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Characterization of ultrasonic transducers using cholesteric liquid crystals
Characterization of ultrasonic transducers using cholesteric liquid crystals R. DENIS
This paper relates to two methods for the characterization of ultrasonic transducers by means of a visualization technique which exploits the thermotropic properties of cholesteric liquid crystals. Examples illustrating these methods are used to demonstrate the validity of the results obtained whilst maintaining the advantages of a simple and economic technique.
Introduction
What is a liquid crystal?
The manufacturers and users of transducers, in all the fields in which ultrasonic techniques are applied, must be able to have means of controlling the quality of their apparatus. The characterization of a transducer is, in any case, a very important problem to solve and the visualization of the ultrasonic beam is certainly a very practical means of control. The visualization techniques at present available entail relatively costly installations. Of these, the ‘Schlieren’ technique exploits the optical diffraction properties of a medium crossed by an ultrasonic wave and enables a black and white visualization to be made of the profile of the beam emitted by the transducer’ ; an index of intensity is available in the colour version.
The fourth state of matter, intermediate between the crystalline solid and the isotropic liquid is commonly called ‘liquid crystal’ or more exactly mesophase. Certain organic composites may have one, or sometimes two, mesomorphic phases within a well defined temperature interval depending on the nature of the composite.
Then, there are the surface techniques which allow the cross sections of the ultrasonic beam to be observed and are based on the fact that light reflected on the surface of a liquid is modified by the presence of an acoustic wave’ . In the last few years, research workers in this field have been interested in setting up techniques based on the particular properties of liquid crystals’ 4. We have directed our work towards the application of the thermotropic properties of cholesteric liquid crystals, with the aim of providing the users of ultrasonic transducers with a simple and inexpensive means of controlling their apparatus.
Mesomorphic structures
There are three types of mesomorphic
structures6J.
The smectic structure, from the greek ‘smegma’ (soap). This is the characteristic structure of the soap bubble. The molecules are located in a direction parallel to their main axis in unimolecular layers. They move perpendicularly to the direction of their main axis. (Fig. 1). The nematic structure, from the greek ‘nema’ (wire). Long fnaments can be seen under a microscope. The molecules are located in a direction parallel to their main axis and move in this direction. (Fig. 2) The cholesteric structure. This can be considered as a particular case of nematic structure. It is typical of cholesterol esters from which it takes its name. The molecules are set in a direction parallel to their main axis, in parallel planes
Liquid crystals Liquid crystals although discovered at the end of the last century, were for a long time listed among laboratory curiosities. It was not until 1958 that an effective study programme was undertaken by the Westinghouse Research Laboratory with Fergason' . Since then, a large number of applications have been developed, especially in the nondestructive control of materials as well as in medicine, aerospace, electronics, optics, etcs7
The author is at the Joint Research Centre, Materials Division, NDT Section, lspra Establishment, Italy, Paper received 19 September 1977.
ULTRASONICS.
JANUARY
1978
Fig. 1
Smectic structure
37
This is a medical type transducer with a nominal operating frequency of 1.5 MHz. After having carefully degreased the surface of the lens, a coat of black paint is applied with the aim of absorbing the non-reflected part of the incident light after it has passed through the layer of liquid crystals (Fig. 4b). Normally a hydrosoluble paint is used which has the advantage of not being attacked by the liquid crystal solvent and, in addition, will be easy to remove with water once the measurement has been made. The paint dries in ten to fifteen minutes in a normal atmosphere but drying can be easily accelerated by a current of hot air. Fig. 2
Nematic
structure
The layer of liquid crystals can then be applied(Fig. 4~). These crystals are in the form of a solution in a solvent, chloroform for example, it will thus be possible to apply them easily by aerosol or more simply with a brush. Before proceeding with the next phase, it is very important to ascertain that the solvent has completely evaporated. A protective Mylar sheet will then be applied to the layer (Fig. 4d) in order to protect the liquid crystals from possible contamination by the dust in the atmosphere as well as by the gasses or vapours of volatile products which could chemically react with the liquid crystals. The film of Mylar also plays an essential role in the preparation of a sensitive film, especially for the homogenization of the surface and of the thickness of the layer of liquid crystals. Excellent results can be obtained using a nylon roller providing a slightly higher temperature than the fusion temperature of liquid crystal is used.
Fig. 3
Cholesteric
structure
which are oriented with respect to one another by a variable angle going round a perpendicular axis, thus composing a helical structure. (Fig. 3) Liquid crystal properties
As in the case of smectic and nematic structures, the cholesteric structure is birefringent. Unlike the other two, it is optically negative, that is to say, the velocity of light perpendicular to the layers is greater than that parallel. It has a considerable rotatory power (up to 60,000 degrees mm-’ ) and an extremely selective optic diffusion, which is very sensitive to temperature as well as to pressure and chemical pollutant?. We will return to discuss these properties in more detail in the next section. Characterization
of ultrasonic transducers
Two methods have already been tested. They are both based on the thermotropic properties of cholesteric liquid crystals. Direct method
Basis-The first method which we shall call the ‘direct method’ consists of observing a layer of liquid crystals on the lens of the transducer to be characterized. The heat will be supplied by the absorption of the acoustic wave at the lens-air interface and the visualization of the thermal flux will be possible thanks to the selective reflection of the light through the layer of liquid crystals. Preparation of the transducer-This technique is suitable for the characterization of plane lens transducers such as the one which was used to illustrate the method (Fig.4a).
38
We would point out that the thickness of the layer of liquid crystals is generally 15-20 pm, and that of the Mylar sheet 12 pm. Taking into account the small quantity of heat available and to maximise the advantage of the great sensitivity of the liquid crystals, it would be of interest to work at a temperature that is very close or just slightly above room temperature. For this reason, a mixture of cholesteric esters will be used so that the mesophase will take place at this temperature. Generally, in the laboratory, the dosage of this mixture is extremely difficult but precautions can be taken if the differences in temperature are about one degree centigrade - spraying alcohol on to the Mylar film if the transducer is too hot - if it is too cold, the transducer may be heated by supplying it earlier at its approximate operating frequency during the time required for the mesophase to take place. Example of the characterization of a plane lens transducerThe installation includes (Fig. 5); - a function generator; - a frequency meter, which is not essential if one is satisfied with the precision provided by the generator vernier reading; - an optical fibre illuminator which has the advantage of giving a cold light. The measurement consists of powering the transducer while varying the applied frequency close to the nominal one and observing the changes in colour of the surface of the liquid crystal film. The latter will reflect, depending on the temperature, a wavelength going from red to violet changing through yellow, green and blue. The range of temperature to which the entire spectrum corresponds, depends on the dosage of the cholesteric mixture and may vary from half a degree to several tens of degrees centigrade. In this way,
ULTRASONICS.
JANUARY
1978
Fig. 4
Direct method
of preparation
it is possible to detect on the one hand, the hot points on the surface of the lens which correspond to heterogeneities of the lens or of the crystal and on the other, the energy peaks at the corresponding frequency. Results-The following figures give the results of measurements made on the medica transducer prepared as described earlier with a cholesteric mixture, the mesophase of which takes place between 22’ and 24°C. At a frequency of 1.18 MHz, the surface becomes slightly heated with a hotter point at the centre. (Fig. 6a)
of the transducer
can at once be stated that this method gives a precise characterization of the plane lens transducer, stressing on the one hand, the energy peaks at the corresponding frequencies and on the other, some defects presented by the transducer. (Fig. 7) It is expected that this method could be improved by working in a thermostatic ambient with a liquid crystal, the thermal characteristics of which have been perfectly determined. It will thus be possible to obtain quantitative measurements that are exact for the different levels of the energy peaks.
At 1.43 MHz, an energy peak occurs; the hottest point at the centre corresponds to a heterogeneity of the lens. (Fig. 6b) A maximum energy peak occurs at 1.47 MHz with a reasonably good distribution. (Fig. 6c) At a frequency of 1.5 MHz, a decrease in the energy level is ascertained. (Fig. 6d) Another measurement at the same frequency and the defect mentioned previously is clearly noticed. (Fig. 6e) At a frequency of 2.27 MHz, which is not close to the nominal frequency, the hot point corresponding to the lens defect disappears and another hot point appears probably caused by a heterogeneity at the crystal level. (Fig. 6f) Another energy peak is obtained MHz. (Fig. 6g)
at a frequency
of 4.67
By comparing these results and those illustrated in the diagram obtained by spectrometry on the same transducer, it
ULTRASONICS.
JANUARY
1978
Fig. 5
Characterization
of a plane lens transducer
-
installation
39
b
*
1.50 MHZ
blur
1.43
MHz
bb.10 lOlla
rod
C
red
~dlow
viqlri
I. 47 MHZ
blue
4.67MHz
d
blyr
1.50
MHz
r:d
blue
I
red Plane Ions transducer
Frequency Crystal
Nominal
Actual
I.SMHz
1.47 MHz
diameter
20mm Icm
I
I
Fig. 6
Direct
method
-
characterization
of a plane lens transduce!
I
I
Liquid crystals cc~er~~cn cell
Thermostated water out
I
Transducer
Contain&
II
\
Interchangeable ring ’
Transducer suppert
T
Thermostated water in
Fig. 8
0 0.1
I
1.41
3
2
4
4.65
[MHz] Fig. 7
Comparison of the results obtained by spectrometry those obtained by the ‘direct method’
and
Tomography method
Basis-The wavelength reflected by a liquid crystal film depends not only on the temperature but also on the angle of incidence the luminous rays make with the observed surface. If the latter is not plane, the difect method cannot be applied. Furthermore, since it entails visualizing the ultrasonic beam itself in the case, for example, of a focusing transducer, the second method wiIl have to be resorted to. This uses the same thermotropic properties of
40
Tomography
method -
thermostated
container
the cholesteric liquid crystal and consists of effecting some tomographies of the ultrasonic beam. Placed in a thermostatic container composed of a Plexiglas cylinder, the transducer moves horizontally (Fig. 8). The front of the container is composed of a plane cell producing a coloured image of the cross-section of the beam emitted by the transducer. The back of the container is hermetically sealed by a flange in which the support of the transducer can slide. Description of the installation-As for the preceding method, the apparatus used consists of a function generator, a frequency meter which, as for the first method, is not essential and an optical fibre illuminator. (see Fig. 9). The installation is completed by a circuit of thermostatic water, a mechanism for the horizontal and angular displacement system of the transducer and a position indicator.
U LTRASON ICS . JANUARY
1978
Preparation of the acoustical to optical conversion cellThe layer of liquid crystal is sandwiched between an optical glass disc and a thin Mylar sheet on the back of which a coating of paraffin is applied to act as an absorber of the ultrasonic wave and to transmit the heat produced to the layer of liquid crystals (Fig. 10).
b
The preparation technique of liquid crystal films is similar to that used for the direct method, except for the dark ground which is here formed by making the container itself opaque using a coating of optic black. A cholosteric mixture is used, the mesophase of which takes place at a temperature close to ambient. In this way, the gradients in the container are minimized. The absorbing paraffin coating may consist of a sheet applied on the Mylar film after some Vaseline oil has been applied in order to superficially dissolve the paraffin and to ensure that the latter completely adheres to the Mylar film. As another solution a paraffin, the melting point of which is approximately SO”C, could be used. Experience has shown that such overheating does not modify the characteristics of the liquid crystals. Example of the characterization of a focusing transducerIn order to illustrate the tomography method, a transducer was used, the characteristics of which supplied by the manufacturer are given in Table 1. Since the transducer is fixed on the sliding support and angularly ascertained, a preliminary examination is made of the beam in the area close to the lens by varying the applied frequency to approximately the nominal frequency. The limit operating frequencies of the transducer are thus determined, especially the frequency at which the maximum is emitted. (Fig. 11)
Fig. 9
Tomography method - installations. a - Function generator; b - Frequency meter; c - Optical fibre illuminator; d - Thermostated container; e - Displacement system and position indicator; f - Circuit of thermostated water
ri
The transducer is then moved to the approximate focal distance by supplying it at the previously defined frequency and by reducing the amplitude in order to obtain the minimum diameter of the violet spot. The whole beam is then inspected. (Fig. 12)
Optic glass(Imm) Mylar spacer (15 to 20pm) Mylar sheet (25 lo 50pm)
Results-In order to give the results of this measurement in a more concrete way, a longitudinal section of the beam has been represented, on the basis of the tomographies. (Fig. 13).This picture shows the division of the acoustic intensities along the beam and particularly emphasizes the focal area (from 55 - 65 mm). Also, a null intensity area can be observed (from 10 - 30 mm). It has thus been possible to define with this method, on the one hand, the frequency band to which the transducer responds as well as the frequency corresponding to the maximum acoustic intensity and on the other, the distribution of the acoustic intensities along the ultrasonic beam. Lastly, providing a reticle is centred on the mechanical axis of the transducer support, it is possible to obtain
Table
1.
Characteristics
Operating
Focal length Crystal
Fig. 10 Tomography
method -
acoustical-to-optical
conversion
cell
of the optical axis of the
Actual
Conclusion
5MHz
4.93
76.2 mm
52 mm
The comparison of the results obtained by this method to those obtained with the ‘Schlieren’ technique demonstrated that it is possible to obtain an exact characterization of an ultrasonic transducer using a much cheaper installation (Fig. 14).
diameter
12.7 mm
JANUARY
layer(-O.lmm)
Liquid crysfals
Nominal
Case and lens diameter
ULTRASONICS.
IAbsorbent
an exact geometric definition beam.
of the transducer
frequency
1
16mm
1978
41
3MHr
4.93
MHz
7 MHz
Fig. 11 Tomography
4MHt
rad \
green
VlOkl
green \
violet \
method
-
blue
grren
red
vlolrt \
rod \
characterization
of a focusing transducer
In conclusion, thanks to cholesteric liquid crystals, it is possible to make available to manufacturers and users of ultrasonic transducers, a visualization technique with which they can effect complete, exact and cheap characterizations of their transducers.
42
rrd
- determination
blue
red
green
I
of the limits of operating
red
\
I
frequencies
References 1 2
Greguss, P. Real time acoustical to optical convertors, State of the Art, Ultras Int 1973 Conference Proc (1973) 265 Greguss, P. New Liquid crystals acoustical to optical display, Acustica, 29 (1973) 52
ULTRASONICS.
JANUARY
1978
d 5
d 40
d
red
blu,e
I
rod
Fig. 12 Tomography
method
-
characterization
30
40
d 70
d 100
d SO
red blue vlolet
d80
de0
d50
blur violet
50
of a focusing transducer
- visualization
of the beam
12 II IC 9 6 7 6 5 4 3 2 F’ LO I 2 3 4 5 6 7 6 9 IC II 12
,Y , , 01
5
IO
I
I
20
50
Ml
I
60
Fig. 13 Tomography method - characterization - longitudinal section of the beam
3
4
70
I
60
90
of a focusing transducer
Cook, D., Werchan, R.E. Mapping ultrasonic fields with cholesteric liquid crystals, Ultrasonics, 9 (2) (1971), 101
JANUARY
1978
01
5 IO
I
I
20
30
I
40
50
60
I
I
70
60
I
90
loo
6 m3
Manaranche, J.C., Henri, P. Vizualization of an ultrasonic beam by means of liquid crystals, Btit J of NDT, 18 (4) (July 1976) 107
ULTRASONICS.
100
Fig. 14
5 6 7
Characterization of a focusing transducer, the ‘Schlieren’ technique
comparison
with
Fergason, J. L. Liquid crystals in NDT, Applied Optics, 7 (9) (1968) Assoultine, G., Leiba, E. Cristaux liquides, Revenue technique Thomson - CSF 1 (4) (1969) J.C. Les cristaux liquides et le con&e non destructifs, Mesure, r~~lution,automatisme, 36 (7) (1971)
Manaranche
43
This paper relates to two methods for the characterization of ultrasonic transducers by means of a visualization technique which exploits the thermotropic properties of cholesteric liquid crystals. Examples illustrating these methods are used to demonstrate the validity of the results obtained whilst maintaining the advantages of a simple and economic technique.
Introduction
What is a liquid crystal?
The manufacturers and users of transducers, in all the fields in which ultrasonic techniques are applied, must be able to have means of controlling the quality of their apparatus. The characterization of a transducer is, in any case, a very important problem to solve and the visualization of the ultrasonic beam is certainly a very practical means of control. The visualization techniques at present available entail relatively costly installations. Of these, the ‘Schlieren’ technique exploits the optical diffraction properties of a medium crossed by an ultrasonic wave and enables a black and white visualization to be made of the profile of the beam emitted by the transducer’ ; an index of intensity is available in the colour version.
The fourth state of matter, intermediate between the crystalline solid and the isotropic liquid is commonly called ‘liquid crystal’ or more exactly mesophase. Certain organic composites may have one, or sometimes two, mesomorphic phases within a well defined temperature interval depending on the nature of the composite.
Then, there are the surface techniques which allow the cross sections of the ultrasonic beam to be observed and are based on the fact that light reflected on the surface of a liquid is modified by the presence of an acoustic wave’ . In the last few years, research workers in this field have been interested in setting up techniques based on the particular properties of liquid crystals’ 4. We have directed our work towards the application of the thermotropic properties of cholesteric liquid crystals, with the aim of providing the users of ultrasonic transducers with a simple and inexpensive means of controlling their apparatus.
Mesomorphic structures
There are three types of mesomorphic
structures6J.
The smectic structure, from the greek ‘smegma’ (soap). This is the characteristic structure of the soap bubble. The molecules are located in a direction parallel to their main axis in unimolecular layers. They move perpendicularly to the direction of their main axis. (Fig. 1). The nematic structure, from the greek ‘nema’ (wire). Long fnaments can be seen under a microscope. The molecules are located in a direction parallel to their main axis and move in this direction. (Fig. 2) The cholesteric structure. This can be considered as a particular case of nematic structure. It is typical of cholesterol esters from which it takes its name. The molecules are set in a direction parallel to their main axis, in parallel planes
Liquid crystals Liquid crystals although discovered at the end of the last century, were for a long time listed among laboratory curiosities. It was not until 1958 that an effective study programme was undertaken by the Westinghouse Research Laboratory with Fergason' . Since then, a large number of applications have been developed, especially in the nondestructive control of materials as well as in medicine, aerospace, electronics, optics, etcs7
The author is at the Joint Research Centre, Materials Division, NDT Section, lspra Establishment, Italy, Paper received 19 September 1977.
ULTRASONICS.
JANUARY
1978
Fig. 1
Smectic structure
37
This is a medical type transducer with a nominal operating frequency of 1.5 MHz. After having carefully degreased the surface of the lens, a coat of black paint is applied with the aim of absorbing the non-reflected part of the incident light after it has passed through the layer of liquid crystals (Fig. 4b). Normally a hydrosoluble paint is used which has the advantage of not being attacked by the liquid crystal solvent and, in addition, will be easy to remove with water once the measurement has been made. The paint dries in ten to fifteen minutes in a normal atmosphere but drying can be easily accelerated by a current of hot air. Fig. 2
Nematic
structure
The layer of liquid crystals can then be applied(Fig. 4~). These crystals are in the form of a solution in a solvent, chloroform for example, it will thus be possible to apply them easily by aerosol or more simply with a brush. Before proceeding with the next phase, it is very important to ascertain that the solvent has completely evaporated. A protective Mylar sheet will then be applied to the layer (Fig. 4d) in order to protect the liquid crystals from possible contamination by the dust in the atmosphere as well as by the gasses or vapours of volatile products which could chemically react with the liquid crystals. The film of Mylar also plays an essential role in the preparation of a sensitive film, especially for the homogenization of the surface and of the thickness of the layer of liquid crystals. Excellent results can be obtained using a nylon roller providing a slightly higher temperature than the fusion temperature of liquid crystal is used.
Fig. 3
Cholesteric
structure
which are oriented with respect to one another by a variable angle going round a perpendicular axis, thus composing a helical structure. (Fig. 3) Liquid crystal properties
As in the case of smectic and nematic structures, the cholesteric structure is birefringent. Unlike the other two, it is optically negative, that is to say, the velocity of light perpendicular to the layers is greater than that parallel. It has a considerable rotatory power (up to 60,000 degrees mm-’ ) and an extremely selective optic diffusion, which is very sensitive to temperature as well as to pressure and chemical pollutant?. We will return to discuss these properties in more detail in the next section. Characterization
of ultrasonic transducers
Two methods have already been tested. They are both based on the thermotropic properties of cholesteric liquid crystals. Direct method
Basis-The first method which we shall call the ‘direct method’ consists of observing a layer of liquid crystals on the lens of the transducer to be characterized. The heat will be supplied by the absorption of the acoustic wave at the lens-air interface and the visualization of the thermal flux will be possible thanks to the selective reflection of the light through the layer of liquid crystals. Preparation of the transducer-This technique is suitable for the characterization of plane lens transducers such as the one which was used to illustrate the method (Fig.4a).
38
We would point out that the thickness of the layer of liquid crystals is generally 15-20 pm, and that of the Mylar sheet 12 pm. Taking into account the small quantity of heat available and to maximise the advantage of the great sensitivity of the liquid crystals, it would be of interest to work at a temperature that is very close or just slightly above room temperature. For this reason, a mixture of cholesteric esters will be used so that the mesophase will take place at this temperature. Generally, in the laboratory, the dosage of this mixture is extremely difficult but precautions can be taken if the differences in temperature are about one degree centigrade - spraying alcohol on to the Mylar film if the transducer is too hot - if it is too cold, the transducer may be heated by supplying it earlier at its approximate operating frequency during the time required for the mesophase to take place. Example of the characterization of a plane lens transducerThe installation includes (Fig. 5); - a function generator; - a frequency meter, which is not essential if one is satisfied with the precision provided by the generator vernier reading; - an optical fibre illuminator which has the advantage of giving a cold light. The measurement consists of powering the transducer while varying the applied frequency close to the nominal one and observing the changes in colour of the surface of the liquid crystal film. The latter will reflect, depending on the temperature, a wavelength going from red to violet changing through yellow, green and blue. The range of temperature to which the entire spectrum corresponds, depends on the dosage of the cholesteric mixture and may vary from half a degree to several tens of degrees centigrade. In this way,
ULTRASONICS.
JANUARY
1978
Fig. 4
Direct method
of preparation
it is possible to detect on the one hand, the hot points on the surface of the lens which correspond to heterogeneities of the lens or of the crystal and on the other, the energy peaks at the corresponding frequency. Results-The following figures give the results of measurements made on the medica transducer prepared as described earlier with a cholesteric mixture, the mesophase of which takes place between 22’ and 24°C. At a frequency of 1.18 MHz, the surface becomes slightly heated with a hotter point at the centre. (Fig. 6a)
of the transducer
can at once be stated that this method gives a precise characterization of the plane lens transducer, stressing on the one hand, the energy peaks at the corresponding frequencies and on the other, some defects presented by the transducer. (Fig. 7) It is expected that this method could be improved by working in a thermostatic ambient with a liquid crystal, the thermal characteristics of which have been perfectly determined. It will thus be possible to obtain quantitative measurements that are exact for the different levels of the energy peaks.
At 1.43 MHz, an energy peak occurs; the hottest point at the centre corresponds to a heterogeneity of the lens. (Fig. 6b) A maximum energy peak occurs at 1.47 MHz with a reasonably good distribution. (Fig. 6c) At a frequency of 1.5 MHz, a decrease in the energy level is ascertained. (Fig. 6d) Another measurement at the same frequency and the defect mentioned previously is clearly noticed. (Fig. 6e) At a frequency of 2.27 MHz, which is not close to the nominal frequency, the hot point corresponding to the lens defect disappears and another hot point appears probably caused by a heterogeneity at the crystal level. (Fig. 6f) Another energy peak is obtained MHz. (Fig. 6g)
at a frequency
of 4.67
By comparing these results and those illustrated in the diagram obtained by spectrometry on the same transducer, it
ULTRASONICS.
JANUARY
1978
Fig. 5
Characterization
of a plane lens transducer
-
installation
39
b
*
1.50 MHZ
blur
1.43
MHz
bb.10 lOlla
rod
C
red
~dlow
viqlri
I. 47 MHZ
blue
4.67MHz
d
blyr
1.50
MHz
r:d
blue
I
red Plane Ions transducer
Frequency Crystal
Nominal
Actual
I.SMHz
1.47 MHz
diameter
20mm Icm
I
I
Fig. 6
Direct
method
-
characterization
of a plane lens transduce!
I
I
Liquid crystals cc~er~~cn cell
Thermostated water out
I
Transducer
Contain&
II
\
Interchangeable ring ’
Transducer suppert
T
Thermostated water in
Fig. 8
0 0.1
I
1.41
3
2
4
4.65
[MHz] Fig. 7
Comparison of the results obtained by spectrometry those obtained by the ‘direct method’
and
Tomography method
Basis-The wavelength reflected by a liquid crystal film depends not only on the temperature but also on the angle of incidence the luminous rays make with the observed surface. If the latter is not plane, the difect method cannot be applied. Furthermore, since it entails visualizing the ultrasonic beam itself in the case, for example, of a focusing transducer, the second method wiIl have to be resorted to. This uses the same thermotropic properties of
40
Tomography
method -
thermostated
container
the cholesteric liquid crystal and consists of effecting some tomographies of the ultrasonic beam. Placed in a thermostatic container composed of a Plexiglas cylinder, the transducer moves horizontally (Fig. 8). The front of the container is composed of a plane cell producing a coloured image of the cross-section of the beam emitted by the transducer. The back of the container is hermetically sealed by a flange in which the support of the transducer can slide. Description of the installation-As for the preceding method, the apparatus used consists of a function generator, a frequency meter which, as for the first method, is not essential and an optical fibre illuminator. (see Fig. 9). The installation is completed by a circuit of thermostatic water, a mechanism for the horizontal and angular displacement system of the transducer and a position indicator.
U LTRASON ICS . JANUARY
1978
Preparation of the acoustical to optical conversion cellThe layer of liquid crystal is sandwiched between an optical glass disc and a thin Mylar sheet on the back of which a coating of paraffin is applied to act as an absorber of the ultrasonic wave and to transmit the heat produced to the layer of liquid crystals (Fig. 10).
b
The preparation technique of liquid crystal films is similar to that used for the direct method, except for the dark ground which is here formed by making the container itself opaque using a coating of optic black. A cholosteric mixture is used, the mesophase of which takes place at a temperature close to ambient. In this way, the gradients in the container are minimized. The absorbing paraffin coating may consist of a sheet applied on the Mylar film after some Vaseline oil has been applied in order to superficially dissolve the paraffin and to ensure that the latter completely adheres to the Mylar film. As another solution a paraffin, the melting point of which is approximately SO”C, could be used. Experience has shown that such overheating does not modify the characteristics of the liquid crystals. Example of the characterization of a focusing transducerIn order to illustrate the tomography method, a transducer was used, the characteristics of which supplied by the manufacturer are given in Table 1. Since the transducer is fixed on the sliding support and angularly ascertained, a preliminary examination is made of the beam in the area close to the lens by varying the applied frequency to approximately the nominal frequency. The limit operating frequencies of the transducer are thus determined, especially the frequency at which the maximum is emitted. (Fig. 11)
Fig. 9
Tomography method - installations. a - Function generator; b - Frequency meter; c - Optical fibre illuminator; d - Thermostated container; e - Displacement system and position indicator; f - Circuit of thermostated water
ri
The transducer is then moved to the approximate focal distance by supplying it at the previously defined frequency and by reducing the amplitude in order to obtain the minimum diameter of the violet spot. The whole beam is then inspected. (Fig. 12)
Optic glass(Imm) Mylar spacer (15 to 20pm) Mylar sheet (25 lo 50pm)
Results-In order to give the results of this measurement in a more concrete way, a longitudinal section of the beam has been represented, on the basis of the tomographies. (Fig. 13).This picture shows the division of the acoustic intensities along the beam and particularly emphasizes the focal area (from 55 - 65 mm). Also, a null intensity area can be observed (from 10 - 30 mm). It has thus been possible to define with this method, on the one hand, the frequency band to which the transducer responds as well as the frequency corresponding to the maximum acoustic intensity and on the other, the distribution of the acoustic intensities along the ultrasonic beam. Lastly, providing a reticle is centred on the mechanical axis of the transducer support, it is possible to obtain
Table
1.
Characteristics
Operating
Focal length Crystal
Fig. 10 Tomography
method -
acoustical-to-optical
conversion
cell
of the optical axis of the
Actual
Conclusion
5MHz
4.93
76.2 mm
52 mm
The comparison of the results obtained by this method to those obtained with the ‘Schlieren’ technique demonstrated that it is possible to obtain an exact characterization of an ultrasonic transducer using a much cheaper installation (Fig. 14).
diameter
12.7 mm
JANUARY
layer(-O.lmm)
Liquid crysfals
Nominal
Case and lens diameter
ULTRASONICS.
IAbsorbent
an exact geometric definition beam.
of the transducer
frequency
1
16mm
1978
41
3MHr
4.93
MHz
7 MHz
Fig. 11 Tomography
4MHt
rad \
green
VlOkl
green \
violet \
method
-
blue
grren
red
vlolrt \
rod \
characterization
of a focusing transducer
In conclusion, thanks to cholesteric liquid crystals, it is possible to make available to manufacturers and users of ultrasonic transducers, a visualization technique with which they can effect complete, exact and cheap characterizations of their transducers.
42
rrd
- determination
blue
red
green
I
of the limits of operating
red
\
I
frequencies
References 1 2
Greguss, P. Real time acoustical to optical convertors, State of the Art, Ultras Int 1973 Conference Proc (1973) 265 Greguss, P. New Liquid crystals acoustical to optical display, Acustica, 29 (1973) 52
ULTRASONICS.
JANUARY
1978
d 5
d 40
d
red
blu,e
I
rod
Fig. 12 Tomography
method
-
characterization
30
40
d 70
d 100
d SO
red blue vlolet
d80
de0
d50
blur violet
50
of a focusing transducer
- visualization
of the beam
12 II IC 9 6 7 6 5 4 3 2 F’ LO I 2 3 4 5 6 7 6 9 IC II 12
,Y , , 01
5
IO
I
I
20
50
Ml
I
60
Fig. 13 Tomography method - characterization - longitudinal section of the beam
3
4
70
I
60
90
of a focusing transducer
Cook, D., Werchan, R.E. Mapping ultrasonic fields with cholesteric liquid crystals, Ultrasonics, 9 (2) (1971), 101
JANUARY
1978
01
5 IO
I
I
20
30
I
40
50
60
I
I
70
60
I
90
loo
6 m3
Manaranche, J.C., Henri, P. Vizualization of an ultrasonic beam by means of liquid crystals, Btit J of NDT, 18 (4) (July 1976) 107
ULTRASONICS.
100
Fig. 14
5 6 7
Characterization of a focusing transducer, the ‘Schlieren’ technique
comparison
with
Fergason, J. L. Liquid crystals in NDT, Applied Optics, 7 (9) (1968) Assoultine, G., Leiba, E. Cristaux liquides, Revenue technique Thomson - CSF 1 (4) (1969) J.C. Les cristaux liquides et le con&e non destructifs, Mesure, r~~lution,automatisme, 36 (7) (1971)
Manaranche
43