An interference microscope is described, constructed in 1951–56, in which three-dimensional objects can be reconstructed, correctly incorporating amplitudes and phases, from two photographs simultaneously taken on one plate. These photographs are “holograms,” that is to say, records of the interference of the image-carrying wave, split in two, with a coherent background wave, also split in two. A phase difference of a quarter wave is produced, for the two otherwise identical photographs, between the image-carrying waves and their respective coherent backgrounds. The two photographs are in sine–cosine or “quadrature” relation; between them they contain the full optical information. If they are illuminated in such a way that there is a difference of a quarter wave in the phases at two corresponding points, and if the two beams are united, the original image-carrying wave is restored correctly in amplitude and in phase. An essential part of the instrument is a “quadrature prism”; a beam splitter with a three-layer sandwich, which establishes the quadrature relation in the taking of the holograms.
The microscope has the advantage that it need not be focused, as it gives a three-dimensional reconstruction. Moreover, photographs can be taken with 1/10 or even 1/100 of the light required for exposure going through the object; the rest of the energy is supplied by the background beam, which is 10–100 times stronger, and which goes around the object.
Many details of the instrument are now out of date owing to the invention of the laser. Alternative methods which have now become possible are discussed for realizing the principle of total reconstruction.
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