<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	O
used	O
for	O
projecting	O
images	O
of	O
transparent	O
objects	O
(	O
such	O
as	O
films	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
they	O
must	O
project	O
the	O
reflected	O
light	O
,	O
opaque	B-General_Concept
projectors	I-General_Concept
require	O
brighter	O
bulbs	O
and	O
larger	O
lenses	O
than	O
overhead	B-Device
projectors	I-Device
.	O
</s>
<s>
Opaque	B-General_Concept
projectors	I-General_Concept
are	O
not	O
as	O
common	O
as	O
the	O
overhead	B-Device
projector	I-Device
.	O
</s>
<s>
Opaque	B-General_Concept
projectors	I-General_Concept
are	O
typically	O
used	O
to	O
project	O
images	O
of	O
book	O
pages	O
,	O
drawings	O
,	O
mineral	O
specimens	O
,	O
leaves	O
,	O
etc	O
.	O
</s>
<s>
Swiss	O
mathematician	O
,	O
physicist	O
,	O
astronomer	O
,	O
logician	O
and	O
engineer	O
Leonhard	O
Euler	O
demonstrated	O
an	O
opaque	B-General_Concept
projector	I-General_Concept
around	O
1756	O
.	O
</s>
<s>
Around	O
1872	O
Henry	O
Morton	O
used	O
an	O
opaque	B-General_Concept
projector	I-General_Concept
in	O
demonstrations	O
for	O
huge	O
audiences	O
,	O
for	O
example	O
in	O
the	O
Philadelphia	O
Opera	O
House	O
which	O
could	O
seat	O
3500	O
people	O
.	O
</s>
<s>
The	O
light	O
source	O
in	O
early	O
opaque	B-General_Concept
projectors	I-General_Concept
was	O
often	O
limelight	O
.	O
</s>
<s>
In	O
the	O
early	O
and	O
middle	O
parts	O
of	O
the	O
20th	O
century	O
,	O
low-cost	O
opaque	B-General_Concept
projectors	I-General_Concept
were	O
produced	O
and	O
marketed	O
as	O
toys	O
for	O
children	O
.	O
</s>
<s>
In	O
educational	O
settings	O
,	O
the	O
specific	O
role	O
of	O
the	O
opaque	B-General_Concept
projector	I-General_Concept
has	O
been	O
superseded	O
first	O
by	O
the	O
overhead	B-Device
projector	I-Device
and	O
later	O
the	O
document	B-Application
camera	I-Application
,	O
a	O
lighted	O
table	O
with	O
a	O
fixed	O
video	O
camera	O
above	O
it	O
.	O
</s>
<s>
The	O
document	B-Application
camera	I-Application
is	O
also	O
called	O
a	O
desktop	O
presenter	O
unit	O
or	O
opaque	B-General_Concept
projector	I-General_Concept
.	O
</s>
<s>
Opaque	B-General_Concept
projectors	I-General_Concept
are	O
still	O
in	O
use	O
for	O
tracing	O
,	O
called	O
tracing	O
projectors	O
.	O
</s>
<s>
Two	O
main	O
classes	O
of	O
opaque	B-General_Concept
projectors	I-General_Concept
thus	O
existed	O
:	O
</s>
