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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80

4 AUTHORS, INCLUDING:

Frank Shih

Seattle University

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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

28 7


3/7

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.

28 8


4/7

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.

28 9


5/7

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

290


6/7

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.

291


7/7

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

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