Model
5528A
Di
stance Measurements
6-24
Di
stance Measurement Error Sources
Sources
of
error in a Distance measurement arc -
1.
Measurement
of
air tempera
tur
e,
pressure, and humidity to determine the
ve
locity of light
(V.O.
l.
Compensation)
2.
Material temperature measurement
3.
Mis-alignment between
ma
chine travel and
Lase
r Measurement
ax
is (
Cos
ine Error)
4. Uncompensated
mea
s
ur
ement path lengths (Dcadpa
lh
Error)
The accumulation
of
all the
crror
s listed abo
ve
determin
es
the overa
ll
acc
ur
acy
of Ihe
La
se
r
Measurement
System
in yo
ur
application.
An "E
rror
Budget
Work
sheet" h
as
been included
at
the end
of
this section to
hclpyou
keep track
of
each potential
error
and its magnitude f
or
eac
h measurement you make.
Velocity-Of-Light Compensation
Description
The wavelength
of
the light
produ
ced by the
Laser
is
used
as
the reference for D
ls
lilnce
measurements.
Thc
waveler
'8th
of
any
light
,
how
ever, depends on the velocity
of
light
through
air. Thi
s,
in
turn
,
depend
s
on
the temperature,
pr
essu
r
e,
and (to a l
esse
r
elC
tent)
humidit
y.
Thus the
abso
lut
e accuracy
of
the laser
Mea
su
rement
Syste
m
is
dir
ed
ly determined
by
how
accurately the ambient co
nd
itions are
kno
wn. SpeCifically, a Distance m
eas
urement c
an
v
ar
y
one
pa
rt
per
million
f
or
anyo
ne of the
following
-
• An
a
ir
te
mp
era
tur
e change
of
1 degree
Ce
l
si
us (2 degrees Farenheit), or -
• An a
ir
pr
ess
ur
e change
of
2.5
mm
(0.1 inch)
of
merc
ur
y, or -
• A change in
humi
di
ty of 80
pe
r
cen
t.
Minimizing
Eff
ects
The Measurement
Di
splay unit
multipli
es
the number of wavelengths it counts (
from
the
la
se
r
Head) by
a compensati
on
f
aClo
r
for
the
velocity-ol.
li
ght.
thereb
y correc
ting
for
any
difference
between the
"c
urr
e
nl
"
ve
locity
of
li
ght (
in
a
ir
) and Ihe
ve
locity
of
light
in
a vacu
um
.
This
ca
libration
fa
c
tor
ca
n be determined a
nd
entered either manually or automatically.
Attaching the
10
75
1 Air
Se
n
so
r
wi
ll automatically
pr
ovide the calibration data; otherwise, you
mu
st calculate the correct
ca
l
ib
rat
ion
numb
ers
and enter them man
ua
ll
y.
(Reme
mb
er that
manual e
ntr
y
won
't
provide
au
t
omatic
correction fo r changing conditions.)
For
manual e
ntr
y of the compens
at
io
n factor, you
mu
st -
• Dete
rmin
e
thc
correct
co
mpensation
fa
c
tor
fr
om
Sect
ion 1
9,
"V.
O
.L.
Ca
mp
en·
sa
tion Factor
".
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