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Acoustical holography and its applications
Acoustical
holography
and its applications
D. HOLT AND J. R. COLDRICK*
A device capable of producing images of objects immersed in opaque media could find application in such areas as engineering inspection, underwater viewing, and medical diagnosis. The results of some experiments in acoustical holography as a viewing system are presented here, together with an assessment of the problems which must be solved before the technique can find useful application.
Conventional techniques for visualisation of sound patterns are mainly of two classes: (1) a direct image-forming system, in which an acoustical lens or its equivalent forms an acoustical image that can be transduced into a visual image; and (2) a pulse-echo system, well known in underwater studies, in which the shape and phase of a sound burst reflected from an object are compared with the original burst impinging on the object. Depth of focus, angular field of view, and depth of field are limited in a direct image forming system, and losses are increased if wide aperture sonic lenses are to be used. The reliability of the pulse-echo system for identification is generally recognised to be limited. Furthermore, the resolution of direct image systems is limited by geometrical aberrations as well as sensitivity to the turbidity of the medium. The advantages of acoustical holography over other conventional image-forming techniques are simplicity, enormous depth of field, less sensitivity to turbulence and turbidity of the medium, and the capability of retrieving information about the target from discrete sampling points. The quality of the reconstructed image is influenced by several parameters: number of sampling points, turbidity and turbulence of the medium, hologram size, sonic wavelength, size and distance of object, and surface finish of object. Although acoustical holography is analogous to optical holography, it poses quite different problems to the research worker. Progress in optical holography was hampered for years because, until the laser was invented, coherent light sources were not available. Recording the holograms was simply a matter of using a high resolution photographic plate. In acoustical holography, however, most sound sources are coherent. The difficulty has been in recording the interference pattern as there is no acoustical equivalent of the photographic plate. Also, if an acoustical hologram is converted into an optical pattern and then placed in a laser beam, the
* British Aircraft Corporation Ltd, Guided Weapons Division, GPO Box No 77, Filton House, Bristol, England 240
ULTRASONLCS October 1969
reconstructed optical image is extremely small because the wavelength of the light is much shorter than that of the sound. If the image is to be viewed full size, the acoustical hologram must first be reduced in the ratio of light wavelength to sound wavelength before the hologram is placed in the laser beam. In practice this process would result in an optical hologram of about 0. lmm square and the reconstructed image would be extremely faint. A compromise is usually made, the hologram is reduced in size to about 5mm square and the reconstruction viewed through a low power microscope or an eyepiece. The mathematical relationships of acoustical holograms and reconstructions were given by Halsteadl together with an analysis of the intrinsic aberrations of holograms. METHODS OF RECORDING ACOUSTICAL HOLOGRAMS The various methods available for recording acoustical holograms were listed by Halsteadr, who evaluated them by the parameters of sensitivity, resolution, and image-formation time. The methods listed came under the headings of electrical detectors, thermal, mechanical, and chemical detectors. Of all these, only three have found widespread use with research workers, the surface deformation technique has been used by Mueller et al,2 the ultrasound camera by Marom et a1,3 and the scanning detector method by Metherell et aL4 Preston and Kreutzer,s and most other workers. Several factors must be considered in the design of a mechanical scanning frame. The size of the scanning frame is governed by the size of hologram required, which in turn is governed by the acoustic wavelength (A), range (R,) and resolutions (r). A simple formula relating aperature (A) to the above parameters is often used: A=R,; This equation gives the correct results when the case of diffuse radiation from an object is considered (normal for optics) but may give incorrect values in the case of specular reflection (normal for acoustical imaging). In the latter case,
quency signal is further amplified and used to modulate the Fig 1 Photographof scanningframe
it is better to calculate hologram size from geometrical considerations of beam angle, object size and range. Using these criteria, the hologram for a beam angle of 30” and source-tohologram distance of 40cm should be 21cm diameter. The scanning frame used in our experiments (see Fig 1) was designed to give a maximum hologram scan of 30cm x 30cm in order to allow for different geometrical configurations. A frequency of operation of 1MHz was chosen because it was the highest frequency at which a beam angle of 30” at reasonable power levels could be achieved with the equipment available. The scanning rate and the rate of sampling the hologram have some effect on the reconstruction. High-speed scanning causes severe disturbance of the water surface, and multiple acoustic reflections can degrade the hologram recording. A hologram is conventionally thought of as a carrier wavelength d = X/o (o is the angle of the reference beam) modulated by a diffraction pattern of the object. Therefore, an oft-used sampling procedure sets the interval between scans at Ax = h/20 to satisfy the Rayleigh Theorem. However, Carter6 shows that some of the spatial frequencies are under-sampled resulting in aliasing. Aliasing is analogous to the production of Moire patterns: super-position of a diffraction pattern (hologram) and a sampling raster produces a ghost pattern. On reconstruction, symmetrical images are produced containing images and defocused diffraction patterns super-imposed. Thus, image quality is further degraded. A sampling interval of AX = k/4a should overcome this effect. Another factor of great importance in scanning frame design is that of stability. All vibrations must be reduced so that amplitudes are less than x/8 if successful holograms are to be produced. The registration between the scanned probe and the recording lamp or oscilloscope beam scan must be held to similar limits of accuracy.
Camera f (Time exposure)
Electronics
Lamp
Detector \
/Object Signal
source
Fig 2 Acoustical reference system
LF
amp
Rectify
and smooth
PHASE AND AMPLITUDE ACOUSTICAL HOLOGRAPHY The acoustical reference holography system is analogous to the optical,holography system and is illustrated in Fig 2. Here, one source illuminates the object, and diffracted sound falls on the hologram plane, or water surface. A separate source, acting as a reference beam, illuminates the hologram plane directly. Since the two sources are driven by the same master oscillator, the sound beams are coherent, and their interaction at the water surface forms an interference pattern or hologram. The interference pattern is recorded by scanning the sound field with a pick-up, amplifying the received signal, detecting the modulation, and applying the low frequency signal to an electro-luminescent diode or tungsten lamp. The brightness variations of the lamp are recorded by a time exposure photograph as the raster is scanned.
Fig 3 Phase and amplitudeelectronic reference system ULTRASONICS October 1969
241
current through a gallium phosphide electro-luminescent diode mechanically linked with the scanned receiver.
indicates that the image resolution obtainable is of the order of an acoustic wavelength.
From a signal processing viewpoint, the first problem is to distinguish between the true acoustic signal and the unwanted electrical pick-up on the receiver leads. A differential amplifier with a common mode rejection ratio of 40dB is used to achieve this. The electrical pick-up then constitutes common mode signal and its effect is greatly reduced, whereas the true acoustic signal is the difference signal which is amplified. An Elliott Type 7702C linear integrated circuit operating in a differential amplifier configuration with a differential voltage gain of 100 gives good results. The amplified receiver signal is then summed electronically with a reference signal derived from the same master oscillator that drives the transmitting sound source. If the phase of this reference signal is the same as that of the sound source, a co-planar reference wavefront is simulated. However, if phase differences are introduced into the reference signal, it is possible to simulate an off-axis reference wavefront’. For the experimental work described in this article, a coplanar electronic reference was used, ie no phase shifts were introduced into the reference signal. The summed signal is rectified and smoothed using a simple diode and C/R network and the sound field variations across the hologram plane are thus converted into a slowly varying dc level. This dc level is amplified by a second Elliott Type 7702C integrated circuit used as a non-inverting amplifier with a voltage gain of 20. The amplified signal is then used to current drive a Ferranti Type XP50 gallium phosphide diode lamp which emits in the green wavelength band when passing current in the forward direction. Early experiments using a small tungsten filament lamp showed small fringe displacements on the holograms which were due to the temporal delays caused by the thermal time constant being excessive at the scanning speeds employed.
Fig 6 shows the reconstructions of two objects placed at different depths as shown in the photograph of Fig 1. The objects were the letter F previously described and a similar letter C and they were separated by about 4in in the tank. On reconstruction, the letters came into focus at different positions on the optical level, illustrating that a two-dimensional acoustic hologram stores three dimensional information about an object. The figure shows the F in focus in one photograph and the C in focus in the other photograph. The out-of-focus reconstructions of the C and F respectively may also be seen in the two photographs. Fig 7 is the hologram and reconstruction of a stencil of the letters BAC cut out of 3/16in hardboard. The stroke width of these letters is only 6.5 acoustic wavelengths. An interesting feature of this hologram is that an off-axis reference was achieved, not by introducing phase shifts into the electronic reference signal, but by maintaining a co-planar reference and tilting the hologram plane.
The much faster gallium phosphide lamp eliminates this problem and has the advantage that its light output varies linearly with forward current. However, due to its very low intensity, it is necessary to use a very fast photographic emulsion in order to record the modulation patterns that constitute the hologram. Polaroid Type 57 Land film with a daylight speed rating of 3,000 ASA is suitable. The use of a plane reference wavefront enables some of the classic interference experiments to be carried out. A typical example is the formation of Newton’s rings formed by interference of the spherical wavefront from the acoustic source with the electronically simulated plane reference wavefront. When an object is placed between the acoustic source and the water surface, the acoustic wavefront is diffracted by the object and the ring pattern is modulated. This is illustrated in Fig 4 which shows a hologram and reconstruction of a letter F cut out of 3/16in hardboard and having a stroke width of 16 acoustic wavelengths. The reconstruction was
An off-axis reference acoustic hologram has two advantages
over the in-line hologram: firstly, the reconstructed image may be formed partly or wholly out of the zero-order light on reconstruction, depending on the angle of the reference beam and on the degree of photographic reduction and secondly, the main advantage is that many more fine fringes are present in the hologram plane, and therefore better resolution may be obtained in the reconstruction since any given object disturbs more fringes in the hologram.
PHASE-ONLY ACOUSTICAL HOLOGRAPHY It is clear that if acoustic holograms can be made by recording only the phase information of the object wave, then the data acquisition requirements are reduced. This could lead to a useful reduction in the hologram recording time when a scanning or sampling technique is employed. Metherells has shown that the quality of reconstructions from phaseonly acoustic holograms is comparable with that obtainable from phase and amplitude holograms. We have developed a simple phase-only system which permits linear recording of phase information in acoustic holography and which we have
Conjugate image
Flg 5 Optical reconstruction
Fig 4 Hologram and reconstruction
of letter F
obtained by placing the reduced hologram in the optical system of Fig 5. This system provides a convergent beam of coherent light (wavelength 0.63pm) and the reconstructed image is seen when the zero order (undiffracted) light is removed by a stop. The sequence of reconstructions and foci down the optic axis is as shown in Fig 5. There was an irregularity in the top edge of the cut-out about lmm deep and this is recognisable in the reconstruction in Fig 4. This 242
apparatus
ULTRASONICS October 1969
Fig 6 Reconstructionsof holograms of the letters F and C at different depths
REFLECTION HOLOGRAMS Fig 10 shows the reconstruction of the hologram of a steel spanner in the reflection mode. The phase-only recording technique was used. The quality of the reconstruction is not as good a8 that of the transmission results already described. This is probably due to the problems of specular reflection discussed in the next section.
Fig 7 OH-axis hologram and reconstruction
of letters BAC
w
Camera
LOWzo Is). amp
I :
swd%,C
-
used to produce several phase-only acoustic holograms in water at 1MHz. Fig 8 shows the phase-only system. It will be noted that a scanned source and stationary receiver configuration is illustrated rather than the more conventional scanned receiver and stationary source. It is possible to obtain an entirely equivalent hologram by using the scanning transducer .as a source and recording the signal received by the stationary transducer as has been demonstrated by Metherell and Spinake and we have used both configurations in the experimental work reported here. The sound field diffracted by the object is detected by the receiver, and the receiver signal is amplified differentially as before. The next problem is to obtain an output voltage ,which is linearly proportional to the phase difference between this receiver signal and a reference signal derived from the master oscillator used to drive the sound source. This output voltage must also be independent of the amplitude of the receiver signal, These requirements are achieved by applying the two sinusoidal waves whose phase difference is to be determined to two separate but identical channels where they are converted into square waves. The square waves are then combined to give the desired output voltage proportional to phase. The squaring circuits are based on a tunnel diode switch: the sinusoidal signals are amplified and then fed via low output impedance amplifiers (complementary emitter followers) to tunnel diode switches as shown in Fig 8. These approximate to crossover detectors. The resuIting square waves are amplified by pulse amplifiers and further squared by NAND gates. The final square waves have a rise time of 29ns and a constant amplitude of 3V. The reference square wave is then inverted and summed with the signal square wave. The rectified output of the summer is smoothed and the resulting low frequency signal is found to be very nearly proportional to the required phase difference. The smalI departure from linearity is attributed to the non-linear characteristics of the rectifier diode. The low frequency signal is further amplified and then used to current-modulate the gallium phosphide lamp as in previously described systems. Since the light output from the lamp is linear with forward current, the system provides a means of linearly recording phase. Unfortunately, the characteristics of the recording film are somewhat unpredictable and it is inevitable that non-lfnearities are introduced in the photographic stage. Fig 9 is an example of a phase-only hologram and reconstruction of a letter C cut out of hardboard and having a stroke width of 15 wavelengths. It will be observed that the hologram fringes are of equal contrast. This is because the phase-only system makes it possible to use the full linear range of the recording equipment for each fringe. In phase and amplitude holograms, the fringe contrast decreases towards the edges of the hologram since the full linear range of the recording equipment can only be used at localised positions where the object wave intensity is at a maximum. It will be noted that the reconstructed letter C in Fig 9 emphasises the sharp edges of the letter as bright regions. This has been predicted by Wade et allo who have performed a computer study of phase-only holography. Thus, if a certain amount of image distortion (ie edges emphaeised and internal details lost) can be tolerated, phase only recording provides a useful simplification in the signal processing and data acquisition requirements.
1OOk
F’ig 8 Phase-only
3oop
recording system
Fig 9 Phrrse-only hologram and reconstruction
of letter C
Fig 10 Reconstruction of reflection hologram of steel spanner ULTRASONICS October 1969
243
APPLICATIONS OF ACOUSTICAL HOLOGRAPHY Before discussing the feasibility and probable performance of the proposed applications of acoustical holography, it is essential to consider the nature of acoustical images, and the best performance that could be expected from an imaging system assuming that the holographic process was perfect. Any distortion or imperfection caused by the holographic recording and reconstructing process can only further degrade the image. The nature of a sound image is difficult to predict quantitatively, but an assessment of the factors determining the image formation enables a qualitative prediction to be made. The major factors determining image quality are acoustic wavelength and the surface finish of the object. The acoustic wavelength governs the detail on the object which will add to the information content of the reflected wave. In general, object detail smaller than an acoustic wavelength will not be ‘seen’ by an acoustic wave. The quality of the surface finish determines how an object surface reacts to an incident acoustic wave. If the surface finish is better than an acoustic wavelength the surface will act as a mirror, reflecting the whole of the acoustic wave according to the laws of reflection (specular reflection). If the surface is rough, the surface will scatter the sound in all directions (diffuse reflection). The aperture of the acoustic receiver is now of great importance in the quality of the image. In the case of a ‘smooth’ surface, only that part of the surface which is oriented in such a way as to directly reflect sound from the source to the receiver will be recorded, ie only the highlights will be observed. In the case of a ‘rough’ surface, much scattered radiation will be collected by the receiver and image quality will depend on the collecting aperture. The intrinsic aberrations of the holographic procesg which further degrade the image will not be discussed here, since they were described in some detail by Halstead’ in his excellent paper. The application of acoustical holography to underwater viewing is receiving some attention from research workers and system designers, who see it as a useful tool for certain civil engineering applications, and as an inspection device in very muddy water. However, there are several severe limitations to be faced. If the viewing distance is limited to l-2m the size of hologram required for a resolution of a few wavelengths will be of the order of lm square. If the operating frequency is lMHz, and Carter’s criterion6 for element spacing is used (spacing X/4a), the element spacing for a co-planar reference is 0.24mm. A lm square array would require 4000 elements per side, or a total of 16,000,OOO elements. Clearly, this is prohibitive from an engineering and economic viewpoint. A system such as the Mills Cross array11 may reduce the number of components required to about 8000, but the number of measurements would remain the same. If only one cycle of received signal were measured at each resolution point, the time required to form a hologram would be some 16s minimum (on a serial processing basis), and during this time the hologram and object must be held stationary relative to each other to at least 0.2mm. The availability of a lm square, sensitive, real-time, acousticaloptical or acoustical-electronic converter with a resolution of 0.25mm would eliminate many of the engineering problems. From an engineering viewpoint, the problems associated with medical diagnosis aids and flaw detection equipment are less severe than for underwater viewing, since the environment may be controlled to a large extent. The problems of interpreting the received images, however, may preclude the use of acou.stical holography for these applications. The viewing distance in a medical viewing aid would be from l-15cm, and the objects to be viewed would be from 0.51Ocm across. The frequency of operations would have to be increased to 2-1OMHz to give the necessary resolution, but the hologram size would need to be only 20cm square as a maximum. It is possible that for certain applications, ie viewing tumours in the breast, smaller holograms of perhaps 1Ocm square would record sufficient information to give meaningful reconstructions. Holograms as small as 1Ocm square make techniques other than mechanical’ scanning, 244
ULTRASONICS October 1969
discrete arrays, or Mills Cross arrays more attractive. The ultrasound camera may be constructed with apertures of 10cm diameter, and may be a useful method of recording acoustical holograms in near real-time. Liquid surface ultrasound holography being developed by Brenden and Smith12 shows great promise as a technique for real-time recording of acoustical holograms of limited size. The technique has the great advantage that real-time reconstruction of the holograms may be achieved. Two other properties of the method are that an acoustical reference beam must be used, and that higher sound intensities are required than for other recording techniques. The inhomogeneity of the human body may cause such severe distortion of the received signals as to make recognition of the reconstructed image extremely difficult. In addition, signals reflected from 1Ocm deep in the body will be 40dB down on a signal reflected from the body surface, therefore the reconstructed image of the object will be masked by a very bright reconstruction of the body surface. This may be thought of as analogous to attempting to view an object through a brightly lit 99% reflecting mirror. It is possible that a combination of pulsed ultrasound, and the focused image hologram technique described by Brenden and Smith12 may alleviate this problem. Most of the above remarks apply equally to flaw detection as to medical viewing aids. It is probable that acoustical holography will find its first application in one of these two fields.
ACKNOWLEDGEMENTS The authors acknowledge useful discussions rasiewicz.
with B. M. Wat-
REFERENCES 1
Halstead, J., ‘Ultrasound holography’, Ultrasonics, April (1968)
2
Mueller, R. K., and Sheridon, N. K., ‘Sound Holograms and optical reconstructions’, Applied Physics Letters, Vol 9, No 9, (1966)
3
Marom, E., et al, ‘Ultrasonic holography by electronic scanning of a piezoelectric crystal’, Applied Physics Letters, Vol 12, No 2, (1968)
4
Metherell, A. F., et al, ‘Introduction to acoustical holography’, Journal of the Acoustical Society of America, Vol 42, No 4, (1967) p 733
5
Preston, K., and Kreutzen, J. L., ‘Ultrasonic imaging using a synthetic holographic technique’, Applied Physics Letters, Vol 10, No 5, (1967)
6
Carter, W. H., ‘Aliasing in sampled holograms’, Proceedings of the Institution of Electrical and Electronic Engineers, (January 1968)
7
MacAnally, R. B., ‘Inclined reference acoustic holography’, Applied Physics Letters, Vol 11, (1967) p 266
8
Metherell, A. F., ‘The relative.importance of phase and amplitude in acoustical holography;, Proceedings of 1st International Symposium on Acoustical Holography, Plenum Press, New York, (1968)
9
Metherell, A. F., and Spinak, S., ‘Acoustical holography of non-existent wavefronts detected at a single point in space’, Applied Physics Letters, Vol 13, (1968) p 22
10
Wade, G., Powers, J., and Landry, J., ‘Image distortion in reconstructions from phase-only holograms’, Proceedings of Institution of Electrical and Electronics Engineers Symposium on Sonics and Ultrasonics, New York, (1968)
11
Skattebol, L. V., ‘Acoustic holograms’, Electronics ters Vol 4, No 26, (1968) p 583
12
Smith, R. B., and Brenden, B. B., ‘Refinements and variations in liquid surface and scanned ultrasound holography’, Ultrasonics (April 1969)
Let-
holography
and its applications
D. HOLT AND J. R. COLDRICK*
A device capable of producing images of objects immersed in opaque media could find application in such areas as engineering inspection, underwater viewing, and medical diagnosis. The results of some experiments in acoustical holography as a viewing system are presented here, together with an assessment of the problems which must be solved before the technique can find useful application.
Conventional techniques for visualisation of sound patterns are mainly of two classes: (1) a direct image-forming system, in which an acoustical lens or its equivalent forms an acoustical image that can be transduced into a visual image; and (2) a pulse-echo system, well known in underwater studies, in which the shape and phase of a sound burst reflected from an object are compared with the original burst impinging on the object. Depth of focus, angular field of view, and depth of field are limited in a direct image forming system, and losses are increased if wide aperture sonic lenses are to be used. The reliability of the pulse-echo system for identification is generally recognised to be limited. Furthermore, the resolution of direct image systems is limited by geometrical aberrations as well as sensitivity to the turbidity of the medium. The advantages of acoustical holography over other conventional image-forming techniques are simplicity, enormous depth of field, less sensitivity to turbulence and turbidity of the medium, and the capability of retrieving information about the target from discrete sampling points. The quality of the reconstructed image is influenced by several parameters: number of sampling points, turbidity and turbulence of the medium, hologram size, sonic wavelength, size and distance of object, and surface finish of object. Although acoustical holography is analogous to optical holography, it poses quite different problems to the research worker. Progress in optical holography was hampered for years because, until the laser was invented, coherent light sources were not available. Recording the holograms was simply a matter of using a high resolution photographic plate. In acoustical holography, however, most sound sources are coherent. The difficulty has been in recording the interference pattern as there is no acoustical equivalent of the photographic plate. Also, if an acoustical hologram is converted into an optical pattern and then placed in a laser beam, the
* British Aircraft Corporation Ltd, Guided Weapons Division, GPO Box No 77, Filton House, Bristol, England 240
ULTRASONLCS October 1969
reconstructed optical image is extremely small because the wavelength of the light is much shorter than that of the sound. If the image is to be viewed full size, the acoustical hologram must first be reduced in the ratio of light wavelength to sound wavelength before the hologram is placed in the laser beam. In practice this process would result in an optical hologram of about 0. lmm square and the reconstructed image would be extremely faint. A compromise is usually made, the hologram is reduced in size to about 5mm square and the reconstruction viewed through a low power microscope or an eyepiece. The mathematical relationships of acoustical holograms and reconstructions were given by Halsteadl together with an analysis of the intrinsic aberrations of holograms. METHODS OF RECORDING ACOUSTICAL HOLOGRAMS The various methods available for recording acoustical holograms were listed by Halsteadr, who evaluated them by the parameters of sensitivity, resolution, and image-formation time. The methods listed came under the headings of electrical detectors, thermal, mechanical, and chemical detectors. Of all these, only three have found widespread use with research workers, the surface deformation technique has been used by Mueller et al,2 the ultrasound camera by Marom et a1,3 and the scanning detector method by Metherell et aL4 Preston and Kreutzer,s and most other workers. Several factors must be considered in the design of a mechanical scanning frame. The size of the scanning frame is governed by the size of hologram required, which in turn is governed by the acoustic wavelength (A), range (R,) and resolutions (r). A simple formula relating aperature (A) to the above parameters is often used: A=R,; This equation gives the correct results when the case of diffuse radiation from an object is considered (normal for optics) but may give incorrect values in the case of specular reflection (normal for acoustical imaging). In the latter case,
quency signal is further amplified and used to modulate the Fig 1 Photographof scanningframe
it is better to calculate hologram size from geometrical considerations of beam angle, object size and range. Using these criteria, the hologram for a beam angle of 30” and source-tohologram distance of 40cm should be 21cm diameter. The scanning frame used in our experiments (see Fig 1) was designed to give a maximum hologram scan of 30cm x 30cm in order to allow for different geometrical configurations. A frequency of operation of 1MHz was chosen because it was the highest frequency at which a beam angle of 30” at reasonable power levels could be achieved with the equipment available. The scanning rate and the rate of sampling the hologram have some effect on the reconstruction. High-speed scanning causes severe disturbance of the water surface, and multiple acoustic reflections can degrade the hologram recording. A hologram is conventionally thought of as a carrier wavelength d = X/o (o is the angle of the reference beam) modulated by a diffraction pattern of the object. Therefore, an oft-used sampling procedure sets the interval between scans at Ax = h/20 to satisfy the Rayleigh Theorem. However, Carter6 shows that some of the spatial frequencies are under-sampled resulting in aliasing. Aliasing is analogous to the production of Moire patterns: super-position of a diffraction pattern (hologram) and a sampling raster produces a ghost pattern. On reconstruction, symmetrical images are produced containing images and defocused diffraction patterns super-imposed. Thus, image quality is further degraded. A sampling interval of AX = k/4a should overcome this effect. Another factor of great importance in scanning frame design is that of stability. All vibrations must be reduced so that amplitudes are less than x/8 if successful holograms are to be produced. The registration between the scanned probe and the recording lamp or oscilloscope beam scan must be held to similar limits of accuracy.
Camera f (Time exposure)
Electronics
Lamp
Detector \
/Object Signal
source
Fig 2 Acoustical reference system
LF
amp
Rectify
and smooth
PHASE AND AMPLITUDE ACOUSTICAL HOLOGRAPHY The acoustical reference holography system is analogous to the optical,holography system and is illustrated in Fig 2. Here, one source illuminates the object, and diffracted sound falls on the hologram plane, or water surface. A separate source, acting as a reference beam, illuminates the hologram plane directly. Since the two sources are driven by the same master oscillator, the sound beams are coherent, and their interaction at the water surface forms an interference pattern or hologram. The interference pattern is recorded by scanning the sound field with a pick-up, amplifying the received signal, detecting the modulation, and applying the low frequency signal to an electro-luminescent diode or tungsten lamp. The brightness variations of the lamp are recorded by a time exposure photograph as the raster is scanned.
Fig 3 Phase and amplitudeelectronic reference system ULTRASONICS October 1969
241
current through a gallium phosphide electro-luminescent diode mechanically linked with the scanned receiver.
indicates that the image resolution obtainable is of the order of an acoustic wavelength.
From a signal processing viewpoint, the first problem is to distinguish between the true acoustic signal and the unwanted electrical pick-up on the receiver leads. A differential amplifier with a common mode rejection ratio of 40dB is used to achieve this. The electrical pick-up then constitutes common mode signal and its effect is greatly reduced, whereas the true acoustic signal is the difference signal which is amplified. An Elliott Type 7702C linear integrated circuit operating in a differential amplifier configuration with a differential voltage gain of 100 gives good results. The amplified receiver signal is then summed electronically with a reference signal derived from the same master oscillator that drives the transmitting sound source. If the phase of this reference signal is the same as that of the sound source, a co-planar reference wavefront is simulated. However, if phase differences are introduced into the reference signal, it is possible to simulate an off-axis reference wavefront’. For the experimental work described in this article, a coplanar electronic reference was used, ie no phase shifts were introduced into the reference signal. The summed signal is rectified and smoothed using a simple diode and C/R network and the sound field variations across the hologram plane are thus converted into a slowly varying dc level. This dc level is amplified by a second Elliott Type 7702C integrated circuit used as a non-inverting amplifier with a voltage gain of 20. The amplified signal is then used to current drive a Ferranti Type XP50 gallium phosphide diode lamp which emits in the green wavelength band when passing current in the forward direction. Early experiments using a small tungsten filament lamp showed small fringe displacements on the holograms which were due to the temporal delays caused by the thermal time constant being excessive at the scanning speeds employed.
Fig 6 shows the reconstructions of two objects placed at different depths as shown in the photograph of Fig 1. The objects were the letter F previously described and a similar letter C and they were separated by about 4in in the tank. On reconstruction, the letters came into focus at different positions on the optical level, illustrating that a two-dimensional acoustic hologram stores three dimensional information about an object. The figure shows the F in focus in one photograph and the C in focus in the other photograph. The out-of-focus reconstructions of the C and F respectively may also be seen in the two photographs. Fig 7 is the hologram and reconstruction of a stencil of the letters BAC cut out of 3/16in hardboard. The stroke width of these letters is only 6.5 acoustic wavelengths. An interesting feature of this hologram is that an off-axis reference was achieved, not by introducing phase shifts into the electronic reference signal, but by maintaining a co-planar reference and tilting the hologram plane.
The much faster gallium phosphide lamp eliminates this problem and has the advantage that its light output varies linearly with forward current. However, due to its very low intensity, it is necessary to use a very fast photographic emulsion in order to record the modulation patterns that constitute the hologram. Polaroid Type 57 Land film with a daylight speed rating of 3,000 ASA is suitable. The use of a plane reference wavefront enables some of the classic interference experiments to be carried out. A typical example is the formation of Newton’s rings formed by interference of the spherical wavefront from the acoustic source with the electronically simulated plane reference wavefront. When an object is placed between the acoustic source and the water surface, the acoustic wavefront is diffracted by the object and the ring pattern is modulated. This is illustrated in Fig 4 which shows a hologram and reconstruction of a letter F cut out of 3/16in hardboard and having a stroke width of 16 acoustic wavelengths. The reconstruction was
An off-axis reference acoustic hologram has two advantages
over the in-line hologram: firstly, the reconstructed image may be formed partly or wholly out of the zero-order light on reconstruction, depending on the angle of the reference beam and on the degree of photographic reduction and secondly, the main advantage is that many more fine fringes are present in the hologram plane, and therefore better resolution may be obtained in the reconstruction since any given object disturbs more fringes in the hologram.
PHASE-ONLY ACOUSTICAL HOLOGRAPHY It is clear that if acoustic holograms can be made by recording only the phase information of the object wave, then the data acquisition requirements are reduced. This could lead to a useful reduction in the hologram recording time when a scanning or sampling technique is employed. Metherells has shown that the quality of reconstructions from phaseonly acoustic holograms is comparable with that obtainable from phase and amplitude holograms. We have developed a simple phase-only system which permits linear recording of phase information in acoustic holography and which we have
Conjugate image
Flg 5 Optical reconstruction
Fig 4 Hologram and reconstruction
of letter F
obtained by placing the reduced hologram in the optical system of Fig 5. This system provides a convergent beam of coherent light (wavelength 0.63pm) and the reconstructed image is seen when the zero order (undiffracted) light is removed by a stop. The sequence of reconstructions and foci down the optic axis is as shown in Fig 5. There was an irregularity in the top edge of the cut-out about lmm deep and this is recognisable in the reconstruction in Fig 4. This 242
apparatus
ULTRASONICS October 1969
Fig 6 Reconstructionsof holograms of the letters F and C at different depths
REFLECTION HOLOGRAMS Fig 10 shows the reconstruction of the hologram of a steel spanner in the reflection mode. The phase-only recording technique was used. The quality of the reconstruction is not as good a8 that of the transmission results already described. This is probably due to the problems of specular reflection discussed in the next section.
Fig 7 OH-axis hologram and reconstruction
of letters BAC
w
Camera
LOWzo Is). amp
I :
swd%,C
-
used to produce several phase-only acoustic holograms in water at 1MHz. Fig 8 shows the phase-only system. It will be noted that a scanned source and stationary receiver configuration is illustrated rather than the more conventional scanned receiver and stationary source. It is possible to obtain an entirely equivalent hologram by using the scanning transducer .as a source and recording the signal received by the stationary transducer as has been demonstrated by Metherell and Spinake and we have used both configurations in the experimental work reported here. The sound field diffracted by the object is detected by the receiver, and the receiver signal is amplified differentially as before. The next problem is to obtain an output voltage ,which is linearly proportional to the phase difference between this receiver signal and a reference signal derived from the master oscillator used to drive the sound source. This output voltage must also be independent of the amplitude of the receiver signal, These requirements are achieved by applying the two sinusoidal waves whose phase difference is to be determined to two separate but identical channels where they are converted into square waves. The square waves are then combined to give the desired output voltage proportional to phase. The squaring circuits are based on a tunnel diode switch: the sinusoidal signals are amplified and then fed via low output impedance amplifiers (complementary emitter followers) to tunnel diode switches as shown in Fig 8. These approximate to crossover detectors. The resuIting square waves are amplified by pulse amplifiers and further squared by NAND gates. The final square waves have a rise time of 29ns and a constant amplitude of 3V. The reference square wave is then inverted and summed with the signal square wave. The rectified output of the summer is smoothed and the resulting low frequency signal is found to be very nearly proportional to the required phase difference. The smalI departure from linearity is attributed to the non-linear characteristics of the rectifier diode. The low frequency signal is further amplified and then used to current-modulate the gallium phosphide lamp as in previously described systems. Since the light output from the lamp is linear with forward current, the system provides a means of linearly recording phase. Unfortunately, the characteristics of the recording film are somewhat unpredictable and it is inevitable that non-lfnearities are introduced in the photographic stage. Fig 9 is an example of a phase-only hologram and reconstruction of a letter C cut out of hardboard and having a stroke width of 15 wavelengths. It will be observed that the hologram fringes are of equal contrast. This is because the phase-only system makes it possible to use the full linear range of the recording equipment for each fringe. In phase and amplitude holograms, the fringe contrast decreases towards the edges of the hologram since the full linear range of the recording equipment can only be used at localised positions where the object wave intensity is at a maximum. It will be noted that the reconstructed letter C in Fig 9 emphasises the sharp edges of the letter as bright regions. This has been predicted by Wade et allo who have performed a computer study of phase-only holography. Thus, if a certain amount of image distortion (ie edges emphaeised and internal details lost) can be tolerated, phase only recording provides a useful simplification in the signal processing and data acquisition requirements.
1OOk
F’ig 8 Phase-only
3oop
recording system
Fig 9 Phrrse-only hologram and reconstruction
of letter C
Fig 10 Reconstruction of reflection hologram of steel spanner ULTRASONICS October 1969
243
APPLICATIONS OF ACOUSTICAL HOLOGRAPHY Before discussing the feasibility and probable performance of the proposed applications of acoustical holography, it is essential to consider the nature of acoustical images, and the best performance that could be expected from an imaging system assuming that the holographic process was perfect. Any distortion or imperfection caused by the holographic recording and reconstructing process can only further degrade the image. The nature of a sound image is difficult to predict quantitatively, but an assessment of the factors determining the image formation enables a qualitative prediction to be made. The major factors determining image quality are acoustic wavelength and the surface finish of the object. The acoustic wavelength governs the detail on the object which will add to the information content of the reflected wave. In general, object detail smaller than an acoustic wavelength will not be ‘seen’ by an acoustic wave. The quality of the surface finish determines how an object surface reacts to an incident acoustic wave. If the surface finish is better than an acoustic wavelength the surface will act as a mirror, reflecting the whole of the acoustic wave according to the laws of reflection (specular reflection). If the surface is rough, the surface will scatter the sound in all directions (diffuse reflection). The aperture of the acoustic receiver is now of great importance in the quality of the image. In the case of a ‘smooth’ surface, only that part of the surface which is oriented in such a way as to directly reflect sound from the source to the receiver will be recorded, ie only the highlights will be observed. In the case of a ‘rough’ surface, much scattered radiation will be collected by the receiver and image quality will depend on the collecting aperture. The intrinsic aberrations of the holographic procesg which further degrade the image will not be discussed here, since they were described in some detail by Halstead’ in his excellent paper. The application of acoustical holography to underwater viewing is receiving some attention from research workers and system designers, who see it as a useful tool for certain civil engineering applications, and as an inspection device in very muddy water. However, there are several severe limitations to be faced. If the viewing distance is limited to l-2m the size of hologram required for a resolution of a few wavelengths will be of the order of lm square. If the operating frequency is lMHz, and Carter’s criterion6 for element spacing is used (spacing X/4a), the element spacing for a co-planar reference is 0.24mm. A lm square array would require 4000 elements per side, or a total of 16,000,OOO elements. Clearly, this is prohibitive from an engineering and economic viewpoint. A system such as the Mills Cross array11 may reduce the number of components required to about 8000, but the number of measurements would remain the same. If only one cycle of received signal were measured at each resolution point, the time required to form a hologram would be some 16s minimum (on a serial processing basis), and during this time the hologram and object must be held stationary relative to each other to at least 0.2mm. The availability of a lm square, sensitive, real-time, acousticaloptical or acoustical-electronic converter with a resolution of 0.25mm would eliminate many of the engineering problems. From an engineering viewpoint, the problems associated with medical diagnosis aids and flaw detection equipment are less severe than for underwater viewing, since the environment may be controlled to a large extent. The problems of interpreting the received images, however, may preclude the use of acou.stical holography for these applications. The viewing distance in a medical viewing aid would be from l-15cm, and the objects to be viewed would be from 0.51Ocm across. The frequency of operations would have to be increased to 2-1OMHz to give the necessary resolution, but the hologram size would need to be only 20cm square as a maximum. It is possible that for certain applications, ie viewing tumours in the breast, smaller holograms of perhaps 1Ocm square would record sufficient information to give meaningful reconstructions. Holograms as small as 1Ocm square make techniques other than mechanical’ scanning, 244
ULTRASONICS October 1969
discrete arrays, or Mills Cross arrays more attractive. The ultrasound camera may be constructed with apertures of 10cm diameter, and may be a useful method of recording acoustical holograms in near real-time. Liquid surface ultrasound holography being developed by Brenden and Smith12 shows great promise as a technique for real-time recording of acoustical holograms of limited size. The technique has the great advantage that real-time reconstruction of the holograms may be achieved. Two other properties of the method are that an acoustical reference beam must be used, and that higher sound intensities are required than for other recording techniques. The inhomogeneity of the human body may cause such severe distortion of the received signals as to make recognition of the reconstructed image extremely difficult. In addition, signals reflected from 1Ocm deep in the body will be 40dB down on a signal reflected from the body surface, therefore the reconstructed image of the object will be masked by a very bright reconstruction of the body surface. This may be thought of as analogous to attempting to view an object through a brightly lit 99% reflecting mirror. It is possible that a combination of pulsed ultrasound, and the focused image hologram technique described by Brenden and Smith12 may alleviate this problem. Most of the above remarks apply equally to flaw detection as to medical viewing aids. It is probable that acoustical holography will find its first application in one of these two fields.
ACKNOWLEDGEMENTS The authors acknowledge useful discussions rasiewicz.
with B. M. Wat-
REFERENCES 1
Halstead, J., ‘Ultrasound holography’, Ultrasonics, April (1968)
2
Mueller, R. K., and Sheridon, N. K., ‘Sound Holograms and optical reconstructions’, Applied Physics Letters, Vol 9, No 9, (1966)
3
Marom, E., et al, ‘Ultrasonic holography by electronic scanning of a piezoelectric crystal’, Applied Physics Letters, Vol 12, No 2, (1968)
4
Metherell, A. F., et al, ‘Introduction to acoustical holography’, Journal of the Acoustical Society of America, Vol 42, No 4, (1967) p 733
5
Preston, K., and Kreutzen, J. L., ‘Ultrasonic imaging using a synthetic holographic technique’, Applied Physics Letters, Vol 10, No 5, (1967)
6
Carter, W. H., ‘Aliasing in sampled holograms’, Proceedings of the Institution of Electrical and Electronic Engineers, (January 1968)
7
MacAnally, R. B., ‘Inclined reference acoustic holography’, Applied Physics Letters, Vol 11, (1967) p 266
8
Metherell, A. F., ‘The relative.importance of phase and amplitude in acoustical holography;, Proceedings of 1st International Symposium on Acoustical Holography, Plenum Press, New York, (1968)
9
Metherell, A. F., and Spinak, S., ‘Acoustical holography of non-existent wavefronts detected at a single point in space’, Applied Physics Letters, Vol 13, (1968) p 22
10
Wade, G., Powers, J., and Landry, J., ‘Image distortion in reconstructions from phase-only holograms’, Proceedings of Institution of Electrical and Electronics Engineers Symposium on Sonics and Ultrasonics, New York, (1968)
11
Skattebol, L. V., ‘Acoustic holograms’, Electronics ters Vol 4, No 26, (1968) p 583
12
Smith, R. B., and Brenden, B. B., ‘Refinements and variations in liquid surface and scanned ultrasound holography’, Ultrasonics (April 1969)
Let-