Optical: systems and elements – Holographic system or element – Using modulated or plural reference beams
Patent
1990-12-31
1993-03-30
Arnold, Bruce Y.
Optical: systems and elements
Holographic system or element
Using modulated or plural reference beams
359 27, 359 30, G03H 104, G03H 110
Patent
active
051989158
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 to 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 nanometres across as compared to micrometres. 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
REFERENCES:
patent: 3442566 (1969-05-01), Forward et al.
patent: 3490827 (1970-01-01), Van Ligten et al.
patent: 3533056 (1970-10-01), Clark
patent: 3539242 (1970-11-01), Burch
patent: 3561838 (1971-02-01), Gabor
patent: 3600935 (1971-08-01), Baum
patent: 3837726 (1974-09-01), Suzuki et al.
Andreev et al., "Image Pulsed Holography in the Green Region of the Spectrum", Opt. Spectrosc. (USSR), vol. 49, No. 5, Nov. 1980, pp. 514 to 515.
P. Hariharan, Optical Holography, Cambridge University Press, Cambridge, 1986, pp. 69 to 71.
Arnold Bruce Y.
Lerner Martin
National Research Development Corporation
LandOfFree
Underwater inspection apparatus and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Underwater inspection apparatus and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Underwater inspection apparatus and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1285254