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Underwater visual inspection and measurement using optical holography
Optics and Lasers in Engineering 16 (1992) 375-390
Underwater Visual Inspection and Measurement using Optical Holography John Watson Department of Engineering, Fraser-Noble Building, University of Aberdeen, Aberdeen, UK AB9 2UE (Received 18 February 1991; revised version received 10 August 1991; accepted 31 August 1991)
ABSTRACT In offshore oil and gas exploration, visual inspection and measurement often has to be carried out under conditions of poor visibility and at great depths. Recording a hologram of an underwater scene with its subsequent replay in the laboratory could provide enormous benefits to the inspection engineer. When replayed in the real image mode, dimensional measurement may be performed directly on the image formed in real space using, for example, measuring microscopy or TV cameras. Potential applications include general archiving, measurement of corrosion pitting and cracking, examination and measurement of damage sites, structural profiling and examination of marine growth. In this paper, we outline the general concepts of underwater holography. A rdsumd is given of the work undertaken in the past, the problems experienced and their solution and an indication of our general direction leading to the establishment of holography as a unique method of subsea inspection.
INTRODUCTION In the offshore oil and gas industry, quality inspection of subsea installations often has to be carried out in poor visibility and under potentially hazardous conditions. With drilling taking place at ever greater depths, the difficulties increase and more emphasis is being placed on remote, rather than diver held, inspection. Optical methods are extensively utilised using, primarily, conventional photography, 375 Optics and Lasers in Engineering 0143-8166/92/$05.00 (~) 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
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stereo-photography and videography. These techniques, though, all suffer drawbacks, such as possession of only moderate resolution, lack of three-dimensionality, loss of parallax information, perspective distortion and limited depth-of-field. For precise dimensional analysis, sophisticated photogrammetric techniques often have to be employed. Holography, which suffers from none of the above limitations, could provide enormous benefits to the inspection engineer. Remote recording of a hologram of an underwater scene followed by replay in the laboratory, produces a full size, high resolution, three-dimensional image which is optically indistinguishable from the original object. Furthermore, by replaying in the real image mode of reconstruction, the image is located in 'real space' in front of the holographic film rather than behind it as in a virtual image. Utilising this mode of replay allows optical inspection and dimensional measurement to be performed directly upon the image using, for example, measuring microscopy or TV-videography. Such uses of holography are more akin to photogrammetry than interferometry, and the technique has, consequently, come to be known as 'holographic-photogrammetry' or 'hologrammetry'. Potential applications include archiving, measurement of corrosion pitting and cracking, examination and measurement of damage sites, structural profiling and examination of marine growth. The impetus for our work is the desire to apply holography to the high resolution imaging and measurement of subsea components and structures. The work that has been performed in the laboratory, so far, has shown that high fidelity images can be obtained of submerged objects under conditions simulating those obtained subsea. The aberrations which are likely to be encountered have been identified and ways of minimising their effect have been explored. Progress has developed to such an extent that it is now possible to conceive of holography as a legitimate method of underwater inspection and to move towards its general availability and acceptance as a unique service to the offshore industry. In this paper, we review the current state of progress in underwater holography and outline the future direction of the technique.
H O L O G R A M M E T R Y IN V I S U A L INSPECTION The underlying principles of hologrammetry as a means of high resolution visual inspection lie in the holographic recording of a scene of interest followed by its replay in the real image mode of reconstruction. ~3 Although the basic processes of holography are
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well-known, real image formation is not usually so well documented. Because of the importance of this m e t h o d of reconstruction to our work, some of the relevant points are worth outlining. Figures 1 and 2 show the essential steps in recording and replaying an off-axis hologram of a three-dimensional, opaque object. Essentially, the hologram records the superposition of two coherent wavefronts at a known film plane. One wavefront, the object wave, is generated by illuminating the object with monochromatic light. The second wavefront, the reference wave, is a uniform background wave, coherent with the first, incident on the film plane at an off-axis angle with respect to the normal. The amplitude and phase of the reference wave are modulated, at the film plane, by the distribution of light received from the object to produce a complex interference field. By recording the interference field permanently on fine-grain photographic film, we obtain the hologram. Illuminating this hologram with a replica of the original reference wave generates a wave which propagates in line with the apparent direction of original object wave. The generated wave is a replica of the original object wave. When the observer views this wave he cannot distinguish between it and that from the original scene, even though there is an obvious time lapse in history between the two waves. A life size image of the original object appears to be located behind the 'Virtual' image of origina[ object
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photographic plate. A 'virtual' image of the original subject has been formed. In hologrammetry, however, it is the conjugate (real) image that provides the basis of the inspection technique rather than virtual image reconstruction. By illuminating the film with the exact conjugate of the original reference wave, a conjugate image is formed in 'real space' in front of the film (Fig. 3). The image so created is optically identical to the original, save that it appears to be reversed left-to-right and back-to-front when viewed from the space in front of the hologram. This is known as 'Pseudoscopic' image formation. Because of its location in real space in front of the observer, visual inspection can be carried out directly on this image using most conventional optical techniques, such as measuring microscopy, photography and videography. 'Optical sectioning' can be performed by merely placing a piece of film or other optical sensor through the reconstructed image in the plane of interest (Fig. 3).
I M A G E F I D E L I T Y IN H O L O G R A M M E T R Y To allow precise dimensional measurements to be made from any hologram, the reconstructed real image should be low in degrading aberrations. 1'2 Optical aberrations are, of course, present in any imaging system; holography is no exception, underwater or not. 4 The primary monochromatic aberrations, which combine to reduce the overall fidelity of an image giving it a blurred appearance, are those of spherical aberration, coma and astigmatism. Additionally, field curva-
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ture and distortion can cause image deformation. In holography, resolution loss and image degradation can arise at every stage of operation, from recording through chemical processing to replay. Some of these causes of lost resolution are inherent in the optics of the recording and reconstruction process themselves. For example, poor optical quality of lenses and mirrors can cause distortion of the recorded wavefronts, whereas, positional or wavelength mismatch between recording and reconstruction wavefronts can influence the fidelity of the replayed wavefronts. Other degrading effects, such as non-uniform distortion of the fringe pattern recorded in the emulsion, can occur in the chemical processing stage. Emulsion or substrate uniformity is also known to influence image quality. These factors have all been studied in some detail by several laboratories and their likely influence on underwater hologrammetry can be inferred from much of this work. 5"6 In spite of the above, high fidelity images can be produced, provided care is exercised at all stages of recording, processing and replay. The dominant factor in maintaining high resolution is the quality of the reconstruction beam and how closely it matches the recording reference beam. To minimise aberrations in the replayed image, the reconstructing reference beam should possess the same wavelength as that used in recording but with the opposite curvature. U n d e r such conditions, lateral, longitudinal and angular magnifications of the image will all equal unity. 7"8
I M A G E F I D E L I T Y IN U N D E R W A T E R
HOLOGRAMMETRY
Recording holograms underwater introduces additional constraints upon which image fidelity depends. 9-21 Some of these effects relate to the characteristics and physical quality of the water in which the object is located. Potential sources of image degradation include those which arise from local refractive index variations such as thermal gradients or turbulence in the water and those which are due to the attenuating properties of the medium, like scattering and absorption of the incident beams. A n o t h e r important source of aberration arises from the mismatch between the refractive indices of the recording and replay spaces. Inevitably these processes combine to reduce the resolving power of the image below that of the equivalent situation in air. Localised refractive index gradients, for example, can occur in sea-water due to the presence of thermal currents and turbulence. 1°-~2"21 Such variations across the field of view will influence the path length
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and direction of light travelling through the water and accordingly manifest themselves as interference fringes superimposed upon the hologram. Such fringes, if severe enough, might be expected to obscure the inspection area and degrade image quality. This can indeed be shown to be the case at exposure durations of the order of tenths of a second to a few seconds or longer. However, at pulsed laser durations of typically 10 to 50 ns, refractive index gradients encountered offshore are unlikely to be of a sufficient magnitude to result in the appearance of secondary fringes in the hologram. Several holograms taken, in the laboratory, with both pulsed ruby and frequency-doubled N d - Y A G lasers have verified these conclusions, as has an analysis of fringe formation, x2"21 A similar analysis applied to localised turbulence gives the same conclusions. Attenuation of light as it passes through water occurs as the result of two main processes: scattering and absorption. 22 Scattering in sea-water arises from interaction with suspended particles or transparent microorganisms and causes the light to deviate from its original path. Both backscatter and forward scatter can be problematic in underwater holography. Light from the irradiating beam can be backscattered towards the film creating a 'luminous fog' through which the target is viewed; also, light reflected from the brighter parts of the target is foward scattered into the line of sight of darker parts. The darker parts of the object appear brighter than they really are, relative to the bright areas, resulting in a decrease in contrast of the image. This loss of contrast ultimately degrades perceived resolution. In sea-water, suspended particles are much larger than the wavelength of light and thus, unlike atmospheric scattering, sea-water scattering is relatively independent of wavelength over the visible spectrum. Absorption in water, on the other hand, is strongly wavelength dependent. In sea-water, wavelengths in the blue-green region (around 480 to 5 2 0 n m ) of the visible spectrum are less heavily absorbed than those at the violet and red ends. At red wavelengths, attenuation is about three to four times greater than in the blue-green depending on the type of water. As the turbidity increases the window shifts more towards the red. Image contrast in the recorded hologram is further d e p e n d e n t on fringe visibility, which is itself d e p e n d e n t upon the relative planes of polarisation of the interfering beams. When recording a hologram, only object light which is polarised in the same plane of vibration as the reference beam fully contributes to the required hologram. In seawater, light received at the holographic film on reflection from the object can exhibit depolarisation to as much as half the intensity of the original incident polarisation. Reflection of light from a rough object
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surface is a known source of depolarisation, as is light backscattered by the water towards the film on its path to the object. Although this depolarised light cannot contribute to the recording of the hologram it does raise the overall background level at the film and reduces the signal-to-noise ratio. These effects can be conceivably minimised if a vertically orientated linear polariser is placed in front of the film plane so that only light polarised in the required plane reaches the film.
REFRACTIVE INDEX MISMATCH When a hologram is recorded underwater and replayed in air, the reconstructed image will suffer from optical aberrations due to the difference in refractive index between the two media. 16'17 Although the aberrations that are introduced are no different to those which appear in the in-air situation, their effect underwater is more pronounced. The source of these aberrations is, however, not connected with the holographic recording process, but is related purely to the different light paths undergone between objects located in air and those in identical positions in water. In real-image reconstruction, image resolution is limited, in theory, only by the quality of the reproduced hologram. In practice, many effects, like those described earlier, will combine to reduce the overall fidelity of the image. The resolving power of a holographic image, in the absence of all aberrations, is given by the following condition as, = lO-3/(1.22z~./O)
[mm -1]
where ~. is the reconstruction wavelength, z is the separation between hologram and reconstructed image and D is the effective aperture of the hologram. The presence of speckle effects introduced by the coherence of the light and the finite aperture of the viewing system influences the resolving power. The speckle size sets the lower limit to the resolution. In practice, calculated values of ~ are usually decreased by a factor of 2 to 3 to take account of this. Measured values of resolution of holograms of underwater objects have been recorded as high as 1 8 m m -1 under certain conditions compared with the in-air case of 22mm-~. ~6"21 The holograms were recorded, in air and in water, at target-to-film distances of 550 mm. The in-water hologram was positioned in a water-filled observation tank. The wall of the tank was nominally 10 m m thick and the distance between its front wall and the object in water was 300 mm. A second pair of holograms at overall target-to-film distances of 1 0 0 0 m m with
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the corresponding distance of the object in water being 750 m m showed resolutions of 9 and 7 m m -~ for in-air and the in-water cases, respectively. In all cases, the film and target were parallel to each other and on the same optic axis. Although these figures show a decrease in resolving power of underwater holograms over the equivalent holograms recorded in air, they should be contrasted with those obtained by underwater photogrammetry which indicate a resolving power of around 0.5 m m -~ for similar viewing conditions in sea-water. = What is not apparent from the above discussion is the image shift which is also observed in replay of underwater holograms. For points on-axis the image will replay closer to the film plane in the simple ratio of the refractive indices. The reason for this being that light rays travelling from the hologram to the image position traverse paths which are substantially different from those encountered in recording. In particular, the refraction of light which takes place at the air/water interface during recording does not occur in replay. The replayed rays do not converge to the same point from which they e m a n a t e d in recording. On replay, in air, retracing the rays back to their apparent origin shows a spread in the image point location (Fig. 4). The waist of the ray bundle defines the m i n i m u m diameter of the image spot. For points on or near the optic axis, the ratio of image to object distance from the boundary, zi/Zo, corresponds approximately to the ratio of the respective refractive indices of air and water, na/nw. As the point of observation on the object is moved laterally with respect to the optic axis, the measured foreshortening increases beyond
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that given by a simple refractive index ratio (Fig. 5). Off-axis points replay closer to the film plane than on-axis points giving rise to 'field curvature' of the image inwards towards the extremities of the field-of-view. Furthermore, ray bundles lying in mutually perpendicular planes converge to different focal points. The consequence of this, is that two distinct images are formed on reconstruction: one for the 'sagittal' plane and one for the 'tangential' plane. The image is 'astigmatic'. As the object point is moved further from the optic axis the difference between the two image positions increases. The remaining two aberrations, coma (asymmetric flare in off-axis image points) and distortion (non-uniform image magnification across the field-ofview), appear in our situation to be much less significant than the others. They are usually masked by the presence of the other three aberrations. 17 The overall effect of all these aberrations is similar to that observed by viewing, from air, an object submerged in water. The scene exhibits characteristic foreshortening of the image, together with curvature at the edges of the field-of-view and astigmatism in off-axis image points. The holographic process faithfully records the aberrated image which can be seen by any observer.
R E D U C T I O N OF A B E R R A T I O N S We have seen then that a hologram of an underwater scene will possess all the aberrations encountered in viewing such a scene from air. For accurate measurement to be performed on the hologram it is essential that all such aberrations be minimised. Amongst the possible methods
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of aberration reduction being considered is the following simple scheme based on a coincidence of wavelengths. 23'24 When a hologram is formed underwater, the in-air wavelength, Za, of the laser beam will be decreased in proportion to the refractive index of water, nw, to an effective value given by, Zw = }ta/nw
The resulting interference field is, thus, characterised by the in-water wavelength of the laser, ~.w, and not by its in-air wavelength. Replaying this hologram in air will produce a mismatch between the effective recording wavelength, Zw, and the actual replay wavelength, ~'a- The resulting holographic image will suffer from all the limiting aberrations outlined above. However, if a laser with a wavelength corresponding to the in-water value of Zw were available, the hologram could be replayed at its effective recording wavelength, as if the water were still present. The effect of the refractive index change between recording and replay is negated and, accordingly, the major source of aberrations is removed. However, a practical limitation in the above argument is that as soon as we take the film plane out of water and place it in air inside the camera, as we would have to do in any real underwater holographic system, we would seem to undo the very process we are trying to bring about. The object and reference beams pass through water but return to the film via a layer of glass followed by one of air. This would seem to imply that once again the interference pattern is characterised by the in-air wavelength of the laser. Fortunately, a closer look reveals this not to be the case. The fringe pattern which is recorded by the hologram in-water is, as we have said, characteristic of the in-water wavelength of the laser but is also d e p e n d e n t on the angle of orientation of the beam, 0, with respect to the holographic film, that is, Pw = Aw/2 sin 0 If the film is moved into air with the rest of the optical system remaining in-water, the effective wavelength of the object beam increases back to its in-air value as it leaves the water. However, the corresponding recording beam angle also increases by refraction at the water/glass/air boundary. Snell's Law tells us that the increase in beam angle is sufficient to exactly counteract the increase in recording wavelength such that the fringe spacing for the partly in-air case remains the same as for the in-water case. Clearly then the aberration correction still applies if the film is placed in air.
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This is not yet the end of the story. Since the object beam must first travel through a layer of glass followed by a layer of air to reach the film, aberrations will be introduced back into the system at the interfaces. The relative spacing of the air/glass interface will influence the reduction of aberrations brought about by the wavelength change. However, since the refractive index of water lies between that of air and glass, by adjusting the relative lengths of the air and glass paths, we can use the effect of one to cancel the effect of the other. Following, for example, a simple geometric ray tracing analysis, 24 we can show that for aberration cancellation to take effect we need the air thickness in front of the film to be about a fifth of the wall thickness of the window. In much of our work on underwater holography, holograms are recorded using pulsed ruby and frequency-doubled N d - Y A G lasers. In these cases, to minimise aberrations in replay, the corresponding replay wavelengths would be, for ruby, A,v = Aa/nw = 694 nm/1.33 = 522 nm
and for N d - Y A G , '~w = '~a/nw
=
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where the refractive index of water is taken to be 1.33. For holograms recorded using a ruby laser, we have three options: replay at 514 nm using an argon-ion laser, at 532 nm using a continuous frequencydoubled N d - Y A G laser or, for exact correspondence, match the replay wavelength exactly by replaying with an argon-ion p u m p e d dye laser tuned to 522 nm. For the N d - Y A G case, the possible choices are limited to HeCd, at 442 nm, or again, a tunable dye laser. Because of the much more convenient replay wavelength, we used a ruby laser in the experiments to verify the aberration correction procedure. A series of holograms recorded using a ruby laser demonstrated the procedure. 25 A grid target consisting of an array of 10 m m squares marked onto a plastic sheet was placed at the back of the tank 230 m m from the inside of the tank wall (Fig. 6). When replayed in-air at 6 9 4 n m (using an argon-ion p u m p e d dye laser D C M dye), the real image revealed field curvature and a fall-off in resolution towards the extremities of the field-of-view (Fig. 7). When the hologram was replayed at 5 1 4 n m (argon-ion) the degree of field curvature and resolution improved markedly (Fig. 8). Confirmation of these results took place in a further series of experiments using a linear arrangement of U S A F bar resolution targets. The targets were positioned such that they were respectively at different distances off-axis. Target 1 and 77 m m off-axis, corresponding to a field
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angle of 18.5 °, and Target 4 was 151 m m off-axis, corresponding to a field angle of 33 ° . Before correction, the level of astigmatism was so great as to render an estimation of resolution difficult. The astigmatic difference in focal positions for Target 1 was about 14 m m increasing to 50 m m for Target 4. On correction, the observed resolution of the bar targets were m e a s u r e d as 16 m m -1 and 11 m m -1 for Targets 1 and 4, respectively. The astigmatic difference for Target 1 reduced to an indiscernable level, whereas, for Target 4, it was only about 2 mm.
Fig. 7. Grid target replayed without aberration correction.
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Fig. 8. Grid target replayed with aberration correction. FUTURE DEVELOPMENTS The authors' work so far has highlighted the potential of optical holography as a means of performing high resolution inspection and measurement of subsea structures and components. The thrust of the authors' current work now falls into two main areas, namely, development of a prototype underwater holographic camera and establishment of an associated reconstruction facility. Because of the unique and intimate relationship between recording and replay, these two developments should go hand-in-hand. Some of the features that need to be taken into account are choice of laser, output energy requirements, reference beam angle and m e t h o d of aberration correction. In relation to the development of an underwater camera, knowledge of the blue-green window suggests that, for long-range inspection from stand-off distances of a few metres, a frequency-doubled N d - Y A G with its 532 nm output would be the most appropriate choice. However, if the wavelength coincidence m e t h o d of aberration correction is adopted, then recording with a ruby laser and replaying with a dye laser at 5 2 2 n m may be a better option. Considering also that stand-off distances of less than a metre are likely to be typical, then, the lower attenuation obtained by recording in green may not be particularly significant. Also of major relevance is the size and weight of the laser since the camera will undoubtedly have to
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be mounted on a remotely-operated vehicle (ROV). Current commercially available ruby lasers are still big, bulky and utilise even larger power supplies. Diode-pumped Y A G lasers although significantly smaller with more modest power requirements, are still limited in available output energy, particularly, in Q-switched and frequencydoubled modes. Implicit in our camera development is the design and utilisation of a compact and robust laser delivering at least 250 mJ in a Q-switched pulse. It is likely then, at this stage, that any prototype will be built around a ruby laser. In parallel with camera development goes reconstruction; any design features incorporated in the camera will have implications on reconstruction geometry. Once again, mode of aberration correction is a significant decision, effectively determining reconstruction wavelength, reference beam angle and whether the reference beam should be divergent or collimated. Reconstruction is a crucial step in the utilisation of holography for visual inspection: there is little point designing a holographic camera to give the highest quality recording to throw all benefits away in replay. To this end, the final configuration of the replay system is reliant on an understanding of the nature of optical aberrations and on their minimisation. The final end-product should be a unique, dedicated inspection system coupled with on-line analysis of the holographic image.
SUMMARY It is now possible to claim with some degree of confidence that, because of its unique image forming properties, holography has an important role to play in the future of visual inspection of subsea structures and components. Initial fears that the presence of thermal gradients in sea-water coupled with its attenuating properties would make it impossible to record high fidelity holograms have proved largely groundless. High resolution images of on-axis targets have been recorded under a wide variety of sea-water conditions and water path lengths. The major source of degrading aberrations has proven to be caused by the refractive index mismatch between recording and replay spaces, rather than factors introduced by the medium itself. These aberrations, not surprisingly, increase the further the object observation point is from the optic axis. Astigmatism and field curvature turned out to be, in extreme off-axis cases, the most serious causes of image degradation rendering resolution measurement almost impossible. However, work on establishing the nature and extent of the abberrations
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revealed a few potential methods of minimising the aberrations on replay. One particularly simple m e t h o d based on a coincidence of wavelengths has shown that a high degree of correction can be obtained. The m e t h o d , however, may not be so simple to implement in practice since it places some restrictions on camera geometry. Although, then, suffering some degradation over the equivalent situation in air the resolving power obtained from underwater holograms is superior to that obtained by stereo photogrammetry under similar conditions and leads to considerable optimism regarding future application of the technique.
ACKNOWLEDGEMENTS The author wishes to thank the Marine Technology Directorate Ltd. (SERC), UK, C O N O C O (UK) Ltd. and the The British Technology Group for their financial support of this work, the former C E G B Laboratories, Marchwood, UK, for their interest and the University of Aberdeen for its continued encouragement.
REFERENCES 1. Caulfield, H. J., Handbook of Optical Holography, Academic Press, New York, 1979. 2. Hariharan, P., Optical Holography, Cambridge University Press, Cambridge, UK, 1984. 3. Watson, J., Optoelectronics, Van Nostrand Reinhold, Wokingham, UK, 1987. 4. Hecht, E., Optics, Addison-Wesley, Reading, Mass., USA, 1987. 5. Glanville, R., Lewin, L., Little, M. J., Tozer, B. A. & Webster, J. M., Proc. Electro-Optics~Laser International '84 UK, Brighton, Cahners, Twickenham, UK, 1984. 6. Tozer, B. A., Glanville, R., Gordon, A. L., Little, M. J., Webster, J. M. & Wright, D. G., Proc. SP1E, Vol. 523: Applications of Holography, SPIE, Bellingham, Washington, 1985. 7. Meier, R. W., Magnifications and third-order aberrations in holography. JOSA, 55 (1965) 987-92. 8. Champagne, E. B., Non-paraxial imaging, magnification and aberration properties in holography. JOSA, 57 (1967) 51-5. 9. Watson, J., Holography--Can it be used underwater? J. Soc. Underwater Technology, 7 (Winter 1981) 16-20. 10. Watson, J. & Britton, P. W., Applications of holography to underwater inspection. Proc. SUBTECH 83: Design and Operation of Underwater Vehicles, Paper 5.4, Soc. for Underwater Tech., London, 1983.
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11. Watson, J. & Britton, P. W., Preliminary results on underwater holography. Optics and Laser Tech., 15 (4) (August 1983) 215-16. 12. Britton, P. W. & Watson, J., Investigation of the effects of thermal gradients and turbulence on the quality of holograms taken underwater. Proc. Electro-Optics~Laser International '84 UK, ed. H. G. Jerrard. Brighton, Cahners, Twickenham, UK, 1984, pp. 440-7. 13. Watson, J. & Britton, P. W., Engineering measurement from underwater optical holography. J. Photog. Science, 33 (1985) 167-73. 14. Watson, J. & Britton, P. W., Applications of optical holography to underwater visual inspection. Proc. SPIE, Vol. 599: Optics in Engineering Measurement, ed. W. F. Fagan. SPIE, Bellingham, Washington, 1985, pp. 26-31. 15. Watson, J., Britton, P. W. & Cran, A. S., Optical holography applied to underwater visual inspection. Proc. SPIE, Vol. 7 0 1 : 1 9 8 6 European Conference on Optics, Optical Systems and Applications, ed. S. Sottini & S. Trigari, SPIE, Bellingham, Washington, 1987, pp. 49-55. 16. Watson, J., Britton, P. W. & Cran, A. C. S., Resolution of holographic images of underwater objects. Optics and Laser Technology, 19(2) (April 1987) 97-101. 17. Kilpatrick, J. M. & Watson, J., Underwater hologrammetry: aberrations in the real image of an underwater object when played in air. J. Phys. D: Appl. Phys., 21 (1988) 1701-05. 18. Watson, J., Hologrammetry and its applications. J. Imaging Tech., 15 (1989) 38-46. 19. Watson, J., High resolution visual inspection using underwater holography. In International Advances in Non-Destructive Testing, Vol. 14, ed. W. J. McGonnagle. Gordon and Breach, New York, 1989, pp. 335-60. 20. Watson, J., Application of holography to underwater visual inspection. In Industrial Applications of Holography, ed. J. Robillard & H. J. Caulfield. Oxford University Press, New York, 1990. 21. Britton, P. W., A feasibility study of underwater holography. M Phil Thesis, University of Aberdeen, 1991. 22. Turner, J. D., J. Soc. Underwater Tech., Winter (1983) 7-13. 23. Armour, I. A., Kilpatrick, J. M. & Watson, J., Reduction of aberrations in underwater holography. 2rid Annual Conference on Holographic Systems, Components and Applications, Proc. lEE No. 311, Institute of Electrical, Engineers, London, 1989, pp. 21-4. 24. Kilpatrick, J. M., An evaluation of optical aberrations in underwater hologrammetry. PhD Thesis, to be published, 1991. 25. Watson, J. & Kilpatrick, J. M., Proc SPIE: Vol 1461: Practical Holography V, ed. S. A. Bentin. SPIE, Bellingham, Washington, 1991.
Underwater Visual Inspection and Measurement using Optical Holography John Watson Department of Engineering, Fraser-Noble Building, University of Aberdeen, Aberdeen, UK AB9 2UE (Received 18 February 1991; revised version received 10 August 1991; accepted 31 August 1991)
ABSTRACT In offshore oil and gas exploration, visual inspection and measurement often has to be carried out under conditions of poor visibility and at great depths. Recording a hologram of an underwater scene with its subsequent replay in the laboratory could provide enormous benefits to the inspection engineer. When replayed in the real image mode, dimensional measurement may be performed directly on the image formed in real space using, for example, measuring microscopy or TV cameras. Potential applications include general archiving, measurement of corrosion pitting and cracking, examination and measurement of damage sites, structural profiling and examination of marine growth. In this paper, we outline the general concepts of underwater holography. A rdsumd is given of the work undertaken in the past, the problems experienced and their solution and an indication of our general direction leading to the establishment of holography as a unique method of subsea inspection.
INTRODUCTION In the offshore oil and gas industry, quality inspection of subsea installations often has to be carried out in poor visibility and under potentially hazardous conditions. With drilling taking place at ever greater depths, the difficulties increase and more emphasis is being placed on remote, rather than diver held, inspection. Optical methods are extensively utilised using, primarily, conventional photography, 375 Optics and Lasers in Engineering 0143-8166/92/$05.00 (~) 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
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stereo-photography and videography. These techniques, though, all suffer drawbacks, such as possession of only moderate resolution, lack of three-dimensionality, loss of parallax information, perspective distortion and limited depth-of-field. For precise dimensional analysis, sophisticated photogrammetric techniques often have to be employed. Holography, which suffers from none of the above limitations, could provide enormous benefits to the inspection engineer. Remote recording of a hologram of an underwater scene followed by replay in the laboratory, produces a full size, high resolution, three-dimensional image which is optically indistinguishable from the original object. Furthermore, by replaying in the real image mode of reconstruction, the image is located in 'real space' in front of the holographic film rather than behind it as in a virtual image. Utilising this mode of replay allows optical inspection and dimensional measurement to be performed directly upon the image using, for example, measuring microscopy or TV-videography. Such uses of holography are more akin to photogrammetry than interferometry, and the technique has, consequently, come to be known as 'holographic-photogrammetry' or 'hologrammetry'. Potential applications include archiving, measurement of corrosion pitting and cracking, examination and measurement of damage sites, structural profiling and examination of marine growth. The impetus for our work is the desire to apply holography to the high resolution imaging and measurement of subsea components and structures. The work that has been performed in the laboratory, so far, has shown that high fidelity images can be obtained of submerged objects under conditions simulating those obtained subsea. The aberrations which are likely to be encountered have been identified and ways of minimising their effect have been explored. Progress has developed to such an extent that it is now possible to conceive of holography as a legitimate method of underwater inspection and to move towards its general availability and acceptance as a unique service to the offshore industry. In this paper, we review the current state of progress in underwater holography and outline the future direction of the technique.
H O L O G R A M M E T R Y IN V I S U A L INSPECTION The underlying principles of hologrammetry as a means of high resolution visual inspection lie in the holographic recording of a scene of interest followed by its replay in the real image mode of reconstruction. ~3 Although the basic processes of holography are
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well-known, real image formation is not usually so well documented. Because of the importance of this m e t h o d of reconstruction to our work, some of the relevant points are worth outlining. Figures 1 and 2 show the essential steps in recording and replaying an off-axis hologram of a three-dimensional, opaque object. Essentially, the hologram records the superposition of two coherent wavefronts at a known film plane. One wavefront, the object wave, is generated by illuminating the object with monochromatic light. The second wavefront, the reference wave, is a uniform background wave, coherent with the first, incident on the film plane at an off-axis angle with respect to the normal. The amplitude and phase of the reference wave are modulated, at the film plane, by the distribution of light received from the object to produce a complex interference field. By recording the interference field permanently on fine-grain photographic film, we obtain the hologram. Illuminating this hologram with a replica of the original reference wave generates a wave which propagates in line with the apparent direction of original object wave. The generated wave is a replica of the original object wave. When the observer views this wave he cannot distinguish between it and that from the original scene, even though there is an obvious time lapse in history between the two waves. A life size image of the original object appears to be located behind the 'Virtual' image of origina[ object
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photographic plate. A 'virtual' image of the original subject has been formed. In hologrammetry, however, it is the conjugate (real) image that provides the basis of the inspection technique rather than virtual image reconstruction. By illuminating the film with the exact conjugate of the original reference wave, a conjugate image is formed in 'real space' in front of the film (Fig. 3). The image so created is optically identical to the original, save that it appears to be reversed left-to-right and back-to-front when viewed from the space in front of the hologram. This is known as 'Pseudoscopic' image formation. Because of its location in real space in front of the observer, visual inspection can be carried out directly on this image using most conventional optical techniques, such as measuring microscopy, photography and videography. 'Optical sectioning' can be performed by merely placing a piece of film or other optical sensor through the reconstructed image in the plane of interest (Fig. 3).
I M A G E F I D E L I T Y IN H O L O G R A M M E T R Y To allow precise dimensional measurements to be made from any hologram, the reconstructed real image should be low in degrading aberrations. 1'2 Optical aberrations are, of course, present in any imaging system; holography is no exception, underwater or not. 4 The primary monochromatic aberrations, which combine to reduce the overall fidelity of an image giving it a blurred appearance, are those of spherical aberration, coma and astigmatism. Additionally, field curva-
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ture and distortion can cause image deformation. In holography, resolution loss and image degradation can arise at every stage of operation, from recording through chemical processing to replay. Some of these causes of lost resolution are inherent in the optics of the recording and reconstruction process themselves. For example, poor optical quality of lenses and mirrors can cause distortion of the recorded wavefronts, whereas, positional or wavelength mismatch between recording and reconstruction wavefronts can influence the fidelity of the replayed wavefronts. Other degrading effects, such as non-uniform distortion of the fringe pattern recorded in the emulsion, can occur in the chemical processing stage. Emulsion or substrate uniformity is also known to influence image quality. These factors have all been studied in some detail by several laboratories and their likely influence on underwater hologrammetry can be inferred from much of this work. 5"6 In spite of the above, high fidelity images can be produced, provided care is exercised at all stages of recording, processing and replay. The dominant factor in maintaining high resolution is the quality of the reconstruction beam and how closely it matches the recording reference beam. To minimise aberrations in the replayed image, the reconstructing reference beam should possess the same wavelength as that used in recording but with the opposite curvature. U n d e r such conditions, lateral, longitudinal and angular magnifications of the image will all equal unity. 7"8
I M A G E F I D E L I T Y IN U N D E R W A T E R
HOLOGRAMMETRY
Recording holograms underwater introduces additional constraints upon which image fidelity depends. 9-21 Some of these effects relate to the characteristics and physical quality of the water in which the object is located. Potential sources of image degradation include those which arise from local refractive index variations such as thermal gradients or turbulence in the water and those which are due to the attenuating properties of the medium, like scattering and absorption of the incident beams. A n o t h e r important source of aberration arises from the mismatch between the refractive indices of the recording and replay spaces. Inevitably these processes combine to reduce the resolving power of the image below that of the equivalent situation in air. Localised refractive index gradients, for example, can occur in sea-water due to the presence of thermal currents and turbulence. 1°-~2"21 Such variations across the field of view will influence the path length
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and direction of light travelling through the water and accordingly manifest themselves as interference fringes superimposed upon the hologram. Such fringes, if severe enough, might be expected to obscure the inspection area and degrade image quality. This can indeed be shown to be the case at exposure durations of the order of tenths of a second to a few seconds or longer. However, at pulsed laser durations of typically 10 to 50 ns, refractive index gradients encountered offshore are unlikely to be of a sufficient magnitude to result in the appearance of secondary fringes in the hologram. Several holograms taken, in the laboratory, with both pulsed ruby and frequency-doubled N d - Y A G lasers have verified these conclusions, as has an analysis of fringe formation, x2"21 A similar analysis applied to localised turbulence gives the same conclusions. Attenuation of light as it passes through water occurs as the result of two main processes: scattering and absorption. 22 Scattering in sea-water arises from interaction with suspended particles or transparent microorganisms and causes the light to deviate from its original path. Both backscatter and forward scatter can be problematic in underwater holography. Light from the irradiating beam can be backscattered towards the film creating a 'luminous fog' through which the target is viewed; also, light reflected from the brighter parts of the target is foward scattered into the line of sight of darker parts. The darker parts of the object appear brighter than they really are, relative to the bright areas, resulting in a decrease in contrast of the image. This loss of contrast ultimately degrades perceived resolution. In sea-water, suspended particles are much larger than the wavelength of light and thus, unlike atmospheric scattering, sea-water scattering is relatively independent of wavelength over the visible spectrum. Absorption in water, on the other hand, is strongly wavelength dependent. In sea-water, wavelengths in the blue-green region (around 480 to 5 2 0 n m ) of the visible spectrum are less heavily absorbed than those at the violet and red ends. At red wavelengths, attenuation is about three to four times greater than in the blue-green depending on the type of water. As the turbidity increases the window shifts more towards the red. Image contrast in the recorded hologram is further d e p e n d e n t on fringe visibility, which is itself d e p e n d e n t upon the relative planes of polarisation of the interfering beams. When recording a hologram, only object light which is polarised in the same plane of vibration as the reference beam fully contributes to the required hologram. In seawater, light received at the holographic film on reflection from the object can exhibit depolarisation to as much as half the intensity of the original incident polarisation. Reflection of light from a rough object
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surface is a known source of depolarisation, as is light backscattered by the water towards the film on its path to the object. Although this depolarised light cannot contribute to the recording of the hologram it does raise the overall background level at the film and reduces the signal-to-noise ratio. These effects can be conceivably minimised if a vertically orientated linear polariser is placed in front of the film plane so that only light polarised in the required plane reaches the film.
REFRACTIVE INDEX MISMATCH When a hologram is recorded underwater and replayed in air, the reconstructed image will suffer from optical aberrations due to the difference in refractive index between the two media. 16'17 Although the aberrations that are introduced are no different to those which appear in the in-air situation, their effect underwater is more pronounced. The source of these aberrations is, however, not connected with the holographic recording process, but is related purely to the different light paths undergone between objects located in air and those in identical positions in water. In real-image reconstruction, image resolution is limited, in theory, only by the quality of the reproduced hologram. In practice, many effects, like those described earlier, will combine to reduce the overall fidelity of the image. The resolving power of a holographic image, in the absence of all aberrations, is given by the following condition as, = lO-3/(1.22z~./O)
[mm -1]
where ~. is the reconstruction wavelength, z is the separation between hologram and reconstructed image and D is the effective aperture of the hologram. The presence of speckle effects introduced by the coherence of the light and the finite aperture of the viewing system influences the resolving power. The speckle size sets the lower limit to the resolution. In practice, calculated values of ~ are usually decreased by a factor of 2 to 3 to take account of this. Measured values of resolution of holograms of underwater objects have been recorded as high as 1 8 m m -1 under certain conditions compared with the in-air case of 22mm-~. ~6"21 The holograms were recorded, in air and in water, at target-to-film distances of 550 mm. The in-water hologram was positioned in a water-filled observation tank. The wall of the tank was nominally 10 m m thick and the distance between its front wall and the object in water was 300 mm. A second pair of holograms at overall target-to-film distances of 1 0 0 0 m m with
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the corresponding distance of the object in water being 750 m m showed resolutions of 9 and 7 m m -~ for in-air and the in-water cases, respectively. In all cases, the film and target were parallel to each other and on the same optic axis. Although these figures show a decrease in resolving power of underwater holograms over the equivalent holograms recorded in air, they should be contrasted with those obtained by underwater photogrammetry which indicate a resolving power of around 0.5 m m -~ for similar viewing conditions in sea-water. = What is not apparent from the above discussion is the image shift which is also observed in replay of underwater holograms. For points on-axis the image will replay closer to the film plane in the simple ratio of the refractive indices. The reason for this being that light rays travelling from the hologram to the image position traverse paths which are substantially different from those encountered in recording. In particular, the refraction of light which takes place at the air/water interface during recording does not occur in replay. The replayed rays do not converge to the same point from which they e m a n a t e d in recording. On replay, in air, retracing the rays back to their apparent origin shows a spread in the image point location (Fig. 4). The waist of the ray bundle defines the m i n i m u m diameter of the image spot. For points on or near the optic axis, the ratio of image to object distance from the boundary, zi/Zo, corresponds approximately to the ratio of the respective refractive indices of air and water, na/nw. As the point of observation on the object is moved laterally with respect to the optic axis, the measured foreshortening increases beyond
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that given by a simple refractive index ratio (Fig. 5). Off-axis points replay closer to the film plane than on-axis points giving rise to 'field curvature' of the image inwards towards the extremities of the field-of-view. Furthermore, ray bundles lying in mutually perpendicular planes converge to different focal points. The consequence of this, is that two distinct images are formed on reconstruction: one for the 'sagittal' plane and one for the 'tangential' plane. The image is 'astigmatic'. As the object point is moved further from the optic axis the difference between the two image positions increases. The remaining two aberrations, coma (asymmetric flare in off-axis image points) and distortion (non-uniform image magnification across the field-ofview), appear in our situation to be much less significant than the others. They are usually masked by the presence of the other three aberrations. 17 The overall effect of all these aberrations is similar to that observed by viewing, from air, an object submerged in water. The scene exhibits characteristic foreshortening of the image, together with curvature at the edges of the field-of-view and astigmatism in off-axis image points. The holographic process faithfully records the aberrated image which can be seen by any observer.
R E D U C T I O N OF A B E R R A T I O N S We have seen then that a hologram of an underwater scene will possess all the aberrations encountered in viewing such a scene from air. For accurate measurement to be performed on the hologram it is essential that all such aberrations be minimised. Amongst the possible methods
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of aberration reduction being considered is the following simple scheme based on a coincidence of wavelengths. 23'24 When a hologram is formed underwater, the in-air wavelength, Za, of the laser beam will be decreased in proportion to the refractive index of water, nw, to an effective value given by, Zw = }ta/nw
The resulting interference field is, thus, characterised by the in-water wavelength of the laser, ~.w, and not by its in-air wavelength. Replaying this hologram in air will produce a mismatch between the effective recording wavelength, Zw, and the actual replay wavelength, ~'a- The resulting holographic image will suffer from all the limiting aberrations outlined above. However, if a laser with a wavelength corresponding to the in-water value of Zw were available, the hologram could be replayed at its effective recording wavelength, as if the water were still present. The effect of the refractive index change between recording and replay is negated and, accordingly, the major source of aberrations is removed. However, a practical limitation in the above argument is that as soon as we take the film plane out of water and place it in air inside the camera, as we would have to do in any real underwater holographic system, we would seem to undo the very process we are trying to bring about. The object and reference beams pass through water but return to the film via a layer of glass followed by one of air. This would seem to imply that once again the interference pattern is characterised by the in-air wavelength of the laser. Fortunately, a closer look reveals this not to be the case. The fringe pattern which is recorded by the hologram in-water is, as we have said, characteristic of the in-water wavelength of the laser but is also d e p e n d e n t on the angle of orientation of the beam, 0, with respect to the holographic film, that is, Pw = Aw/2 sin 0 If the film is moved into air with the rest of the optical system remaining in-water, the effective wavelength of the object beam increases back to its in-air value as it leaves the water. However, the corresponding recording beam angle also increases by refraction at the water/glass/air boundary. Snell's Law tells us that the increase in beam angle is sufficient to exactly counteract the increase in recording wavelength such that the fringe spacing for the partly in-air case remains the same as for the in-water case. Clearly then the aberration correction still applies if the film is placed in air.
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This is not yet the end of the story. Since the object beam must first travel through a layer of glass followed by a layer of air to reach the film, aberrations will be introduced back into the system at the interfaces. The relative spacing of the air/glass interface will influence the reduction of aberrations brought about by the wavelength change. However, since the refractive index of water lies between that of air and glass, by adjusting the relative lengths of the air and glass paths, we can use the effect of one to cancel the effect of the other. Following, for example, a simple geometric ray tracing analysis, 24 we can show that for aberration cancellation to take effect we need the air thickness in front of the film to be about a fifth of the wall thickness of the window. In much of our work on underwater holography, holograms are recorded using pulsed ruby and frequency-doubled N d - Y A G lasers. In these cases, to minimise aberrations in replay, the corresponding replay wavelengths would be, for ruby, A,v = Aa/nw = 694 nm/1.33 = 522 nm
and for N d - Y A G , '~w = '~a/nw
=
532 nm/1-33 = 400 n m
where the refractive index of water is taken to be 1.33. For holograms recorded using a ruby laser, we have three options: replay at 514 nm using an argon-ion laser, at 532 nm using a continuous frequencydoubled N d - Y A G laser or, for exact correspondence, match the replay wavelength exactly by replaying with an argon-ion p u m p e d dye laser tuned to 522 nm. For the N d - Y A G case, the possible choices are limited to HeCd, at 442 nm, or again, a tunable dye laser. Because of the much more convenient replay wavelength, we used a ruby laser in the experiments to verify the aberration correction procedure. A series of holograms recorded using a ruby laser demonstrated the procedure. 25 A grid target consisting of an array of 10 m m squares marked onto a plastic sheet was placed at the back of the tank 230 m m from the inside of the tank wall (Fig. 6). When replayed in-air at 6 9 4 n m (using an argon-ion p u m p e d dye laser D C M dye), the real image revealed field curvature and a fall-off in resolution towards the extremities of the field-of-view (Fig. 7). When the hologram was replayed at 5 1 4 n m (argon-ion) the degree of field curvature and resolution improved markedly (Fig. 8). Confirmation of these results took place in a further series of experiments using a linear arrangement of U S A F bar resolution targets. The targets were positioned such that they were respectively at different distances off-axis. Target 1 and 77 m m off-axis, corresponding to a field
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angle of 18.5 °, and Target 4 was 151 m m off-axis, corresponding to a field angle of 33 ° . Before correction, the level of astigmatism was so great as to render an estimation of resolution difficult. The astigmatic difference in focal positions for Target 1 was about 14 m m increasing to 50 m m for Target 4. On correction, the observed resolution of the bar targets were m e a s u r e d as 16 m m -1 and 11 m m -1 for Targets 1 and 4, respectively. The astigmatic difference for Target 1 reduced to an indiscernable level, whereas, for Target 4, it was only about 2 mm.
Fig. 7. Grid target replayed without aberration correction.
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Fig. 8. Grid target replayed with aberration correction. FUTURE DEVELOPMENTS The authors' work so far has highlighted the potential of optical holography as a means of performing high resolution inspection and measurement of subsea structures and components. The thrust of the authors' current work now falls into two main areas, namely, development of a prototype underwater holographic camera and establishment of an associated reconstruction facility. Because of the unique and intimate relationship between recording and replay, these two developments should go hand-in-hand. Some of the features that need to be taken into account are choice of laser, output energy requirements, reference beam angle and m e t h o d of aberration correction. In relation to the development of an underwater camera, knowledge of the blue-green window suggests that, for long-range inspection from stand-off distances of a few metres, a frequency-doubled N d - Y A G with its 532 nm output would be the most appropriate choice. However, if the wavelength coincidence m e t h o d of aberration correction is adopted, then recording with a ruby laser and replaying with a dye laser at 5 2 2 n m may be a better option. Considering also that stand-off distances of less than a metre are likely to be typical, then, the lower attenuation obtained by recording in green may not be particularly significant. Also of major relevance is the size and weight of the laser since the camera will undoubtedly have to
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be mounted on a remotely-operated vehicle (ROV). Current commercially available ruby lasers are still big, bulky and utilise even larger power supplies. Diode-pumped Y A G lasers although significantly smaller with more modest power requirements, are still limited in available output energy, particularly, in Q-switched and frequencydoubled modes. Implicit in our camera development is the design and utilisation of a compact and robust laser delivering at least 250 mJ in a Q-switched pulse. It is likely then, at this stage, that any prototype will be built around a ruby laser. In parallel with camera development goes reconstruction; any design features incorporated in the camera will have implications on reconstruction geometry. Once again, mode of aberration correction is a significant decision, effectively determining reconstruction wavelength, reference beam angle and whether the reference beam should be divergent or collimated. Reconstruction is a crucial step in the utilisation of holography for visual inspection: there is little point designing a holographic camera to give the highest quality recording to throw all benefits away in replay. To this end, the final configuration of the replay system is reliant on an understanding of the nature of optical aberrations and on their minimisation. The final end-product should be a unique, dedicated inspection system coupled with on-line analysis of the holographic image.
SUMMARY It is now possible to claim with some degree of confidence that, because of its unique image forming properties, holography has an important role to play in the future of visual inspection of subsea structures and components. Initial fears that the presence of thermal gradients in sea-water coupled with its attenuating properties would make it impossible to record high fidelity holograms have proved largely groundless. High resolution images of on-axis targets have been recorded under a wide variety of sea-water conditions and water path lengths. The major source of degrading aberrations has proven to be caused by the refractive index mismatch between recording and replay spaces, rather than factors introduced by the medium itself. These aberrations, not surprisingly, increase the further the object observation point is from the optic axis. Astigmatism and field curvature turned out to be, in extreme off-axis cases, the most serious causes of image degradation rendering resolution measurement almost impossible. However, work on establishing the nature and extent of the abberrations
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revealed a few potential methods of minimising the aberrations on replay. One particularly simple m e t h o d based on a coincidence of wavelengths has shown that a high degree of correction can be obtained. The m e t h o d , however, may not be so simple to implement in practice since it places some restrictions on camera geometry. Although, then, suffering some degradation over the equivalent situation in air the resolving power obtained from underwater holograms is superior to that obtained by stereo photogrammetry under similar conditions and leads to considerable optimism regarding future application of the technique.
ACKNOWLEDGEMENTS The author wishes to thank the Marine Technology Directorate Ltd. (SERC), UK, C O N O C O (UK) Ltd. and the The British Technology Group for their financial support of this work, the former C E G B Laboratories, Marchwood, UK, for their interest and the University of Aberdeen for its continued encouragement.
REFERENCES 1. Caulfield, H. J., Handbook of Optical Holography, Academic Press, New York, 1979. 2. Hariharan, P., Optical Holography, Cambridge University Press, Cambridge, UK, 1984. 3. Watson, J., Optoelectronics, Van Nostrand Reinhold, Wokingham, UK, 1987. 4. Hecht, E., Optics, Addison-Wesley, Reading, Mass., USA, 1987. 5. Glanville, R., Lewin, L., Little, M. J., Tozer, B. A. & Webster, J. M., Proc. Electro-Optics~Laser International '84 UK, Brighton, Cahners, Twickenham, UK, 1984. 6. Tozer, B. A., Glanville, R., Gordon, A. L., Little, M. J., Webster, J. M. & Wright, D. G., Proc. SP1E, Vol. 523: Applications of Holography, SPIE, Bellingham, Washington, 1985. 7. Meier, R. W., Magnifications and third-order aberrations in holography. JOSA, 55 (1965) 987-92. 8. Champagne, E. B., Non-paraxial imaging, magnification and aberration properties in holography. JOSA, 57 (1967) 51-5. 9. Watson, J., Holography--Can it be used underwater? J. Soc. Underwater Technology, 7 (Winter 1981) 16-20. 10. Watson, J. & Britton, P. W., Applications of holography to underwater inspection. Proc. SUBTECH 83: Design and Operation of Underwater Vehicles, Paper 5.4, Soc. for Underwater Tech., London, 1983.
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11. Watson, J. & Britton, P. W., Preliminary results on underwater holography. Optics and Laser Tech., 15 (4) (August 1983) 215-16. 12. Britton, P. W. & Watson, J., Investigation of the effects of thermal gradients and turbulence on the quality of holograms taken underwater. Proc. Electro-Optics~Laser International '84 UK, ed. H. G. Jerrard. Brighton, Cahners, Twickenham, UK, 1984, pp. 440-7. 13. Watson, J. & Britton, P. W., Engineering measurement from underwater optical holography. J. Photog. Science, 33 (1985) 167-73. 14. Watson, J. & Britton, P. W., Applications of optical holography to underwater visual inspection. Proc. SPIE, Vol. 599: Optics in Engineering Measurement, ed. W. F. Fagan. SPIE, Bellingham, Washington, 1985, pp. 26-31. 15. Watson, J., Britton, P. W. & Cran, A. S., Optical holography applied to underwater visual inspection. Proc. SPIE, Vol. 7 0 1 : 1 9 8 6 European Conference on Optics, Optical Systems and Applications, ed. S. Sottini & S. Trigari, SPIE, Bellingham, Washington, 1987, pp. 49-55. 16. Watson, J., Britton, P. W. & Cran, A. C. S., Resolution of holographic images of underwater objects. Optics and Laser Technology, 19(2) (April 1987) 97-101. 17. Kilpatrick, J. M. & Watson, J., Underwater hologrammetry: aberrations in the real image of an underwater object when played in air. J. Phys. D: Appl. Phys., 21 (1988) 1701-05. 18. Watson, J., Hologrammetry and its applications. J. Imaging Tech., 15 (1989) 38-46. 19. Watson, J., High resolution visual inspection using underwater holography. In International Advances in Non-Destructive Testing, Vol. 14, ed. W. J. McGonnagle. Gordon and Breach, New York, 1989, pp. 335-60. 20. Watson, J., Application of holography to underwater visual inspection. In Industrial Applications of Holography, ed. J. Robillard & H. J. Caulfield. Oxford University Press, New York, 1990. 21. Britton, P. W., A feasibility study of underwater holography. M Phil Thesis, University of Aberdeen, 1991. 22. Turner, J. D., J. Soc. Underwater Tech., Winter (1983) 7-13. 23. Armour, I. A., Kilpatrick, J. M. & Watson, J., Reduction of aberrations in underwater holography. 2rid Annual Conference on Holographic Systems, Components and Applications, Proc. lEE No. 311, Institute of Electrical, Engineers, London, 1989, pp. 21-4. 24. Kilpatrick, J. M., An evaluation of optical aberrations in underwater hologrammetry. PhD Thesis, to be published, 1991. 25. Watson, J. & Kilpatrick, J. M., Proc SPIE: Vol 1461: Practical Holography V, ed. S. A. Bentin. SPIE, Bellingham, Washington, 1991.