Holography from Fantasy to Reality

From classic Star Wars to Iron Man， 3D holographic images are presented to the audience in the form of special effects in many science fiction films. Characters in the films can interact with 3D images formed in the air， which not only brings shocking visual effect， but also arises people's expectation on future technologies. How far is the technology in the films away from our real life？

As the name implies， holography refers to all the information on the light scattered or emitted by humans or stuff in nature （including information on amplitude and phase of light waves）. Holography is a 3D imaging technology indeed， can record information on light intensity， where the light is emitted from， and where the light is emitted to （i.e.， the information on phase） utilizing principles of optical interference and diffraction， while general photography records only the information on the light intensity in a certain direction （or the information on amplitude）. It is different to see a hologram from different angles， and it is like seeing a real object. Therefore， holography is one of the very effective ways in evolving into 3D display.

Based on electrostatic image principle， general photography is able to shoot in natural environment. Holography requires a special light source （typically using laser） to record the scattered light emitted by an object. Laser beam is divided into two beams during recording， one of which is projected onto the recording medium， known as reference beam； another is projected onto the target object to form an object beam after the scattering of the object surface. The object beam is projected onto the same area as the recording medium， an interferometry stacking area is formed on the recording medium by the object beam and the reference beam， thus， interference fringes are formed， and holograms are obtained after treatment of the interference fringes.

A beam of laser is required to irradiate the hologram when reconstructing the object image. When the wavelength and propagation direction of the beam of laser are exactly the same with those of the reference beam， the original light field of an object is restored. So we can see different sides of the object from different angles， just like seeing the real object.

Recording material of holography is also different from that of ordinary photography. Traditional optical holography uses silver halide， dichromate gelatin， and photoresist to make light-sensitive film to record a hologram which needs to go through subsequent development， fixation， rinsing， drying and other treatment， cumbersome and time-consuming. In recent years， the emergence of new photosensitive materials， such as photo conducting thermoplastics， photopolymer， diffracted light crystal， etc. not only eliminates cumbersome chemical treatment， but also improves the information storage capacity and the diffraction efficiency.

More Convenient

Digital Holography

Hologram is essentially an interference pattern. With digital techniques， interference and diffraction process can be simulated， and such interference pattern can be directly "computed"， thus， digital holography formed in the late 1960s. This technique not only avoids harsh optical recording experiment environment， but also simulates various objects which do not actually exist， therefore， it has obvious simplicity and flexibility. Digital holography effectively combines optics and electronics technologies to make them play their respective advantages， thus new prospects for development are shown.

The first generation hologram requires a beam of laser to record and reconstruct original 3D object images. The second generation hologram requires a beam of laser for recording alone， while employs white light to reconstruct 3D images， which extends the conditions to see holograms. For example， reflection holography， image plane holography， rainbow holography， embossed holography， etc. are like this. Substituting photosensitive materials with photosensitive electron graphic devices （such as CCDs）， digital holography technology can record holograms more easily and quickly. Digital hologram uses optical modulators （such as SLMs） or simulates optical reconstruction of a hologram in computer. Therefore， changes in sides or colors of an object can be seen from different angles while reconstructing a hologram

Various Applications

of Holography

Holography has played an important role in people's lives， cultural education， production practice and scientific research. Holographic artworks， holographic trademarks， holographic stamps， and holographic museums have emerged because of artistry and ornamental of holograms； and due to high-tech features of holograms， holographic anti-counterfeiting technique has emerged and been used in forgery prevention in commodities， money， certificate， etc. In recent years， holographic television， holographic movie and other 3D display technologies have been developing. Various holographic products and applications are around the world.

China published a holographic stamp holder in 1987， lunar Ding Mao year （the Year of the Rabbit）， so there is a hologram of three bunnies on the stamp holder of a block of four stamps. Different front and side images of bunnies can be seen from different angles， it is really vivid.

On April 21， 2010， US Treasury and Federal Reserve jointly released the design proposal for new hundred-dollar bills on which holographic anti-counterfeiting technique was used to set a security line. As the name implies， holographic museum refers to a kind of museum exhibiting with holograms instead of real objects. At present， there are holographic museums is some cities in the US， the U.K.， and some other countries. In these museums， some rare treasures are photographed into holograms to reduce damage， theft and other security risks of cultural relics.

Front-projected holographic display is currently widely used in product exhibition， release conference， stage shows， bar entertainment and other situations， and can make virtual scenes imaged， thus it becomes one of virtual imaging technologies. Using this technology can produce 3D aerial mirage， and make the interaction between the mirage and the performers possible to create an illusion of jointly performing and produce shocking performance effects.

Further Development

of Holography

In 1948， Gable， a British Hungary scientist invented holography. Various holograms， such as transmission holography， reflection and transmission， image plane holography， rainbow holography， reconstruction holography， true color holography， dynamic holography， computer-generated holography， digital holography， etc. have emerged in succession after holography developing for half a century. The implementation techniques of holography are more and more and more and more mature. Holographic three-dimensional display gets out of the lab， and has served human society.

In recent years， holography has made some breakthroughs in large size， high resolution， true color， Real Time Kinematic and other key technologies. The aerial image study team of Massachusetts Institute of Technology has successfully developed three holographic three-dimensional video display systems which can display true color three-dimensional holographic videos at a rate of 20 frames per second. German's See Real has developed a 20-inch single-color three-dimensional holographic display device. British QinetiQ gets computer generated holograms by means of computation using super computers so that high-resolution holographic three-dimensional display can be realized.

Because holography requires strict experimental conditions， and digital holography is limited by computer rate and display carrier， holographic three-dimensional video is still far from satisfactory in practicality of walking into millions of households， therefore， "pseudo holography" and other auto stereoscopic display techniques have emerged as the times require. Unlike real images of objects reconstructed in space by means of holographic display， "pseudo holography" mainly imitates the principle of human eye's stereopsis to make the audience perceive the three-dimension effect of objects. Principle of stereopsis refers to the subtle difference existing between the images of left and right eye points because there is a certain distance between the left and right eyes， and images on retinas of left and right eyes come from different eye points when human eyes see the same scene. Cerebral cortex extracts depth information in a scene by the fusion of two images， thereby experiencing three-dimensional effect. Currently， auto stereoscopic display has been able to reach a higher resolution， and is gradually going to the public.

Benton， the inventor of rainbow holography has prospected the future of holography， said， "holography brings our civilization with two meanings， one of which means to realize wave-front reconstruction through interference and diffraction/reflection， more broadly， another of which implies the ultimate three-dimensional imaging method in the future， representing an advance of science and technology closely related to the daily life. It is expected that future innovations are as rich and practical as the evolvement of holography. To the 21st century， various innovations and development of holography are showing holography's immense creativity and enormous impact on the information society.