glass meas qnde 2001
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Determination of glass thickness using laser-based ultrasound
ARTICLE · JANUARY 2001
DOI: 10.1063/1.1373771
CITATION
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4 AUTHORS, INCLUDING:
Frank Shih
Seattle University
9 PUBLICATIONS 130 CITATIONS
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Bruno Pouet
Bossa Nova Technologies, USA, Culver City
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Marvin Klein
Intelligent Optical Systems, Inc.
158 PUBLICATIONS 2,112 CITATIONS
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Available from: Marvin Klein
Retrieved on: 28 September 2015
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DETERMINATION OF GLASS THICKNESS USING LASER BASED
ULTRASOUND
Frank J. Shih
1
, Bruno F. Pouet
1
, Marvin B. Klein
1
, Andrew D. W. McKie
2
^asson Technologies, 6059 Bristol Parkway, Culver City, CA 90230, USA
2
Rockwell Science Center, 1049 Camino Do s Rios, Thousand Oaks, C A 91360, U SA
Abstract. Thickness measurements
of
glass plates
an d
glass bottles using laser-based ultrasound
LBU) a re described. Ultrasound in the glass specimens w as generated thermoelastically with either
a pulsed CO
2
laser, or a Q-switched Nd:YAG laser in the
case
o f colored glass filters. Th e detection
of
ultrasound
wa s
accomplished
by one of the following
methods;
a
spherical Fabry-Perot
interferometer system or a photo-refractive interferometer based on two-wave mixing. A self-
interference effect, utilizing
the
partial reflection from
the
front
an d
back faces
of a
glass plate
wa s
also demonstrated to have sufficient sensitivity under certain conditions. Th e thickness of the glass
plates and colored glass bottles wa s determined using the
fundamental
reverberation
frequency
obtained
from
the
time-domain
waveform
data.
LB U
results were compared
to
physical thickness
measurements
and
showed excellent agreement.
INTRODUCTION
To avoid breakage during transport, the walls of glass containers, such as bottles
and
beakers, must typically meet
a
minimum thickness tolerance. Therefore,
it is of
interest
to
measure
the
container wall thickness, especially during
the
production process,
to ensure that specifications are met. To this end, we explored the possibility of using
laser-based ultrasound LBU) techniques
[1] for
measuring
the
wall thickness
of
glass
specimens
of
varying degrees
of
curvature
and
color.
An
in-line laser-based thickness
measurement technique using laser-optical triangulation is currently available [2]. In this
technique, a laser beam is passed through the glass surface at a known angle; a portion of
the
beam
is
reflected
off the
front
surface,
while
a
portion
of the
same beam
is reflected off
the
back surface
after
transmission through
the
glass.
The
spatial separation between
the
tw o reflections
is
then used
to
calculate
the
plate thickness. However, this technique
is not
well-suited for glass containers since it works poorly with curved or non-parallel
surfaces.
Given that
the
second beam
has to reflect of f the
back
surface after
going through
the
plate,
transparency
of the
glass
can
also
be an
issue.
In
this situation
a
tunable probe laser would
be desirable to handle various colored glasses, which introduces complexity and cost.
EXPERIMENTAL
Our
approach
is to use
laser-based ultrasound
to
measure glass wall thickness.
Ultrasound
in the
glass specimens
was
generated thermoelastically with
a
pulsed
CO
2
laser,
or a Q-switched Nd:YAG laser in the case of a colored glass
filter.
The detection of
ultrasound
is
performed
by one of the following
methods:
a
spherical Fabry-Perot
CP557,
Review o f Progress
in Qua ntitative Nond estructive
Evaluat ion Vo l .
20 ed. by D. O. Thompson and D. E . Chimenti
©2001
American
Institute
of Physics
l-56396-988-2/01/ 18.00
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Wii
FIGURE I Glass specimens with varying degrees
of curvature.
00 §
FIGURE
2.
Experimental setup
of the spherical
Fabry-Perot
interferometer.
interferometer detection system coupled with an argon-ion laser, a photorefractive
interferometer based
on
two-wave mixing,
and a
self-interference technique
that can be
used
for
glass plates
with
parallel faces.
The bulk wave velocity in
glass,
for use as a
reference,
was
established
using a
conventional
immersion pulse-echo
system.
The glass
wall thickness
of various container specimens was
then
measured both
by LBU
technique
and by a
dial-caliper.
Th e types o f glass
specimens used
are shown in Figure 1. First, a flat
window
glass
is tested. The
front
and back faces of the glass plate are
nearly
parallel. This is the type of
specimen that
can be handled by the current
triangulation
technique. To
assess
th e
merit
of
laser-based
ultrasound on
glass with
curved
surfaces,
we
tested
a
1000-mL beaker,
whose
diameter is about 4 inches. Measurement was then made on several colored glass beer
bottles. Th e measurements were made at the lower portion of the
bottle,
where the shape is
cylindrical and the
diameter
of
this
region was about 2 inches.
Attempts
were even made
on a classic Coca-Cola bottle,
where
th e front and
back surface
at any given point is highly
irregular in shape.
Th e spherical Fabry-Perot setup
that
w as
used
for
each
specimen is shown in Figure
2. The excitation laser was a pulsed CO2
laser (10.6 jum).
Th e
COi laser
is a suitable
choice
since
the
glass
is opaque at the operating
wavelength
and
results
in
improved
ultrasonic generation efficiency compared with other available generation lasers. Th e
energy of the excitation beam was ~5
mJ/pulse, resulting
in a thermoelastic excitation.
Thus,
upon
inspection, no damage was observed on the
glass
specimens
after
LB U
measurement. The probe
beam,
an
argon-ion
laser, is
then
directed to the
same spot
as the
excitation
laser
beam.
Th e reflected
light
from the specimen surface is collected,
transmitted
through the
spherical
Fabry-Perot cavity, and
measured with
a
photodetector.
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U- ^ \
FIGURE
3.
Experimental setup
of the
photo-refractive
interferometer
based
on
two-wave mixing.
Another
technique w as
also explored
to look at the same
problem.
In
order
to use a
Q-switched
Nd:YAG
generation
laser that was available at
Lasson,
a KG3
color glass filter
was
used
as a specimen. The detection
system, shown
in Figure 3, is a photorefractive
interferometer based on two-wave mixing. A probe laser (CW diode-pumped,
frequency-
doubled
Nd:YAG at 532 nm), is reflected
back
from the specimen as the signal beam,
carrying
information about the
surface motion, while
a reference beam is
reflected
off a
mirror. The
signal
beam and the
reference beam then interfere
in a
photorefractive
crystal,
producing a
real-time
hologram. Th e signal beam and the diffracted reference beam are
then combined
a t a
photodiode used
for
coherent
detection.
RESULTS
ND
DISCUSSION
The
time-domain
waveform for one of the parallel glass plates is shown in
Figure
4 .
Th e
measurement
was done by spherical
Fabry-Perot
interferometer system. The
laser-
induced ultrasonic
wave reflects
back and
forth
between the two faces,
building
up a
reverberation, which causes both faces of the glass
plate
to move in and out at a
characteristic
frequency. This frequency/is
inversely
proportional
to the thickness of the
plate. Specifically, we have/=v/2L,
where
v is the P-wave velocity and L is the thickness
of the plate. From
Figure
4, the periodicity of the
time-domain
waveform is plainly
visible.
Th e
resonance
is best analyzed in the frequency
domain. Using
the
P-wave
velocity of
5.81
mm/jas
obtained
from an immersion pulse-echo measurement, the
thickness
of the
plate
is
determined
to
be2.2
mm. The experiment is then
repeated
for the
FIGURE 4 Time-domain
waveform
of the flat parallel
glass
plates.
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15 2 25 3 35 4
Time ( M S )
Frequency MHz)
FIGURE 5 .
Time-domain
waveform and the corresponding frequency spectrum of the 1000-mL beaker.
1000-mL beaker. Both
the
time-domain
waveform and the corresponding frequency
spectrum for the
beaker
are plotted in Figure 5. It can be observed that in glass with curved
surfaces,
the signal
strength
in the time-dom ain decays rapidly.
However,
the
fundamental
frequency, determined by the
glass
wall thickness, is still
distinctly
isolated in the
frequency spectrum.
Th e
next specimens were
glass
beer bottles,
which
were
the main
subject
of
interest.
These mass-produced glass
bottles
are
formed
by blowing molten
glasses into
molds. While
the outer dimension of these
glass bottles
are determined by the shape of the
molds, and are relatively well controlled, the inner dimensions of the
glass
bottle are less
uniform.
Th e specimen
used
in the
next
series of experiments is a
Heineken beer
bottle,
which has a
green color.
Th e
same
experiments
were also
p erformed on
clear
and brown
bottles
and
yielded
similar results. Two locations on the opposite
sides
of the
Heineken
bottle were marked for thickness
measurements.
Th e
time-domain
waveform and the
corresponding
frequency
spectrum
for
locations
one are
p lotted
in Figure 6 .
Th e glass
beer bottle
was then
sectioned off just
above the marked
location,
and a
physical
thickness
measurement was made
using
a dial caliper. Using the reference P-
wave velocity
of 5.81
mm/jas described earlier,
we calculate the
wall thickness using LBU,
and comp ared the two sets of measurements. The results are tabulated in Table 1. It can be
seen that
the two sets of
numbers
are in excellent agreement.
Q3 5
Q3
025
02
Q15
Q1
Q0 5
0
0
10
0 5 10
Time
(micro-second)
FIGURE 6 Time-domain
waveform
and the corresponding frequency spectrum of the glass beer bottle.
TABLE 1 Summ ary of the LBU and physical measurement results of the glass beer bottle.
Glass
bottle
wall
thickness
LB U
1.8 mm
(1.37
MHz)
2.1 mm (1.61
MHz)
Physical Measurement
1.8 mm
2.0 mm
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FIGURE 7 .
Time-domain waveform
and the corresponding
frequency
spectrum of the Coca-Cola bottle.
The same approach was then applied to a classic Coca-Cola bottle to test the
limitations
of this technique. The
interior
and
exterior
surfaces of
these classic
beverage
bottles
are highly contoured and irregular in shape. Both the
time
domain waveform and
its corresponding
frequency
spectrum, are given in
Figure
7.
Note
that the
signal strength
is
extremely
small,
on the order of 0.2 mV. The
measurement
was
achieved
only
after
signal averaging a nd
some careful
alignment.
Another
set of LBU
measurements
was made with a photorefractive interferometer
that was
based
on
two-wave
mixing. The
experimental
setup is shown in
Figure
3. For
this
set of
experiments,
a 2 mm
thick
KG3 color glass filter was u sed as the specimen.
This
filter has very high
absorption
at the
1064
nm
wavelength
of the Q-switched
Nd:YAG
generation
laser.
The time-domain
waveform
and its
corresponding frequency
spectrum
are plotted in Figure 8. The
fundamental frequency
of the reverberation is 1.25 MHz. The
fundamental peak
is
clearly
distinguishable and can be used to
determine
the thickness of
the glass plate.
Much
to our
surprise,
when the reference
beam
was removed, there was still a very
strong signal.
Th e
experimental setup, time-domain
waveform and the
corresponding
frequency spectrum
are
shown
in
Figures
9 and 10. The
frequency spectrum
gives the
same
fundamental frequency as in
Figure
8. Therefore, the
thickness
can also be detected
with
jus t a photodetector.
Frequency Spectrum
Fundamental frequency
gives
thi kness
4 6 8 TO
Frequency MHz)
FIGURE
8
Time-domain waveform
and the
corresponding frequency spectrum
of the
2-mm
thick KG3,
obtained using a two-wave mixing scheme.
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Probe
Laser
Speckled
Signal Beam
Detector
Color Glass
Filter
FIGURE 9 Experimental setup in which the
reference
beam was removed from the two-wave mixing setup.
0 . 0 -
a
> 0 . 2 ~
-0.4-
40
30
20-
10
o r̂—
-
——
— — —
— — —
— — —
r i mu3? u ofjf̂ viy u
i
1 1
^
i l «
'*
i<
*
M f m S
°^^
„
^ \^
0 2 4 6 8 10
0 5 10 IS
Time
0/S)
rf€Mt|piftey
M H z )
FIGURE 1 0 Time-domain waveform and the corresponding frequency spectrum of the 2 - m m thick K G 3 ,
obtained using just a photodetector.
Th e same
procedure
w as
repeated
on a
piece
of
window glass.
A
pulsed
C C > 2
laser
was used as the excitation laser, while an argon-ion laser was used as the probe laser. Th e
result
was
similar.
The frequency-doubled CW Nd:YAG
probe laser 5 3 2
nm
actually
transmits fairly well in KG3 glass as
does
the argon-ion
laser
in plain
window glass.
Since
the probe
beam
transmits in the glass
itself,
there is a
self-interference effect,
in which the
glass acts as its own low-finesse etalon.
Specifically, the excitation laser
generates
an ultrasonic resonance in the glass plate,
in
which
the thickness changes periodically. The probe laser partially reflects back and
forth
in the
glass, causing multiple-beam interference.
The
reflected wave
is
then picked
up by a photodetector,
resulting
in a plot like the one shown in Figure 10. The advantag e
of having such
a
system
is its
low-cost
and simplicity. We also
attempted
to use
this
technique on curved
surfaces,
but it was
less
successful. For best results
with this self-
interference method, the two faces of the glass
have
to be reasonably parallel.
CONCLUSION
W e have demonstrated the capability of accurately measuring the
thickness
of
glass
of varying degree of
curvature
using L B U . Th e
described
self-interfering etalon effect
works
well in
transparent
materials with nearly
parallel
surfaces, but is
more
difficult to
implement in
curved surfaces.
In-line determination of glass bottle
thickness
using LBU
appears
feasible.
R F R N S
1.
Scruby,
C. B.,
Drain,
L. E.,
Laser-Ultrasonics: Techniques and
Applications
Adam Hilger, Bristol,
2.
McCullough,
R. W., Bondurant, P. D., Doyle, J. L.,
Ma terials Eva lua t i on
53, 1338-1345 (1995).
3. McKie, A. D. W., Addison, R. C., in Rev i ew o f P rogress
in
Quant i ta t ive Nondestruc t ive
Evaluations
edited by D. O. Thompson, Plenum
Press,
New York, 1 9 9 5 , Vol 14, pp.
523-528
4.
McKie,
A. D. W.,
Addison,
R. C., Ultrasonics 32, 333 (1994).
292