overheating	O
that	O
would	O
fog	O
the	O
image	O
.	O
</s>
<s>
The	O
first	O
of	O
these	O
LCD	B-Device
panels	O
were	O
monochrome-only	O
,	O
and	O
could	O
display	O
NTSC	O
video	O
output	O
such	O
as	O
from	O
an	O
Apple	B-Device
II	I-Device
computer	O
or	O
VCR	O
.	O
</s>
<s>
In	O
the	O
late	O
1980s	O
color	O
models	O
became	O
available	O
,	O
capable	O
of	O
"	O
thousands	O
"	O
of	O
colors	O
(	O
16-bit	O
color	O
)	O
,	O
for	O
the	O
color	O
Macintosh	B-Device
and	O
VGA	B-Protocol
PCs	O
.	O
</s>
<s>
The	O
Do-It-Yourself	O
community	O
has	O
started	O
using	O
this	O
idea	O
to	O
make	O
low-cost	O
home	B-Device
theater	I-Device
projectors	I-Device
.	O
</s>
<s>
By	O
removing	O
the	O
casing	O
and	O
backlight	O
assembly	O
of	O
a	O
common	O
LCD	B-Device
monitor	O
,	O
one	O
can	O
use	O
the	O
exposed	O
LCD	B-Device
screen	O
in	O
conjunction	O
with	O
the	O
overhead	B-Device
projector	I-Device
to	O
project	B-Application
the	O
contents	O
of	O
the	O
LCD	B-Device
screen	O
to	O
the	O
wall	O
at	O
a	O
much	O
lower	O
cost	O
than	O
with	O
standard	O
LCD	B-Device
projectors	I-Device
.	O
</s>
<s>
Due	O
to	O
the	O
mirroring	O
of	O
the	O
image	O
in	O
the	O
head	O
of	O
the	O
overhead	B-Device
projector	I-Device
,	O
the	O
image	O
on	O
the	O
wall	O
is	O
"	O
re-flipped	O
"	O
to	O
where	O
it	O
would	O
be	O
if	O
one	O
was	O
looking	O
at	O
the	O
LCD	B-Device
screen	O
normally	O
.	O
</s>
<s>
Overhead	B-Device
projectors	I-Device
were	O
once	O
a	O
common	O
fixture	O
in	O
most	O
classrooms	O
and	O
business	O
conference	O
rooms	O
in	O
the	O
United	O
States	O
,	O
but	O
in	O
the	O
2000s	O
they	O
were	O
slowly	O
being	O
replaced	O
by	O
document	B-Application
cameras	I-Application
,	O
dedicated	O
computer	O
projection	O
systems	O
and	O
interactive	B-Application
whiteboards	I-Application
.	O
</s>
<s>
Such	O
systems	O
allow	O
the	O
presenter	O
to	O
project	B-Application
video	O
directly	O
from	O
a	O
computer	O
file	O
,	O
typically	O
produced	O
using	O
software	O
such	O
as	O
Microsoft	B-Application
PowerPoint	I-Application
and	O
LibreOffice	B-Application
.	O
</s>
<s>
The	O
relatively	O
expensive	O
printing	O
or	O
photocopying	O
of	O
color	O
transparencies	B-Algorithm
is	O
eliminated	O
.	O
</s>
<s>
The	O
LCD	B-Device
video	O
display	O
panels	O
that	O
were	O
once	O
used	O
as	O
an	O
add-on	O
to	O
an	O
overhead	B-Device
projector	I-Device
have	O
become	O
obsolete	O
,	O
with	O
that	O
combination	O
of	O
display	O
technology	O
and	O
projection	O
optics	O
now	O
optimally	O
integrated	O
into	O
a	O
modern	O
video	B-Device
projector	I-Device
.	O
</s>
<s>
The	O
optical	O
focus	O
,	O
linearity	O
,	O
brightness	O
and	O
clarity	O
of	O
an	O
overhead	O
generally	O
cannot	O
match	O
that	O
of	O
a	O
video	B-Device
projector	I-Device
.	O
</s>
<s>
Video	B-Device
projectors	I-Device
use	O
extremely	O
small	O
picture	O
generation	O
mechanisms	O
,	O
allowing	O
for	O
precision	O
optics	O
that	O
far	O
exceed	O
the	O
plastic	O
fresnel	O
lens	O
 '	O
optical	O
performance	O
.	O
</s>
<s>
Critics	O
feel	O
that	O
there	O
are	O
some	O
downsides	O
as	O
these	O
technologies	O
are	O
more	O
prone	O
to	O
failure	O
and	O
have	O
a	O
much	O
steeper	O
learning	O
curve	O
for	O
the	O
user	O
than	O
a	O
standard	O
overhead	B-Device
projector	I-Device
.	O
</s>
