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LASER BEAM
r
REFERENCE
BEAM
Crq
MEASUREMENT
BEAM
RETROREFLECTOR
INTERFEROMETER
REFERENCE
BEAV
-
RECEIVER
DETECTS
ONLY
OVERLAPPED
PORTION
/'
MEASUREMENT BEAM
-
Figure
2-50.
Effect
of
Optics
Misalignment
A
lateral
offset
of
the
interferometer
has
the
same
effect.
Although
a
maximum
offset
of
fZ.
mm
(M.1
inch)
is
allowable,
every
effort
must
be
made
to
optimize
the
laser
beam
overla
for
maximum
performance.
If
the
measurement
beam
is
not
angularly
aligned
parallel
to
the
mechanical
motion
of
travel
of
the
retroreflector,
there
are
two
effects.
First,
a
cosine
error
is
included
in
the
measure-
ment
of
a
magnitude
directly
related
to
the
angle
of
misalignment.
(For
a
complete
descrip-
tion
of
cosine
error
refer
to
Accuracy
Considerations.)
Second,
the
angular
misalignment
also
causes
a
displacement
of
the
measurement
beam
with
respect
to
the
reference
beam
at
recom-
bination.
This
results
in
additional
signal
loss.
Figure
2-57
illustrates
the
result
of
angular
misalignment.
-
LASER
BEAM
-
RECEIVER
P
I
RETROREFLECTOR
RETROREFLECTOR
POSITION
2
iui>~f--D
DIRECTION
OF TRAVEL
-
INTERFEROMETER
Figure
2-57.
Effects
of
Angular
Misalignment
to
the
Direction
of
Travel
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