EP0210358B1 - Acoustic focussing device - Google Patents

Abstract

An acoustic lens arrangement having at least one transducer for the generation and/or for the reception of a plane acoustic wavefield. The arrangement includes a focusing surface for focusing the acoustic wavefield in an object region and at least one medium for the low-loss transmission of the acoustic wavefield between a transducer, the focusing surface and the object region to be investigated. The longitudinal axis of the focusing surface is inclined relative to the direction of the normal to the acoustic wavefield in such a manner that when the longitudinal axis is positioned normal to the surface of the object region, the acoustic beams incident thereon form a critical angle theta R with the normal to the surface of the object.

Description

The invention relates to an acoustic lens arrangement according to the preamble of claim 1.

A lens arrangement of this type is e.g. known from US-A-4 028 933. A piezoelectric transducer is arranged on one side of a cylindrical sapphire rod and a spherical hollow surface is incorporated on the opposite side. An electrical high-frequency field applied to the transducer generates a flat acoustic wave field in the sapphire rod, which is focused by the spherical hollow surface in an adjacent immersion liquid.

The lens arrangement is part of an acoustic microscope. An object to be examined is brought into the acoustic focus. After interaction of the focused acoustic waves with the object (generation of longitudinal waves, bulk waves), acoustic waves emanate from it, which are collected by the same or another acoustic lens and im piezoelectric transducers can be converted into electrical signals. By scanning the object in a grid-like manner, an image of the object representing the acoustic interaction can be obtained from these electrical signals.

Essentially, the acoustic waves that are regularly reflected or transmitted on the object are used for acoustic microscopy. However, it is known that acoustic waves that hit a surface of the object at a certain material-dependent angle (Rayleigh angle ϑ _R ) excite surface waves in this surface (surface acoustic waves, SAW). Along their path of propagation, the SAW scatter acoustic waves out of the object (leaky waves, leak waves). These waves can also be detected and converted into electrical signals. In acoustic microscopy, they are superimposed on the regular signal, especially when focusing on an object area lying below the object surface. They can also be evaluated separately using special circuit measures (cf. DE Pat. Application P 34 09 929.8).

When the SAW encounters inhomogeneities in the object surface, the SAW are reflected on it, so that they change their direction of propagation. The result of this is that leakage waves also occur increasingly in this direction. Since the SAW penetrate relatively deeply into the object surface, they are now increasingly becoming one used to determine material properties of different objects. The particular advantage is that it is a non-destructive measurement method that also allows quantitative measurements. To do this, however, it is necessary to increase the local resolving power and to improve the signal yield.

There are two major problems to be solved in improving the SAW measurement method. The first problem is to generate the SAW as efficiently as possible in the surface of the material to be examined, which is generally not piezoelectric. The second problem is to focus the generated SAW on the smallest possible spot size.

Several different arrangements have already been proposed specifically for generating SAW on non-piezoelectric surfaces, but these are not suitable for focusing the SAW.

In Appl.Phys.Lett. 42 , pp. 411-413 (1983) describes a method for generating convergent SAW on the surface to be examined, which uses an acoustic lens of the type mentioned at the beginning, but in which the acoustic transducer is designed as a semicircular surface. In the defocused state, this lens generates SAWs that are focused at one point on the optical axis of the acoustic lens. More detailed investigations have shown that the acoustic energy converted into SAW only comes from a very Narrow ring area of the radiation surface of the acoustic lens originates, for which the already mentioned Rayleigh angle is observed with regard to the beam inclination. The remaining energy of the emitted sound wave field is reflected on the object surface or converted into longitudinal waves (bulk waves).

Another method to avoid this disadvantage is in J.Appl.Phys. 55 (Jan. 1984), pp. 75-79. An acoustic lens with a cylindrical starting surface is inclined with its longitudinal axis in relation to the object surface in such a way that the beam axis maintains the Rayleigh angle. An improved conversion ratio of the emitted ultrasound wave field into SAW is achieved with this, but even here, not all irradiated waves are still inclined at the Rayleigh angle and instead of a point focus, there is a line focus over which the SAW energy is distributed.

The invention was therefore based on the object of specifying an acoustic lens arrangement which, with the highest possible conversion rate of the radiated sound wave field into SAW, enables a point-like focusing of the SAW which is simple to manufacture and ensures a high signal yield.

This object is achieved by the features specified in the characterizing part of claim 1 solved. Advantageous refinements result from subclaims 2 to 16.

The angle of inclination ϑ _{R of} the sound rays when they hit the object area depends on the ratio of the phase velocities of the sound waves in the adjacent media. In the case of an immersion liquid following the cylinder surface, V _{I is} the rate of propagation in this medium. If it is a solid transmission medium, V _I can be the propagation speed of the longitudinal waves or the shear waves in the solid, but it can also be the propagation speed in a gaseous medium.

Rayleigh-Wellen

(transversale Oberflächenwellen)

Pseudo-Oberflächenwellen

(bei anisotropen Festkörpern)

Love-Wellen

(bei parallel zur Oberfläche geschichteten Objekten)

Stonely-Wellen

(bei parallel zur Oberfläche geschichteten Objekten)

Sezewa-Wellen

(bei parallel zur Oberfläche geschichteten Objekten)

The speed of sound propagation in the object area depends on different material properties, such as the structure, density, elasticity or a layer structure. A distinction is made between different propagation speeds V _R for

Rayleigh waves

(transverse surface waves)

Pseudo surface waves

(with anisotropic solids)

Love waves

(for objects layered parallel to the surface)

Stonely waves

(for objects layered parallel to the surface)

Sezewa waves

(for objects layered parallel to the surface)

Fig. 1

eine Prinzipdarstellung der Wirkungsweise der akustischen Linsenanordnung;

Fig. 2

eine graphische Darstellung zur Ermittlung der optimalen Brennweite der akustischen Linsenanordnung;

Fig. 3, 3a

das Signal bei ungestörter Oberflächenwellenausbreitung;

Fig. 4, 4a, 4b

das Signal bei einer Störstelle im SAW-Fokus;

Fig. 5, 6, 7

mögliche Ausführungsformen der akustischen Linsenanordnung;

Fig. 8, 9, 10:

Anordnungen mit getrennten Sende- und Empfangssystemen.

Exemplary embodiments of the acoustic lens arrangement according to the invention are shown schematically in the drawing. They are described in more detail below, special advantages compared to the known arrangements also being discussed. In detail show:

Fig. 1

a schematic diagram of the operation of the acoustic lens assembly;

Fig. 2

a graphical representation for determining the optimal focal length of the acoustic lens arrangement;

3, 3a

the signal with undisturbed surface wave propagation;

4, 4a, 4b

the signal at a fault in the SAW focus;

5, 6, 7

possible embodiments of the acoustic lens arrangement;

8, 9, 10:

Arrangements with separate transmission and reception systems.

1, the basic mode of operation of the lens arrangement according to the invention will first be explained. An acoustic beam emitted by a rod-shaped transducer in an immersion liquid generated, falls inclined on a parabolic concave cylindrical surface of a solid. If the angle of incidence is large enough, the total sound power is reflected specularly and no waves are excited in the solid. The reflector acts like a parabolic cylinder mirror.

The incident acoustic beam is to be represented by a plane wave of the form exp [j (k _y · y + k _z · z)]. The y- and z-dependency does not change during reflection, but an x-dependent term arises that takes into account the reflection on the parabolic cylinder surface.

Since a parabolic cylindrical mirror focuses a vertically striking plane wave in a line, an obliquely striking plane wave is focused in a line with a linearly variable phase. The wave fronts are conical and in the present case the axis of the cone coincides with the focal line of the parabolic cylinder.

If an object with a flat surface is arranged perpendicular to this axis, the intersection of the conical wavefronts with the object surface is always circular. In contrast, with the from J.Appl.Phys. 55 , pp. 75-79, known arrangement produces cylindrical wave fronts, the intersection of which is elliptical with the object surface. Due to the geometric limitation of the invention Reflector, the wave fronts reflected on it only comprise a section of a cone, so that the cutting line instead of a circle represents an arc.

As already mentioned, an ultrasound beam excites the more intensely the angle of incidence coincides with the Rayleigh angle as it passes through a liquid / solid interface in the surface of the solid SAW. According to the invention, this fact is combined with the special properties of the reflector described, in that the angle of incidence of the beam generated by the acoustic transducer on the reflector is selected to be equal to the Rayleigh angle. The wavefront running towards the interface with the object then cuts the object surface in an arc with a decreasing radius. Each surface wave generated will amplify the surface wave generated in front of it with a larger radius, since the selected special angle of incidence of the acoustic wavefront matches the k-vector component of the surface wave along the transition interface. It should be emphasized that in this way all of the energy contained in the conical wavefront is converted into a single circular converging wavefront of the SAW. Almost all of the acoustic energy generated by the transducer is concentrated in a diffraction-limited focus point.

In contrast to this, in the case of the obliquely incident cylindrical wavefront known from the prior art, elliptical arcs with a constant shape arise as intersection lines with the object surface, which do not result in a phase-rigid reinforcement of the wave fronts of the SAW which have already been generated. Even a converging, spherically shaped wavefront cannot do this because only a fraction of the incident wavefront fulfills the condition of the Rayleigh angle.

The generated SAW have a limited lifespan and are ultimately scattered back into the liquid layer as longitudinal waves. These waves, also known as leakage waves, can arise as soon as the surface waves are generated. If the surface of the object is perfectly flat and has no impurities, ie there are no surface wave reflectors, almost no leakage waves will return to the transducer. Since the incident beam is limited in diameter and plane waves are also contained in its angular spectrum, SAW can also be excited towards the reflector, ie it runs backwards. The leakage waves resulting from these SAWs will then generate an output signal at the acoustic transducer, even if there are no impurities in the surface. However, this effect is very low and can be further suppressed by appropriate beam expansion and suitable shaping of the reflector. The acoustic transducer only receives a sufficiently strong signal if the direction of propagation the forward running SAW is changed at any point of failure. In the event that such an impurity is present precisely at the focal point, the SAW is reflected on it and runs back as a circularly divergent wave. The waves scattered back into the liquid reassemble in the original conical wavefront and are returned to the acoustic transducer by the reflector as a collimated beam. If the point of impurity is not exactly in the focus point, the wavefront reflected on it will not be able to exactly restore the originally radiated beam, so that the output signal of the converter is smaller than in the in-focus position.
An exemplary embodiment is shown schematically in FIG. 1. The acoustic lens arrangement consists of an acoustic transducer 1, a cylindrical mirror 2 and a mechanical connection 3, with which the angle of inclination and the position of the transducer 1 can be adjusted relative to the mirror 2 so that the transducer sonicates the entire mirror surface regardless of the angle of inclination. The arrangement is immersed in a water bath 4 serving as immersion during operation. The mirror 2 is arranged on the object 5 to be examined so that the longitudinal axis 6 of its cylindrical hollow surface 7 is perpendicular to the object surface. The pulsed sound wave field 8 generated by the transducer 1 falls on the mirror 2 at the Rayleigh angle ϑ _R. After the reflection, the plane phase front produces a conically shaped phase front 9, which also strikes the object surface at the Rayleigh angle ϑ _R and excites SAW 10 in it. The rays reflected from the object surface are picked up by the transducer 1 and converted into corresponding electrical signals which are displayed on an oscilloscope (not shown). A micropositioning system, also not shown, allows a raster-shaped relative displacement between acoustic lens arrangement 1, 2, 3 and object 5 to be examined.

The converter 1 consists of a flat ceramic disk, the thickness of which is designed for a resonance frequency of 1 MHz. The transition surface to the immersion liquid 4 is provided with a λ / 4 adaptation layer, not shown. The transducer is driven by a voltage pulse lasting approximately 0.2 microseconds, which generates a sinusoidally falling pressure pulse. The emitted ultrasound pulse is about 5 microseconds long and has a center frequency of 1 MHz.

In order to produce a conical phase front of the sound wave field, the cylindrical hollow surface 7 should have a parabolic shape. Since this is difficult to manufacture, experiments with a circular cylindrical mirror surface have been successfully carried out as an approximation to this shape. The geometric limitation of this simplified hollow surface was chosen so that when the reflector is sonicated with a flat wavefront, the marginal rays form the central ray have a path difference of not more than λ / 4, where λ is the wavelength of the ultrasound beam in the immersion liquid 4.

wobei V die Schallgeschwindigkeit in der Immersionsflüssigkeit ist und mit V_s, V₁ und V_R die Scher-, die Longitudinal- und die Rayleigh-Schallgeschwindigkeiten in dem zu untersuchenden Festkörper bezeichnet sind.For an optimal construction of the acoustic lens arrangement, a certain focal length must be selected, which depends on the frequency of the ultrasonic wave field used and the material to be examined. The optimal focal length f _opt can be read from FIG. 2. In this figure, f _{opt is} normalized with respect to the Schoch shift Δ _s and plotted as a function of Δ _s / λ, where λ is the sound wavelength in the immersion liquid. According to Brekovskikh (1980), the ratio Δ _s / λ is given by

where V is the speed of sound in the immersion liquid and V _s , V₁ and V _{R are} the shear, the longitudinal and the Rayleigh sound speeds in the solid to be examined.

Δ _s / λ can be calculated using this formula if the relevant physical parameters are used. For aluminum, this gives the value of Δ _s / λ, for example 21.3, for stainless steel 57.85, for molybdenum 90.3, and for aluminum oxide (Al₂O₃) 118.3. It can be shown that the dependence f _opt / Δ _s on Δ _s / λ is fairly relaxed and that one can generally choose f _opt = 0.59 Δ _s . At an ultrasound frequency of 1.5 MHz, f _opt becomes 12.5 mm for aluminum _objects, for example. At an ultrasound frequency of 100 MHz there is an f _opt = 1.05 mm for aluminum oxide (Al₂O₃). If the parabolic cylindrical reflector is approximated by a cylindrical surface with a circular curvature, then f _{opt is} equal to half the radius. An f _opt of 12.5 mm can be realized with a cylinder of 50 mm diameter and an f _opt of 1.05 mm with a cylinder of 4.2 mm diameter.

Once f _opt is determined, the maximum width 2x _{m of} the reflector with no significant cylindrical aberrations can be calculated using the following formula:

With this value, the lens arrangement achieves a maximum resolution. It is 22.4 mm for aluminum at 1.5 MHz ultrasonic frequency and 1.22 mm for Al₂O₃ at 100 MHz.

und ergibt 0,56 für Aluminium und 0,86 für Al₂O₃.The aperture (f number) of the lens arrangement can be determined as follows using the values already determined will:

and gives 0.56 for aluminum and 0.86 for Al₂O₃.

The height H of the reflector should be equal to f _opt · cot ϑ _R when the base of the reflector almost touches the object surface to be examined. The optimal height is 21.7 for aluminum at 1.5 MHz and 4 mm for Al₂O₃ at 100 MHz.

All selected absolute values change in inverse proportion to the frequency ratio when other ultrasonic frequencies are selected.

Suitable as mirror material e.g. Brass, which has a high acoustic impedance compared to the immersion liquid water. In one embodiment, the mirror had a height of 38 mm, a width of 37 mm and a cylinder radius of 50 mm. These dimensions deviate slightly from the theoretical limits for the investigation of aluminum. However, it has been shown that the losses in signal power due to this are negligible.

If aluminum is used as the test object, an ultrasonic frequency of 1 MHz results in a wavelength of the SAW of 2.85 mm, which also means the diameter of the diffraction-limited focus and the layer thickness of the object surface in which the SAW run. Inhomogeneities lying within this layer thickness can be recognized on the basis of the sound waves reflected back at them. A 10 mm thick test plate therefore looks like an almost infinitely thick object for the SAW.

The acoustic lens arrangement should initially be arranged in the middle of a sufficiently large test area. This case is shown schematically in FIG. 3. The oscilloscope image of the measurement signal shown in FIG. 3a shows only one echo pulse 20. This signal is due to the fact already described that the acoustic wave front generated by the acoustic transducer is not exactly flat and that beam components also hit the reflector 2 whose angles of incidence deviate more or less from the Rayleigh angle ϑ _R. These are reflected at the edge between the object surface and the cylindrical hollow surface and generate the echo signal. This signal can be minimized by optimizing the transducer and reflector geometry and setting a suitable detection sensitivity. If the acoustic lens arrangement, as shown in FIG. 4, is shifted to the edge of the test surface such that the focus of the SAW lies exactly on the edge, then a second significantly larger echo pulse 21 is observed in the oscillogram. This is shown in FIG. 4a and enlarged again in FIG. 4b.

The distance between the two echo pulses 20, 21 is 17 microseconds. This corresponds to the running time of the SAW for a distance of 50 mm, ie twice the focal length. From this, a very simple method for the exact setting of the Rayleigh angle ϑ _R between the beam direction of the plane sound wave field emanating from the transducer and the longitudinal axis of the reflector can be derived. The reflector is to be arranged at a distance of its focal length from an edge of the object to be examined and the angle of inclination of the transducer is to be changed until the amplitude of the echo pulse 21 has a maximum.

Measurements on various test objects confirmed that periodically occurring defects with a distance of approximately 2 mm can be detected separately with focused SAW with a wavelength of approximately 3 mm. With the same wavelength, inhomogeneities that were about 2.5 mm below the surface of the object could be clearly recognized, which confirms that the depth of penetration of the SAW corresponds to its wavelength.

The device for adjusting the angle of inclination between the transducer and reflector is used primarily to optimize the object-dependent Rayleigh angle ϑ _R for the almost lossless conversion of the radiated sound wave field into SAW. However, it is known that with certain layer structures, besides the SAW, other waves in the object can also be excited, which also depend on the angle of incidence of the ultrasound beams depend on the liquid / object interface. Such waves are known for example under the designation love waves, Stonely waves and Sezewa waves. If, for example, the object to be examined has several layers of different materials lying on top of one another, these waves can be selectively excited if the angle of incidence in the liquid is set appropriately. The waves entering the object are focused in a similar way to the SAW. This makes it possible to achieve a greater depth of penetration for the acoustic focus than with the SAW.

The device according to the invention has been described above for applications with relatively low ultrasound frequencies. However, it can also be used in acoustic microscopes that use ultrasound frequencies up to the GHz range. A suitable lens arrangement is shown in FIG. 5. A rod 40 made of a material with low acoustic losses, such as sapphire, is provided with parallel, flat polished end faces. An acoustic transducer 41 (ZnO) is located on one side and lies between two gold electrodes 42, 43. The other side is provided with a λ / 4 antireflection coating made of glass or carbon with a suitable acoustic impedance in order to achieve a good adaptation for the transition of the ultrasound rays into the immersion liquid, not shown. The cylindrical, preferably parabolically shaped reflector 44 is glued to this side of the rod 40 so that a certain one Rayleigh angle ϑ _R to its longitudinal axis is created. It consists, for example, of aluminum or another solid material with high acoustic impedance. The geometric dimensions (height and width) and the focal length must be adapted to the intended ultrasound frequency. They decrease almost linearly in proportion to the quantities mentioned for 1 MHz with the increase in the ultrasound frequency. For this reason, it will be expedient to provide different fixed lens arrangements with a reflector inclined in accordance with the required Rayleigh angle ϑ _R for the examination of different materials. In principle, however, the angle of inclination can also be made adjustable here, which allows an individual adaptation to the examination object.

6 shows a very compact and mechanically very stable embodiment of the acoustic lens arrangement. The transducer 1 and the cylindrical surface 7 that is hollow relative to the transducer are formed on the outer surfaces of a solid body 60 suitable for sound transmission. In order to produce a sufficiently high impedance of the reflector surface with respect to the impedance of the sound-transmitting solid body, a metal layer is evaporated onto the cylinder surface 7. An immersion liquid can also be inserted between the exit surface of the lens arrangement and the object surface for better coupling of the focused sound beam to the object surface.

Another embodiment is shown in FIG. 7. With this arrangement, the sound focusing is now generated by refraction on such a surface instead of a reflection on the cylinder surface perpendicular to the object surface. The sound transmission from the transducer 1 takes place through a solid body 70 to the cylindrical hollow surface 7, which in this case is curved towards the transducer and whose longitudinal axis 6 is perpendicular to the object surface 5. The normal direction on the flat sound wave field emanating from the transducer is inclined at an angle ϑ _{i with} respect to the object surface. The space between the hollow surface 7 and the object surface 5 is filled with an immersion liquid, not shown.

wobei mit V_Festkörper und V_Immersion die Phasengeschwindigkeiten der Schallwellen in den beiden Übertragungsmedien bezeichnet sind. Nach der Brechung entsteht wieder eine konische Wellenfront wie bei den zuvor beschriebenen Reflektions-Linsenanordnungen.If the difference in the propagation velocities for the sound rays in the solid 70 and the immersion liquid is large enough, then the hollow surface 7 acts in the horizontal direction like a cylindrical lens. On the other hand, Snell's law of refraction must be observed in the vertical direction:

V _{solid state} and V _immersion denote the phase velocities of the sound waves in the two transmission media. After the refraction, a conical wavefront is created as in the previous ones described reflection lens assemblies.

The inclination of the transducer plane is to be chosen so that the sound waves hit the object surface at the critical angle ϑ _R , taking into account the refraction at the hollow surface 7. Then SAW are generated in the object surface, which are focused at one point. It should also be mentioned that both longitudinal waves and shear waves can be excited in the sound propagation in the solid body 70. V _solid then means the phase velocity for the wave type used in each case. To avoid transmission losses, the hollow surface 7 must be provided with a suitable anti-reflective coating.

The maximum size of the angle (90-ers _R ) is determined by the choice of the solid 70 and the immersion liquid. Due to this fact, the selection of the solid transmission medium is restricted depending on the material properties of the object to be examined. The basic principle is that the sound propagation speed in the solid body 70 must be lower than in the object surface 5.

In the exemplary embodiments described so far, the acoustic transducer is usually used alternately as a transmitter and a receiver in the pulse-echo method. In the case of continuous sound generation, the sound waves returning from the object are combined with the radiated ones interfere so that a phase-modulated signal is produced at the converter.

FIG. 8 shows an embodiment with two confocal lens arrangements, one of which serves as a transmitter and the other as a receiver for the sound waves, as indicated by the arrow directions. Both arrangements are on the same axis of SAW propagation. Such a structure can of course work with both continuous and pulsed sound wave generation. In pulse-echo mode, two signals can be obtained which are assigned to the sound wave components scattered backwards in the direction of the transmitter and to the sound wave components scattered forward in the direction of the receiver.

The arrangement of two confocal lens arrangements shown in FIG. 9 is selected such that the directions of the SAW propagation form an angle φ with one another. This angle can be made adjustable. This arrangement is also suitable for continuous and pulsed sound generation. It can be used to determine anisotropies in the SAW reflection in particular.

The embodiment shown in FIG. 10 works with only one reflector and a two-part converter, one of which can be used as a transmitter and the other as a receiver in both continuous and pulsed operation. The imaging properties of the reflector ensure sufficient directional selection between the transmitted and the received sound beam bundle, so that the two beams do not interfere with each other or only to a very small extent, regardless of the orientation of the dividing line between the transducers.

Claims (16)

1. Acoustic lens arrangement with at least one transducer (1) for the production and/or for the reception of a planar sound-wave field (8), means for the focussing of the sound-wave field in the object region and at least one medium (4) for the low-loss transmission of the sound-wave field (8) between the transducer (1), the means for the focussing and the object region, wherein a cylindrical surface (7) sounded by the sound-wave field (8) is provided opposite the transducer (1), characterised thereby, that the longitudinal axis (6) of the cylindrical surface is so inclined relative to the normal direction onto the sound-wave field (8) that, in one setting of the longitudinal axis (6) normal to the surface (5) of the object region, the sound beams impinging there form an angle ${\text{ϑ}}_{\text{R}}{\text{= sine ⁻¹ V}}_{\text{I}}{\text{/V}}_{\text{R}}$

   

   with the normal on the surface, wherein V_I is the phase velocity of the sound-waves in the transmission medium (4) between the cylindrical surface and the object region and V_R is the phase velocity of the sound-waves in the object region.
2. Acoustic lens arrangement according to claim 1, characterised thereby, that the cylindrical surface (7) opposite the transducer (1) is hollow and utilised in reflection.
3. Acoustic lens arrangement according to claim 2, characterised thereby, that the acoustic impedance of the cylindrical surface (7) is high in comparison with that of the sound-transmitting medium (4, 60).
4. Acoustic lens arrangement according to claim 1, characterised thereby, that the cylindrical surface (7) opposite the transducer (1) is arcuate and utilised as refractive surface in transmission.
5. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that a liquid immersion medium (4) is provided for the transmission of the sound-wave field (8) between the transducer (1), the cylindrical surface (7) and the object region (5).
6. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that a solid medium (60) is provided for the transmission of the sound-wave field (8) between the transducer (1), the cylindrical surface (7) and the object region (5).
7. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that a solid medium (70) is provided between the transducer (1) and the cylindrical surface (7) and a liquid immersion medium (4) is provided in at least a partial region between the cylindrical surface (7) and the object region (5).
8. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that a gaseous medium is provided in a partial region for sound transmission.
9. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that the angle of inclination ϑ_R between the normal direction onto the sound-wave field (8) and the longitudinal axis (6) of the cylindrical surface (7) is adjustable.
10. Acoustic lens arrangement according to claim 1, characterised thereby, that the cross-section of the cylindrical surface (7) forms a parabola.
11. Acoustic lens arrangement according to claim 1, characterised thereby, that the cross-section of the cylindrical surface (7) forms a circular arc.
12. Acoustic lens arrangement according to one of the preceding claims, characterised thereby, that two separate and mutually confocal lens arrangements are provided, of which one serves as sound transmitter and the other as sound receiver.
13. Acoustic lens arrangement according to claim 12, characterised thereby, that the planes, which are spanned by the central beams of the transmitter and receiver arrangement, co-incide (Figure 8).
14. Acoustic lens arrangement according to claim 12, characterised thereby, that the planes, which are spanned by the central beams of the transmitter and receiver arrangement, form an angle ϑ between them (Figure 9).
15. Acoustic lens arrangement according to claim 14, characterised thereby, that the angle ϑ is adjustable.
16. Acoustic lens arrangement according to one of the claims 1 to 11, characterised thereby, that the transducer is divided in two, wherein the one half serves as sound transmitter and the other half as sound receiver (Figure 10).

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