Hologram and Holography
Hologram and Holography
Holography is defined as a method of producing a three-dimensional (3D) impression, or photographic image, of an object. The recording and the image it brings to life are each referred to as holograms. The word hologram is derived from the Greek word holos, or hole, and gram, or message. Holograms were first possible when the laser was commercially available in the early 1960s. Color holograms were commonplace in the latter part of the 1980s, being made from radiation from the microwave to x-ray regions of the electromagnetic spectrum.
This impression is taken by splitting a beam of coherent (that is, uniform over distance as well as over time) radiation along two paths. One is known and stays undisturbed, to act as a reference. Another strikes the object and is diffracted in an unpredictable fashion along the object’s contours. This action can be compared to throwing one rock into a pool of water, which creates a regular pattern of rings, and then scattering smaller stones afterwards, to see what kind of design appears where the expanding rings intersect with each other. Likewise, intersections of radiation waves hold crucial information. The aim is to track and record the pattern of interference of the split rays.
The surface of the hologram acts as a diffraction grating by alternating clear and opaque strips. When one views a common optical hologram, this grating replicates the action of ordinary illumination, capturing the phase and amplitude of the light beam and its interference pattern, in an additive fashion. One can not only see how bright a jewel is but, also, can see how the light sparkles on each facet if one’s position is shifted.
Holograms were being produced by the 1960s in the East and West, but developments in each area followed different paths.
Hungarian-born British physicist Dennis Gabor (1900–1979) developed the theoretical principles of holography in 1947. Gabor’s intention was to improve the resolution of electron microscopes. He wrote on his efforts to tackle the problem in 1948, but since no stable source of coherent light was available, his work excited little interest as an imaging technique. T. A. Mainman at Hughes Aircraft in the United States was the first to demonstrate a ruby laser in 1960. After two other researchers, E. N. Leith and J. Upatnieks, used the laser to make 3D (three-dimensional) images in the early 1960s, Gabor was awarded the Nobel Prize for his research in 1971.
In 1958, Russian physicist Yuri Nikolaevich Denisyuk (1927–2006) had no idea what Gabor had done. He was fond of science fiction, and came across a reference in Efremov’s story “Star Ships” to a mysterious plate, which could show a face in natural dimensions with animated eyes. The Russian researcher was inspired to try to make something just like that, which he referred to as a wave photograph. Denisyuk’s hologram could be seen under white light, because the plate doubled as a color filter.
There are many sorts of holograms, classified by their differences in material (amplitude, thick/thin, absorption), diffraction (phase), orientation of recording (rainbow, transmission and reflection, image plane, Fresnel, Fraunhofer), and optical systems (Fourier and
Amplitude —The range in vibration of any radiation, visible or otherwise, as measured from the mean to the extreme.
Coherent radiation —Electromagnetic waves with a consistent inter-relation; in the case of laser light the waves propagate in phase.
Diffraction grating —A dispersive element consisting of a surface scribed with very fine, closely spaced grooves that cause different wavelengths of light to reflect or refract (bend) different amounts.
Fringe pattern —A recognizable array of light and dark or transparent and opaque bands produced by interference.
Interference —The effect two sets of propagating waves have on each other, and the combined pattern that may be detected as formed by this interaction.
LASER —An acronym for light amplification by stimulated emission of radiation, the process by which a device generates a steady and coherent stream of electromagnetic radiation.
Parallax —The apparent displacement of a viewed three-dimensional object as seen by each eye of the static observer, in a process sometimes called stereoscopic vision.
Phase —A cycle, measured according to the point at which a form of radiation is considered to have completed its full range of harmonic motion.
Refraction —The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.
Wave front —In holography, a surface comprised at a particular instance of the total points reached by a vibration of electromagnetic radiation, during its propagation through a solid, liquid or gaseous substance.
lens-less Fourier). The hologram is usually defined as a record of an interference pattern in a chemical medium, but the pattern does not have to be produced by a light source, nor must the hologram be stored on photographic film. Sonic waves, x-ray waves, and microwaves are used as well, and computers can generate ones just by using mathematical formulas.
Researchers have been experimenting with aspects of the holographic process all along, and new tests are always being devised, in order to explore novel ways to improve the resolution and vibrancy of the images. The most common differences among these methods involve the mechanical setup of the exposure, the chemistry of the recording medium, and the means of displaying the final product. Full color holograms can be made by creating three masters in red, green, and blue, after painting the object in grayscale tones, according to a separation technique already used in art printing. Different shades of gray are interpreted by a combination of the masters as different colors. Fiber optic delivery systems can insure proper illumination and eliminate aberrations that arise during long exposures. Multiplex or multiple-exposure holograms can be in planar or cylindrical form, showing a 360-degree view or even apparent movement.
Ordinary photography only accounts for the intensity of light. The only consideration is whether the light is too bright or too dark. One can usually see the grains in a photographic image, but the features in the fringe pattern of a hologram measure the same as each wavelength of light (1/2000 of a millimeter), recording amplitude in their depth of modulation and phase in their varying positions.
Older 3D imagery constructed from photographs is known as stereoscopy. This method reproduces a single viewpoint with the aid of two images. The two images are superposed to recreate the parallax between the left eye’s view and the right eye’s view, but that is where the options stop. Holography allows for a full range of parallax effects: one can see around, over and even behind objects in a hologram.
Flashbulbs can be uncomfortable, but holograms use laser technology. Direct physical contact with a low-power laser cannot harm a person unless one look directly into the beam, but remove all potentially reflective surfaces from the area, in order to prevent an accident.
The most common holograms are now an everyday occurrence. Embossed holograms are mass produced on mylar—foil and plastic—and can be viewed under the kind of diffused light that renders higher-quality holograms blurry. These can be seen on a variety of consumer goods, but they are also used on credit or identification cards as security measures. Holographic optical elements (HOEs) do not generate images themselves, but are employed to regulate the pattern of a scanning light beam. Supermarket checkout scanners are built out of a collection of HOEs mounted on a spinning disc, which can read a UPC (universal product code) from any angle.
Holographic memory is an emerging technology, which aims to preserve data in a format superior to currently used magnetic ones. Binary computer code (patterns of ones and zeros) could be represented as light and dark spots. Part of a hologram can be defective or destroyed, while the remaining part will still retain all the data intact. Creative use of multiplexing can layer information, recorded from different positions.
Computer-aided design (CAD) imagery would be made more accessible to the average viewer if the full-scale plan appeared in apparent 3D, instead of requiring that a series of linear plots be deciphered visually, which is the current practice. Holograms can be used as visualization aids and screening devices in aviation and automotives as well, since they can be viewed from a particular angle, but not others.
X rays can show detail where an electron microscope would only show dark undifferentiated circles, and would render less damage to a living thing or tissue than electronic bombardment. Subatomic or light-in-flight experiments could be recorded in fully-dimensional imagery, in real time.
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