Holography is the science of recording three-dimensional information on a piece of two-dimensional film. It does this by recording the light that is scattered from an object in such a way that this pattern of light can be reconstructed when the hologram is viewed. The word holography comes from the Greek words holos, meaning “whole,” and gramma, meaning “message.” Holography is one of several techniques for representing images of objects in three dimensions that are attractive to science communicators working in science centers, museums, and similar institutions. In addition, teaching about holography provides an opportunity to convey important concepts in physics.
Dennis Gábor (1900-1979), a physicist born in Hungary, received the Nobel Prize in Physics in 1971 for his work on holography. Gábor was limited by the light sources available at the time, which consisted of mercury arc lamps that lacked the coherence necessary to produce high-quality holograms. Holography was given a significant boost by the development of the laser in the 1960s, which was the enabling technology to allow high-quality, three-dimensional holograms to be made. Significant advances in the field of holography were made by Yuri Denisyuk in the Soviet Union (who developed a technique for making reflection holograms) and by Emmett Leith and Juris Upatnieks in the United States (who worked on a technique for making transmission holograms); all created three-dimensional holograms that are similar to those we are familiar with today.
In a conventional photograph, the film medium or digital sensor captures a focused image, recording for each point a value for the intensity of the light—and in color photography, its color. This results in a static, two-dimensional image. By contrast, in holography, the image that is formed on the plate is an unfocused image. The thing that differentiates this unfocused image is that it is recorded using coherent monochromatic light such as that produced from a laser. First, a reference beam is shone on the plate, which creates a pattern according to the phase of the light at any one point. Then a second beam, known as the object beam, is reflected off the object to be recorded, again onto the plate. As the light is monochromatic, interference occurs between the two light beams. This interference is recorded on a very finely grained photographic emulsion as patterns of light and dark. The plate is then processed to fix the image. To reconstruct the hologram, a light source is used to re-create the reference beam, and light is diffracted from the reference beam in a manner that reconstructs the light field originally created by the objects—thus, the user can see a representation of the original image in three dimensions.
Holograms provide a very visual way for science communicators to advance concepts where three-dimensional representations are especially important; in addition, the technology presents an exciting vehicle for communicating a range of physics concepts. Holography also has the potential to be used in science communication to present a three-dimensional visualization of a three-dimensional object, where the original object is not available for exhibition. The Soviet Union made particularly effective use of holography in cataloguing and preserving its treasures and artifacts as three-dimensional holograms. A particularly notable exhibition of these took place in 1985, titled “Holography, Treasures of the USSR.” Holograms are particularly suitable for static displays in museums and science centers where users can view holograms at their leisure; however, careful attention should be paid to illuminating the hologram so that it is presented at optimum quality to viewers. This will depend on the technique used to produce the original hologram. For lectures or larger audiences, small holograms may present a challenge to display—small copies can be passed around for inspection. Larger holograms that could be viewed by a whole audience are likely to be prohibitively expensive. Fortunately, a range of other technologies is available that can potentially be used, as described below.
Teaching holography is a good vehicle for teaching some fundamental physics regarding interference and light waves, as well as basic photochemistry. One of the things that makes holography such an accessible subject to teach is that it sits both in the arts and sciences, and so it is appealing to a wide audience.
The term holography refers to the specific technique of encoding an unfocused three-dimensional image onto a two-dimensional plate. There are a number of other three-dimensional imaging techniques that are often misconstrued as producing holograms and that can be used to communicate three-dimensional information. Some of these might be appropriate for different applications where holography is not suitable.
A lenticular image is a plastic image that is overprinted with a series of cylindrical plastic lenses. The image behind the plastic lenses consists of a number of images taken from several different viewpoints along a straight line. These images are divided into fine vertical strips, and these strips are then interspersed. Often an image is printed onto a substrate that is then laminated onto a preproduced lens sheet. Viewing the image does not require any glasses or other special equipment.
An anaglyph is a type of three-dimensional image where the images for the right and the left eye are encoded using different color lenses. Normally, either red-green or red-cyan color combinations are used. The anaglyph image is normally encoded onto a single color film or video that contains information for both the right and left eyes. The eye covered by the red lens does not see images projected in red light; however, cyan images are perceived as being dark. The eye covered by the cyan lens, conversely, will not see the cyan image but will perceive the red image as dark. It is also possible to encode a limited amount of pseudocolor information when generating anaglyph images using a computer.
It is possible to make large, color, three-dimensional images by using two projectors with opposing polarizing filters fitted over the projector lens, projecting onto an aluminized screen. The audience then wears polarizing glasses that match the encoding of the polarizing lenses on the projector. This ensures that the left eye receives the left eye information from one projector, while the right eye receives the right eye information from the other. The viewer looking at the screen without glasses will perceive a blurry color image.
A volumetric display forms a three-dimensional representation of an image contained within a volume. A variety of different techniques can be used to accomplish this. In a swept volume display, a rapidly spinning or oscillating display is rapidly updated with information pertaining to the image at that point in three-dimensional space. As the display sweeps through the space, it scans the three-dimensional image, with persistence of vision acting to splice these slices of image together and form the impression of a three-dimensional display. Another alternative method of producing a volumetric display is to have a static volume display. The static volumetric display consists of light-emitting elements that are transparent in ordinary use, but when activated, emit light or turn opaque. Whereas the terminology of pixels is used to indicate a point in two-dimensional computer graphic space, the term voxel is used as a contraction of “VOlumetric piXEL.” Volumetric displays have been created that use pulsed lasers to create a ball of glowing plasma that hovers in the air—displays have also been created that will generate dots within volumes anywhere up to as large as a cubic meter.
Effective Graphics, Science Centers and Science Museums, Visual Images in Science Communication
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