<s>
A	O
reference	B-Algorithm
beam	I-Algorithm
is	O
a	O
laser	O
beam	O
used	O
to	O
read	O
and	O
write	O
holograms	B-Algorithm
.	O
</s>
<s>
It	O
is	O
one	O
of	O
two	O
laser	O
beams	O
used	O
to	O
create	O
a	O
hologram	B-Algorithm
.	O
</s>
<s>
In	O
order	O
to	O
read	O
a	O
hologram	B-Algorithm
out	O
,	O
some	O
aspects	O
of	O
the	O
reference	B-Algorithm
beam	I-Algorithm
(	O
namely	O
its	O
angle	O
of	O
incidence	O
,	O
beam	O
profile	O
and	O
wavelength	O
)	O
must	O
be	O
reproduced	O
exactly	O
as	O
when	O
it	O
was	O
used	O
to	O
write	O
the	O
hologram	B-Algorithm
.	O
</s>
<s>
As	O
a	O
result	O
,	O
usually	O
reference	B-Algorithm
beams	I-Algorithm
are	O
Gaussian	O
beams	O
or	O
spherical	O
wave	O
beams	O
(	O
beams	O
that	O
radiate	O
from	O
a	O
single	O
point	O
)	O
which	O
are	O
fairly	O
easy	O
to	O
reproduce	O
.	O
</s>
<s>
The	O
other	O
beam	O
used	O
to	O
write	O
a	O
hologram	B-Algorithm
is	O
the	O
signal	O
beam	O
or	O
object	O
beam	O
.	O
</s>
