Name: 全息 (Holography)
Uploaded: 2021-01-14T05:50:33.000Z
Duration: 50 min 8 s
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關係子句

Holography is a technique which enables three-dimensional images to be made. It involves the use of

a laser, interference, diffraction, light intensity recording and suitable illumination

of the recording. The image changes as the position and orientation of the viewing system

changes in exactly the same way as if the object were still present, thus making the

image appear three-dimensional. The holographic recording itself is not an

image; it consists of an apparently random structure of either varying intensity, density

Overview and history The Hungarian-British physicist Dennis Gabor,

was awarded the Nobel Prize in Physics in 1971 "for his invention and development of

the holographic method". His work, done in the late 1940s, built on pioneering work in

the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and WL

Bragg in 1939. The discovery was an unexpected result of research into improving electron

microscopes at the British Thomson-Houston Company in Rugby, England, and the company

filed a patent in December 1947. The technique as originally invented is still used in electron

microscopy, where it is known as electron holography, but optical holography did not

really advance until the development of the laser in 1960. The word holography comes from

the Greek words ὅλος and γραφή.

The development of the laser enabled the first practical optical holograms that recorded

3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and

Juris Upatnieks at the University of Michigan, USA. Early holograms used silver halide photographic

emulsions as the recording medium. They were not very efficient as the produced grating

absorbed much of the incident light. Various methods of converting the variation in transmission

to a variation in refractive index were developed which enabled much more efficient holograms

to be produced. Several types of holograms can be made. Transmission

holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser

light through them and looking at the reconstructed image from the side of the hologram opposite

the source. A later refinement, the "rainbow transmission" hologram, allows more convenient

illumination by white light rather than by lasers. Rainbow holograms are commonly used

for security and authentication, for example, on credit cards and product packaging.

Another kind of common hologram, the reflection or Denisyuk hologram, can also be viewed using

a white-light illumination source on the same side of the hologram as the viewer and is

the type of hologram normally seen in holographic displays. They are also capable of multicolour-image

reproduction. Specular holography is a related technique

for making three-dimensional images by controlling the motion of specularities on a two-dimensional

surface. It works by reflectively or refractively manipulating bundles of light rays, whereas

Gabor-style holography works by diffractively reconstructing wavefronts.

Most holograms produced are of static objects but systems for displaying changing scenes

on a holographic volumetric display are now being developed.

Holograms can also be used to store, retrieve, and process information optically.

In its early days, holography required high-power expensive lasers, but nowadays, mass-produced

low-cost semi-conductor or diode lasers, such as those found in millions of DVD recorders

and used in other common applications, can be used to make holograms and have made holography

much more accessible to low-budget researchers, artists and dedicated hobbyists.

It was thought that it would be possible to use X-rays to make holograms of molecules

and view them using visible light. However, X-ray holograms have not been created to date.

Holography is a technique that enables a light field, which is generally the product of a

light source scattered off objects, to be recorded and later reconstructed when the

original light field is no longer present, due to the absence of the original objects.

Holography can be thought of as somewhat similar to sound recording, whereby a sound field

created by vibrating matter like musical instruments or vocal cords, is encoded in such a way that

it can be reproduced later, without the presence of the original vibrating matter.

Laser Holograms are recorded using a flash of light

that illuminates a scene and then imprints on a recording medium, much in the way a photograph

is recorded. In addition, however, part of the light beam must be shone directly onto

the recording medium - this second light beam is known as the reference beam. A hologram

requires a laser as the sole light source. Lasers can be precisely controlled and have

a fixed wavelength, unlike sunlight or light from conventional sources, which contain many

different wavelengths. To prevent external light from interfering, holograms are usually

taken in darkness, or in low level light of a different color from the laser light used

in making the hologram. Holography requires a specific exposure time, which can be controlled

using a shutter, or by electronically timing the laser.

Apparatus A hologram can be made by shining part of

the light beam directly onto the recording medium, and the other part onto the object

in such a way that some of the scattered light falls onto the recording medium.

A more flexible arrangement for recording a hologram requires the laser beam to be aimed

through a series of elements that change it in different ways. The first element is a

beam splitter that divides the beam into two identical beams, each aimed in different directions:

One beam is spread using lenses and directed onto the scene using mirrors. Some of the

light scattered from the scene then falls onto the recording medium.

The second beam is also spread through the use of lenses, but is directed so that it

doesn't come in contact with the scene, and instead travels directly onto the recording

medium. Several different materials can be used as

the recording medium. One of the most common is a film very similar to photographic film,

but with a much higher concentration of light-reactive grains, making it capable of the much higher

resolution that holograms require. A layer of this recording medium is attached to a

transparent substrate, which is commonly glass, but may also be plastic.

Process When the two laser beams reach the recording

medium, their light waves, intersect and interfere with each other. It is this interference pattern

that is imprinted on the recording medium. The pattern itself is seemingly random, as

it represents the way in which the scene's light interfered with the original light source

— but not the original light source itself. The interference pattern can be considered

an encoded version of the scene, requiring a particular key — the original light source

— in order to view its contents. This missing key is provided later by shining

a laser, identical to the one used to record the hologram, onto the developed film. When

this beam illuminates the hologram, it is diffracted by the hologram's surface pattern.

This produces a light field identical to the one originally produced by the scene and scattered

onto the hologram. The image this effect produces in a person's retina is known as a virtual

Holography may be better understood via an examination of its differences from ordinary

photography: A hologram represents a recording of information

regarding the light that came from the original scene as scattered in a range of directions

rather than from only one direction, as in a photograph. This allows the scene to be

viewed from a range of different angles, as if it were still present.

A photograph can be recorded using normal light sources whereas a laser is required

to record a hologram. A lens is required in photography to record

the image, whereas in holography, the light from the object is scattered directly onto

the recording medium. A holographic recording requires a second

light beam to be directed onto the recording medium.

A photograph can be viewed in a wide range of lighting conditions, whereas holograms