Optical: systems and elements – Holographic system or element – For synthetically generating a hologram
Patent
1990-12-31
1993-02-02
Lerner, Martin
Optical: systems and elements
Holographic system or element
For synthetically generating a hologram
359 30, 359901, G03H 300
Patent
active
051842300
DESCRIPTION:
BRIEF SUMMARY
This invention relates to holography and, in particular, to methods of and apparatus for the holographic inspection of underwater objects such as pipelines.
The recovery of oil and gas has presented a significant challenge to the offshore industry as regards routine inspection and maintenance of subsea installations. As drilling now proceeds to even greater depths the problems encountered increase and more emphasis is now being placed on remote, rather than diver held, techniques of inspection. Visual inspection is extensively utilised with the major part of this being carried out using conventional photography, stereo photography and closed-circuit television. These methods all, however, suffer several drawbacks. Conventional photography produces two-dimensional images of moderate resolution but loses parallax information and, particularly in close-up, suffers from perspective distortion and limited depth of field. Stereo-photography improves this situation by producing a three-dimensional image from two fixed viewpoints: parallax information is still lost. Furthermore, if precise dimensional measurements are required, sophisticated photogrammetric techniques are necessary with limited resolution as yet occupancy. Television systems while providing real-time operation are essentially low resolution techniques.
Holography, by comparison, suffers from none of these limitations and gives the observer an infinity of viewpoints from which the view the scene. It is the ability of holography to reproduce, remote from the original scene, a full size three-dimensional image possessing high resolution and low in optical aberrations which make it a potentially powerful method of visual inspection. Applications which can be envisaged include general archiving, measurement of corrosion pitting and cracking, examination and measurement of damage sites, structural profiling and examination of marine growth. In all such applications the required end product is usually a high resolution hologram of a particular scene of interest. From this hologram, inspection can be performed directly on an image reconstructed in real space.
Holographic visual inspection or "hologrammetry" as it has now come to be known, is becoming increasingly important as a means of making high resolution dimensional measurements of engineering components and structures. This is particularly true when the inspection site is located in a hazardous environment or is an area where access is difficult, such as encountered in the nuclear power generating industry or the offshore oil and gas industries.
The basis of holography as a means of high resolution visual inspection is the holographic recording of the scene of interest with the subsequent replay of the processed hologram in the real image mode of reconstruction. Reconstruction of the real image produces an image which is reversed left-to-right and back-to-front when viewed from the space in front of the hologram. Such a representation of the image is known as "pseudoscopic".
In general, the holographic interference field is captured on photographic film. Other media such as thermoplastic film, photochromic materials, non-linear optical crystals and dichromated gelatin may, alternatively, be used. Holographic film differs from the film used in ordinary photography only in that the grains of silver halide are of the order of a few nanometers across as compared to micrometers. Such film is very insensitive to light but has the capacity to record the fine detail inherent in an interference field. Typical sensitivity is around a few millijoules per square meter. The exposed film is chemically processed in a similar, but somewhat more elaborate, way to ordinary film to render the holographic interference permanent.
For purposes of visual inspection, however, creation of the virtual image is not the most suitable form of holographic reconstruction. It so happens that if we turn the plate around and illuminate it from behind with a wave which is the exact conjugate of the reference beam, a conjugate image w
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British Technology Group Limited
Lerner Martin
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