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GLOSSARY

 

 

Channels- These are images in a hologram which change abruptly from one to another as the viewer passes by. They can also fade or overlap as they change. Animated holograms are essentially made up of hundreds of image channels.

 

Computer generated hologram- A hologram made from a 3-D computer model.

 

Cylindricals- Transmission holograms produced with the film curved into the form of a cylinder, so that the image is seen in the center from a full 360° field of view.

 

Depth of image- The distance from the front to the back of the hologram image, which is focused in space by the hologram film. There are several ways to measure image depth precisely, using various types of rangefinders. Image depth is not limited by the lateral dimensions of the hologram film.

 

Diffraction- This is the process that makes holograms work, and refers to the bending of light as it passes through very small openings. Diffraction “patterns” use light wave interference to intricately control the intensity distribution of the light transmitted, so that 3-D, animated images can be formed.  This is different from refraction, which is the bending of light when it passes from one medium to another, like air to glass.

 

Efficiency- In holography, this refers to the brightness of an image. It is the ratio of diffracted to incident light intensity.

 

Embossed holograms- Holograms stamped on foil in large numbers, from a transmission hologram original, and often used for credit cards and packaging.

 

Emulsion- This term is used for the light sensitive surface of photographic and holographic films, although an actual chemical emulsion is sometimes not used.

 

Flash images- Images that appear and disappear suddenly as the viewer walks past a hologram.

 

Full color- Coloring of an image to respresent the natural subject colors as closely as possible, as in color photography.

 

Grating- A pattern of light and dark lines which will diffract light. A simple linear grating will disperse white light into a spectrum of colors. Holographic gratings can produce many spectra, focused to different distances. An image hologram is essentially an extremely complex grating.

 

Halogen lamps- The mr-16 type halogen lamp is usually the preferred light source for holograms. This small bulb is common and inexpensive, but requires the standard 12 volt dc housing. An excellent alternative is the PAR 16 lamp, which operates on ordinary line voltage.

 

HOE's- Holographic Optical Elements are holograms produced to perform the function of traditional optics, such as mirrors and lenses.

 

Hologram- 3-D image recorded on film, and recreated by the process of diffraction. (see FAQ)

 

Holograph- From Webster's "A manuscript in the handwriting of the author." (see FAQ)

 

Holographic- Pertaining to a hologram (not a holograph), such as "holographic art". (see FAQ)

 

Lamination- In holography, this is the permanent application of the hologram film to a rigid substrate like glass or acrylic sheet. This is usually done to keep the film flat during display, and also to protect the emulsion

 

Laser- Source of coherent light, produced by the quantum process of stimulated emission. Coherent light has the purity and organization required to record diffraction patterns, and therefore holograms.

 

Laser transmission holograms- The original type of hologram, viewable only with laser light.  These images are startlingly realistic, but not practical for public display, due to the need for a laser.

 

Lenticular photograph- A series of photographs, cut into strips, combined into a single print and viewed through

a plastic overlay lens sheet to produce a 3-D effect. These non-holograms offer good color and replication, but the 3-D and resolution are very limited.

 

Lighting angle- The vertical angle, usually around 45°, at which light must strike the hologram to produce the brightest image. Approximately equal to the reference beam angle when the hologram is made.

 

Master hologram- The first hologram in a 2-step process, usually viewable only by laser light, and often combined with other masters for the final hologram transfer.

 

Mercury vapor lamp- Short arc mercury vapor lamps make a good light source for laser illuminated holograms. Images are brighter and more evenly lit than with a laser. But cost and maintenance still often make this type of hologram impractical for public disaplay.

 

Mirror mounted hologram- A transmission hologram placed directly in front of an ordinary mirror, so that it can be lit from the front instead of the rear.  This is often convenient where there is limited space.  The image appears the same, although there is usually a slight loss in contrast.

 

Multiplex- The simultaneous recording or transmission of multiple data.  Cylindrical holograms are often called “multiplex,” although this is not always technically the case.

 

Object beam- One of the two parts of a laser beam used to produce the wave interference pattern to be recorded into a hologram. This part carries the information about the subject, and is equivalent to the signal wave in radio communications. The pattern is formed by mixing this beam with the reference beam.

 

Parallax- The difference in appearance in an object when seen from different perspectives.  The parallax between left and right eye views produces 3-D perception. Holograms are unique in providing a wide, continuous range of parallax on the image.

 

Photopolymer holograms- Replicated in large numbers on photopolymer emulsions from a reflection hologram original, and often used for gift items, tickets and packaging.

 

Polarization- The orientation of the electric and magnetic vectors in a light wave.  Many lasers produced linearly polarized light, which is important to their suitability for holography.

 

Projection holograms- There is much confusion over this term. Any imagery focused in front of the hologram film is usually referred to as "projected". Cylindrical holograms can also produce images behind the film which can be seen from a full 360 degrees of view. But complicated and cumbersome holographic "projectors", as imagined in science fiction movies, do not exist. (See FAQ)

 

Pseudoscopic perspective- The appearance of a scene with the depth relationship of objects reversed.  This can be accomplished by viewing a hologram from the reverse side or by certain mechanisms that can switch the left and right eye perspectives.

 

Rainbow hologram- Nickname for white light transmission type hologram, deriving from the spectral image colors.

 

Real images- As opposed to virtual images, these are made up of actual, focused light. These can be seen on a projection screen placed at the image location, or exposed onto film. In holography, this is the imagery in front of the film plane.

 

Reference beam- One of the two parts of a laser beam used to produce the wave interference pattern to be recorded into a hologram. It is the equivalent of the carrier wave in radio communications. The pattern is formed by mixing this beam with the object beam.

 

Reference angle- The angle at which the reference beam strikes the hologram film. It is usually close to 45 degrees, and equates roughly to the illumination angle when the hologram is displayed.

 

Reflection hologram (white light)- Designed to be lit from the front, these holograms have fixed color, parallax in both vertical and horizontal directions, but limited size and brightness.

 

Silver halide holograms- Any hologram made on a silver halide based emulsion, similar to standard photographic film. Emulsions can be coated onto film or glass, but must be extremely fine grain.

 

Stereogram- Hologram produced from movie footage of a rotating subject. Images can be computer generated, animated, reduced or enlarged, or photographed on site. This is an alternative to the original hologram process, in which the subject is imaged directly onto the film with a laser exposure.

 

Stereogram printer- Hologram camera specifically designed to produce a hologram master from a series of photographic images, showing the subject rotating from one to the next. Every printer is designed and built for the particular needs of the lab, and is usually computer automated to perform several hundred step-and-repeat exposures, "building" the master hologram from left to right.

 

Stereo pair- The perspectives required for the left and right eye in order to see an image in 3-D. Stereoscopic cameras provide one pair, for 3-D viewing from one angle only. Lenticular photographs provide several, so the image can appear to rotate slightly. Holograms provide a wide continuum of stereo pairs, for truly 3-D imagery.

 

Transmission hologram (white light)- Also called rainbow holograms, these are designed to be lit from the rear, but can also be mirror-mounted and lit from the front. Images are spectral colored, very bright, and in sizes up to 1.1x1.8 meter.

 

Transfer hologram- The final hologram, viewable with ordinary “white” light, and made from the images focused by one or more masters. Many transfers can be made from the same set of masters, and masters can be archived for later use.

 

Vibration isolation- This refers to the stability of the optical system that is required when a hologram is recorded.  No vibration isolation is needed when the hologram is displayed.

 

Viewing angle- This is the area from which a holographic image can be seen.  Anyone standing too far from center may not see the image.

 

Viewing distance- The ideal distance for viewing a transmission hologram. Although the hologram can seen from any distance, here the colors will be constant from top to bottom. Viewing distance can be tuned to suit the display situation.  

 

Virtual images- As opposed to real images, these are not made up of actual, focused light. Light paths coming from an optical element, such as a hologram, trace back to these positions, so that an image appears to be at that location. These images will not show up on a screen placed at the image position. In holograms, these are all images focused behind the film.

 

Wavelength- The length of one cycle of a wave. In visible light, the wavelength determines the color. On a broader scale, it defines the different types of radiation, such as gamma and x-rays, ultraviolet, microwave and radio waves.

 


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