Glossary
Absorption Hologram: a hologram which diffracts light by means of small patterns of exposed emulsion in the form of silver residue.
Angstrom Unit: one ten billionth of a meter; one tenth of a nanometer, abbrev. A.
Argon Ion Laser: a continuous wave gas laser which is capable of emitting light in various wavelengths of both blue and green light, usually more powerful and more expensive than a helium-neon type laser.
Coherence Volume: that volume of space in which an object may be placed and be expected to make a successful hologram; is defined by the tolerable path difference of the reference and object beams; also referred to as coherence length or depth.
Coherent: implies a definite phase relationship between waves of light or other radiation being emitted.
Constructive Interference: the effects resulting from a superimposition of coherent waves of light, for example, where a crest is superimposed on a crest.
Continuous Wave: CW. In referring to laser this means that the energy emitted is continuous; a CW laser can be turned on and off like a normal light bulb unlike a ruby laser which emits its energy in pulses sometimes lasting less than a fraction of a billionth of a second.
Destructive Interference: The effects of the superimposition of a crest over a trough. The higher amplitude of the crest is cancelled by the lower amplitude of the trough. This occurs frequently when the waves of light are out of phase.
Electron: a stable elementary particle having a negative charge. The electron orbits about the nucleus of the atom at a given distance from the nucleus. If an electron is raised to a higher energy level, i.e. more distant from the nucleus, by an energy input, then the electron will give off energy as it falls back to a lower energy level. This energy is sometimes visible light.
Electromagnetic Radiation: EMR - can be defined as waves of emitted energy charcterized by the coexistence of electric and magnetic fields being propagated through air at approximately 300,000 km per second. The visible light region is a very, very miniscule portion of the entire EM spectrum.
Etalon: an optical component made of fused quarts; sometimes used on a laser to filter out all other modes and insure pure monochromaticity and thus geatry increasing the coherence volume.
Excited State: the condition of an electron when it has been raised to a higher energy level by some external force. In a laser the force is electrical energy from the power supply.
Ground State: the state of lowest energy of an atomic or molecular system.
Helium-Neon Laser: a continuous wave gas laser emitting light in the visible red region at 632,8 nm; also the least expensive and most common laser.
Hertz: a unit of frequency in the International System of Units. It is equal to one cycle per second, 1 CPS.
Hologram: A photograph of interference pattern, which contains information about intensity and phase of light reflected by an object. When illuminated at the correct angle with a sufficiently coherent source it will yield a diffracted wave which is identical in amplitude and phase distribution with the light reflected from the original object. The resultant three dimensional image can be viewed or photographed.
In-Line Hologram: a type of hologram in which the reference beam is brought to the holographic film at the same angle or on axis with the object beam; a single beam arrangement.
Interference pattern or fringe(s): a pattern of light and dark bands caused by the constructive and destructive interference of waves of light which is recorded in the holographic film emulsion.
Krypton-Argon Ion Laser: a continuous wave gas laser which can lase in the blue, green and red region; is more powerful and more expensive that the helium-neon laser.
Laser: an acronym for light amplification by stimulated emission of radiation. It provides a source of light which can be phase coherent and an intense beam can be attained by use of resonance techniques.
Mode: one of several possible states of oscillation which may be sustained in a resonant system.
Multiplex Hologram: a type of hologram which is formed by integrating a large number of photographs in a holographic manner; integral photography; usually only provides horizontal parallax.
Nanometer: one billionth of one meter, 1x10-9 meter.
Noise: any undesirable disturbance or spurious signal.
Object or Scene Beam: that part of the laser beam which is sent to the object being holographed and which is subsequently changed or modulated by the object before interfering which the reference beam on the photosensitive material.
Off-Axis reference Beam: a reference beam which travels a different path from the object or scene beam and which is brought to the holographic film from a different angle than the object or scene modulated beam; requires a twin beam arrangement.
Orthoscopic Image: that reconstructed image which maintains the same spatial realtionships of the object(s) as they were when holographed; usually the virtual image.
Oscillation: a periodical change in the amount of energy contained in an electrical, atomic or mechanical system.
Phase: that portion of a period or cycle through which a quantity (in this case a wave of electromagnetic radiation) has proceeded from an arbitrary point, for example the crest or highest point in amplitude of its wavelength, to the next crest after passing through zero.
Phase Hologram: a hologram which refracts light by means of different thicknesses of a transparent substance; commonly, a bleached hologram.
Photon: a quantum or discrete package of Electromagnetic Radiation.
Plane Wave: a wave of coherent laser light before it is changed or scattered by interfering with an object. A reference beam is essentially a plane wave.
Population Inversion: the condition which occurs when the greater majority of electrons are in the higer energy level rater than the ground state.
Pseudoscopic Image: that reconstructed emage which reverses the spatial relationships of the objects from that which it was when holographed; usually the real image.
Pulse Ruby Laser: a solid state laser the heart of which is a ruby rod comprised of aluminum oxide mixed with a small amount of chromium. When properly excited and amplified the ruby emits light in the visible red region. The ruby laser is a solid state laser. When pulsed it can emit powerful bursts of light energy commonly around 20 billionths of a second or 20 nanoseconds, thus, virtually eliminating the concern with object movement in holography. As well as other pulse lasers.
Rainbow hologram: a hologram, which represents microstructure, or set of periodic microscopic channels. When light illuminates such structure, it will reflect light not as a mirror, but reflect in the other direction (diffraction effect). As result - "rainbow" colour.
Real Image: that image which is projected out from the hologram towards the viewer.
Reference Beam: that part of the laser beam which is not affected or changed by the object being holographed.
Reflection Hologram: a type of hologram which is constructed by causing the object beam and reference beam to interfere from opposite side of the holographic film or plate. In order to view the reconstructed image incoherent light is reflected back towards the viewer from the hologram.
Resonant Cavity: a chamber or enclosure whose design and physical characteristics permit only energy of a specific frequency to be propagated.
Spatial Coherence: that condition in which the light waves traveling through space are not only of the same frequency, but they are in phase in space.
TEM: the lowest mode of oscillation in the laser; preferable because it gives most uniform illumination and is most stable mode of oscillation.
Temporal Coherence: that condition in which the light waves are monochromatic, i.e. each cycle of the wave takes exactly the same time to pass a given point in space.
Transmission Hologram: a type of hologram which is constructed by causing the object beam and reference beam to interfere from the same side of the holographic film or plate. In order to view the reconstructed image, semi-coherent filtered light or very coherent laser light is transmitted to the viewer through the hologram.
Transverse Wave: a wave motion in which the substance of the medium is displaced in a direction at right angles to the direction of propagation of the wave.
Virtual Image: that image which appears in the space behind the hologram.
Volume Hologram: a type of hologram in which the angle difference between the object beam and reference beam is equal to or greater than 90 degrees. All reflection holograms are volume holograms.
Wavelength: the length of distance in the direction of travel of a wave motion between two arbitrary points in neighboring cycles having the same phase.