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Holographic Interferometry

object hologram single separate

SEAN JOHNSTON
University of Glasgow Crichton Campus

Invented at a variety of laboratories during the mid 1960s and successfully commercialized over the following decade, holographic interferometry relies on the optical interference produced between two separate holographic images. By recording two holograms and superimposing the images that they reconstruct, any optical differences can be detected via the resulting pattern of interference fringes. Each fringe represents a shift by half a wavelength of light (scarcely 300nm), and may be produced by motion or deformation of the object between exposures, or by changes in the density of the intervening air. In time-lapse or time-averaged interferometry, a single hologram is recorded while the object moves or oscillates. This can reveal modes of oscillation, such as the vibrations of a musical instrument. In double-exposure interferometry, two separate recordings are made to detect changes that have occurred in the interim. Typical applications include study of aerodynamic flow. In real-time double-exposure interferometry, the image reconstructed by a single hologram is compared with the object itself when sighted through the hologram, yielding so-called live fringes. This permits the object to be mechanically or thermally stressed, for example, to detect strain interactively. In a third variant known as holographic contouring, a single hologram is recorded using a laser that produces two closely spaced frequencies, yielding two separate interference patterns on the same plate. When a single laser frequency subsequently reconstructs the hologram, the images interfere to generate a pattern of fringes that map the depth of the object.

Holography - Introduction, Concepts and Technique, Types of Hologram, Applications, Changing Status [next] [back] Holocaust

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