Optical: systems and elements – Holographic system or element – Having particular recording medium
Patent
1994-09-06
1998-08-18
Henry, Jon W.
Optical: systems and elements
Holographic system or element
Having particular recording medium
359 7, 359 24, G03H 104, G03H 126
Patent
active
057964980
DESCRIPTION:
BRIEF SUMMARY
The present invention provides apparatus for generating real time or near real time 2-D and/or 3-D images of objects, particularly objects obscured by diffusing media.
The apparatus according to the invention is expected to have applications in medical diagnostic equipment for imaging internal tissues in human, animal or plant bodies in-vivo, in micro-fabrication, microscopy and various engineering applications.
Various techniques for producing 3-D holographic images are well known but a brief explanation is included for the sake of completeness and to assist in the readers understanding of the present invention. Briefly, an object is illuminated by an object beam of coherent light from a laser source. An image recording medium, commonly a photographic plate is exposed to the object light reflected from the object. Simultaneously, a reference beam derived from the source, is directed on to the surface of the photographic plate. By careful arrangement of the apparatus the coincidence of the object beam and the reference beam generates a pattern of interference fringes which record the intensity and phase of the wavefront of the object light. By developing the plate and illuminating the resulting interferogram using a reconstruction beam similar to the reference beam, a virtual or a real image of the object can be created.
The closeness of the fringes in a conventional hologram means that the recording will be very sensitive to movement of the object during exposure, causing severe degradation of the hologram record. When photographic plates are used for recording exposure times of seconds are required for the light levels commonly used.
In a recent experiment conducted by Abramson N. H. and Spears K. G. entitled "Single pulse light-in-flight recording by holography" published in Applied Optics/Vol 28, No.10/15:05:89, Abramson and Spears disclose apparatus and a method whereby light-in-flight techniques allow temporal information to be encoded into the hologram as position along the holographic plate. To achieve this a pulsed laser source is used and the reference beam illuminates the holographic plate at oblique incidence so that interference occurs for different relative delays between the object and reference beam. Once the Hologram is recorded the holographic plate can be developed and illuminated with a reconstruction beam (which may be similar to the reference beam although this beam can be a continuous wave and of a different wavelength). For a three dimensional object different views (two dimensional images) of the object at different positions along the holographic plate will correspond to different depths in the object. Thus a contour map can be obtained, giving three dimensional information about the object.
The time needed to record a hologram onto photographic plate seriously limits the usefulness of Abramson and Spears technique. Because of the low light levels, prolonged exposure times of the order of seconds are required. Also, film requires long developing times, of the order of thirty minutes. This is very inconvenient and impractical for many possible applications. The holographic film used is expensive and cannot be reused.
Accordingly there is provided apparatus comprising a light source to generate an object beam and a reference beam whereby an object can be illuminated with the object beam, a real-time recording medium arranged to be exposed to and record an interferogram formed by the coincidence of light reflected from the object and the reference beam, and means for viewing and recording the holographic image which is reconstructed from the interferogram.
The `real-time` recording medium may be a photorefractive crystals (such as crystals of Bi.sub.12 SiO.sub.20) which has the advantageous property of responding to light by locally changing the refractive index from a rest to an excited state. Thus the interferogram pattern to which the photorefractive crystal is exposed is recorded and can be read by illuminating the crystal with a suitable reconstruction beam. Because photorefract
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Henry Jon W.
Imperial College of Science Technology and Medicine
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