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Model
5528A
Distance Measurement
s
6-24
Distance
Measurement Error
Sources
Sources of error
in
a
Distance
measurement arc -
1.
Measurement
of
air
temperature,
pressure,
and humidit
y to determine the veloci ty
of light
(V.O .l.
Compensatio
n)
2.
Material
temperatu re
measurement
3.
Mis-alignment between
mach
ine
travel
and
Laser
Measurement
ax is (Cosine
Error)
4.
Uncompensated
measurement path
lengths
(Dcadpalh
Error)
The accu mulation of
all the
crrors
listed
above determines
the
overall
accuracy o
f Ihe
Laser
Measurement
System
in
your
application.
An
"Error
Budget
Worksheet"
has
been
incl
uded at
the end
of this
section
to hclpyou keep
track
of
each
potential error
and it
s
magnitude
for
each
measurement
you
make.
Velocity-Of-Light Compensation
Description
The
wavelength of
the light
produced
by
the
Laser
is
used
as
the reference for Dlslil
nce
measurements.
Thc waveler'8th of any
light
,
however, depends
on
the
velocity
of
li
ght
through
air.
This,
in
turn, depends
on the
temperature,
pressu re,
and
(to a
lesser
elC
tent) humidity.
Thu
s
th
e absolute accuracy of
the laser
Measu
rement
Syste
m
is
dired ly
determined
by
how
accurately
th
e
ambient
cond
itio ns are know
n.
SpeC
ifi
cally, a
Distance
meas urement
can
vary one
part
per million
for
anyone
o
f
the
following
-
•
An
air
temperature change
of
1
degree Celsi us (2
degrees Farenheit
),
or
-
•
An
air
pressure
change of
2.5
mm
(0.1
inch)
of mercury,
or
-
•
A change
in
humidity
of 80
percent.
Minimizing
Effects
The
Measurement Display unit
multiplies
the number
of
wavelengths
it
counts
(from the
laser
Head)
by
a compensation faClo
r
for
the
velocity-ol. light.
thereby
correcting for any difference
between
the
"currenl
"
veloci ty of light
(in
air)
and
Ih
e velocity of ligh
t
in
a
vacu
um.
This
calibration
factor
ca
n
be determined
and
entered either manua
lly
or
automati
cally.
Attaching
the
10751 Air
Sensor will
automatically
provide
the
calibration
data
;
otherwise,
you
mu
st
calcu
late
th
e
correct
ca
libration numbers
and
ent
er
them
man
ua lly.
(Remember
that
manual
entry
won 't provide
au tomatic co
rrection
fo
r
changing
cond itions.
)
For manual
entry o
f the
compensat ion factor
,
you
must
-
•
Determine
thc
correct
compensation factor
from Sect
ion
19,
"V.
O
.L.
Ca mpen·
sation
Factor".
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