..... Click the link for more information. .) Before reaching the object, the beam is split into two parts; one (the reference beam) is recorded directly on the photographic plate and the other is reflected from the object to be photographed and is then recorded. Since the two parts of the beam arriving at the photographic plate have traveled by different paths and are no longer necessarily coherent, they create an interference interferometer. When the wavelength of the light is known, the interferometer indicates the thickness of the film by the interference patterns it forms. The reverse process, i.e., the measurement of the length of an unknown light wave, can also be carried out by the interferometer.
..... Click the link for more information. pattern, exposing the plate at points where they arrive in phase and leaving the plate unexposed where they arrive out of phase (nullifying each other). The pattern on the plate is a record of the waves as they are reflected from the object, recorded with the aid of the reference beam. When this hologram is later illuminated with coherent light of the same frequency as that used to form it, a three-dimensional image of the object is produced; it can even be photographed from various angles. This technique of image formation is known as wave front reconstruction. Dennis Gabors, a British scientist who in 1948 developed the wave theory of light (itself first suggested by Christopher Huygens in the late 17th cent.) can be viewed as the father of theoretical holography. However, no adequate source of coherent light was available until the invention of the laser in 1960. Holography using laser light was developed during the early 1960s and has had several applications. In research, holography has been combined with microscopy to extend studies of very small objects; it has also been used to study the instantaneous properties of large collections of atmospheric particles. In industry, holography has been applied to stress and vibrational analysis. Color holograms have been developed, formed using three separate exposures with laser beams of each of the primary colors (see color color, effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the
..... Click the link for more information. ). Another new technique is acoustical holography, in which the object is irradiated with a coherent beam of ultrasonic waves (see sound sound, any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body vibrates, or moves back and forth (see vibration ), the oscillation causes a periodic disturbance of the surrounding air or other
..... Click the link for more information. ; ultrasonics ultrasonics, study and application of the energy of sound waves vibrating at frequencies greater than 20,000 cycles per second, i.e., beyond the range of human hearing.
..... Click the link for more information. ); the resulting interference pattern is recorded by means of microphones to form a hologram, and the photographic plate thus produced is viewed by means of laser light to give a visible three-dimensional image.
Bibliography
See G. W. Stroke, An Introduction to Coherent Optics and Holography (2d ed. 1969); T. Okoshi, Three-Dimensional Imaging Techniques (1976); N. Abramson, The Making and Evaluation of Holograms (1981); J. E. Kasper and S. A. Feller, The Complete Book of Holograms (1987).
holography
Method of recording or reproducing a three-dimensional image, or hologram, by means of a pattern of interference produced using a laser beam. To create a hologram, a beam of coherent light (a laser) is split; half the beam falls on a recording medium (such as a photographic plate) unaltered, and the other half is first reflected off the object to be imaged. The two beams together produce an interference pattern of stripes and whorls on the plate. The developed plate is the hologram. When light is shone on the hologram, a three-dimensional image of the original object is produced by the recorded interference pattern. Some holograms require laser light to reproduce the image; others may be viewed in ordinary white light. Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900–1979), who won a 1971 Nobel Prize for his invention.
holography
holography [hə′läg·rə·fē]
Holography
A technique for recording, and later reconstructing, the amplitude and phase distributions of a coherent wave disturbance. Invented by Dennis Gabor in 1948, the process was originally envisioned as a possible method for improving the resolution of electron microscopes. While this original application has not proved feasible, the technique is widely used as a method for optical image formation, and in addition has been successfully used with acoustical and radio waves. See Acoustical holography
The technique is accomplished by recording the pattern of interference between the unknown “object” wave of interest and a known “reference” wave (Fig. 1). In general, the object wave is generated by illuminating the (possibly three-dimensional) subject of concern with a highly coherent beam of light, such as supplied by a laser source. The waves reflected from the object strike a light-sensitive recording medium, such as photographic film or plate. Simultaneously a portion of the light is allowed to bypass the object, and is sent directly to the recording plane, typically by means of a mirror placed next to the object. Thus incident on the recording medium is the sum of the light from the object and a mutually coherent “reference” wave. See Laser
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The photographic recording obtained is known as a hologram (meaning a “total recording"); this record generally bears no resemblance to the original object, but rather is a collection of many fine fringes which appear in rather irregular patterns. Nonetheless, when this photographic transparency is illuminated by coherent light, one of the transmitted wave components is an exact duplication of the original object wave (Fig. 2). This wave component therefore appears to originate from the object (although the object has long since been removed) and accordingly generates a virtual image of it, which appears to an observer to exist in three-dimensional space behind the transparency. The image is truly three-dimensional in the sense that the observer's eyes must refocus to examine foreground and background, and indeed can “look behind” objects in the foreground simply by moving his or her head laterally.
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Holography has been demonstrated to offer the capability of several unique kinds of interferometry. This capability is a consequence of the fact that holographic images are coherent; that is, they have well-defined amplitude and phase distributions. Any use of holography to achieve the superposition of two coherent images will result in a potential method of interferometry. See Interferometry
Optical memories for storing large volumes of binary data in the form of holograms have been developed for commercial use. Such a memory consists of an array of small holograms, each capable of reconstructing a different “page” of binary data. When one of these holograms is illuminated by coherent light, it generates a real image consisting of an array of bright or dark spots, each spot representing a binary digit.
There has been interest in the use of holography for purposes of display of three-dimensional images. Applications have been found in the field of advertising, and there is increased use of holography as a medium for artistic expression.
Microwave holography is microwave imaging by means of coherent continuous-wave electromagnetic radiation in the wavelength range from 1 mm to 1 m. As a long-wavelength imaging modality, it differs from techniques which employ echo timing (for example, conventional radar) by its requirement for phase information. In this respect it resembles optical holography, from which it has departed significantly. The technique usually involves small-scale systems, that is, systems in which the effective data acquisition aperture is of the order of tens or hundreds of wavelengths. Microwave holographic imaging is characterized by high lateral-resolution capability in comparison with images obtained from echo timing. The natural image format of the data it presents to the human observer enhances its diagnostic potential. In particular, it conveniently produces phase imagery which increases further its diagnostic capability.
Microwave holography is useful in applications where images of concealed structure are required. Microwave radiation penetrates a variety of dielectric media to a depth depending on the attenuation of a given wavelength in a particular medium. One such application is the mapping of subsurface pipes and cables. Plastic pipes as well as metal pipes can be imaged. Hence this noninvasive microwave technique has a diagnostic power greater than the normal metal detectors.
The major limitation of the microwave holographic techniques is that the images produced are essentially two-dimensional. The reason is that the microwave wavelength is so long (104–106 times that of light) that the depth of focus of the microwave hologram is prohibitive. This disadvantage is overcome by employing a tomographic mode of imaging which exploits the ability of microwaves to penetrate many materials and thereby characterize their three-dimensional structure more accurately. Microwave holographic tomography requires holograms to be recorded from different views of the object and synthesized.
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