Diffractive Optics and Holography

Abstract

When we take a photograph of something, say an apple, the camera records the light that bounced of the apple and went into the camera lens. But this only records a single perspective of the apple. That’s why photographs look flat. If we want to do a little better, we could take 2 photographs using two cameras at some angel, and then send one of those images to one of your eyes and the other image to your other eye. This is how the 3-D movie works, it gives them depth. But there’s a problem with it. You can’t turn your head and get more perspectives on the apple. We can go one step further now. We know that light has a wave nature. We can deflect a light wave by passing it through a diffraction grating. Finally, we can combine a bunch of these diffraction gratings in order to shape the light wave in whatever shape we want. That’s exactly what Holography is. It’s a combination and superposition of diffraction gratings that are designed to reconstruct the original light wave that bounced of the apple.

Introduction

Holography is a method of reproducing three-dimensional image of an object by means of light wave patterns recorded on a photographic plate or film. Holography is sometimes called lens less photography because no lenses are used to form the image. The plate or film with the recorded wave patterns is called a Hologram. Holography was invented in 1947 by the Hungarian physicist Dennis Gabor while he was carrying out research on electronic microscopy. Gabor received the Nobel Prize for Physics in 1971 for this invention. The most interesting things about this is that laser light had not even invented yet, when he wrote his paper about holograms.

How Holograms Work

The formation of Hologram is done by a process called Reconstruction. The reproduction of a 3-D image from the hologram is done by a process called Construction. Thus, holography consists of 2 process -

1) Recording of hologram

2) Reconstruction of image

1) Recording of Hologram

The recording of hologram is based on the phenomenon of Interference. It requires a laser source, a plane mirror or beam splitter, an object and a photographic plate.

A laser beam from laser source is incident on a plane mirror or beam splitter. The function of the beam splitter is split the laser beam. One part of split beam, after reflection from beam splitter (inclined at 450), strikes on the photographic plate. This beam is called reference beam. While the other part (transmitted from beam splitter) strikes on the photographic plate after suffering reflection from the various points of object. This beam is called object beam. The object beam, reflected from the object, interferes with the reference beam when both the beams reach photographic plate. The superposition of the two beams produces an Interference pattern and this pattern is recorded on the photographic plate (which record the information of photons). The photographic plate with recorded interference pattern is called Hologram. This plate is also known as Gabor Zone Plate in honor of Denis Gabor.

Each and every part of hologram receives light from various points of object. Thus, even if hologram is broken into parts, each part is capable of reconstruction of whole object, although the viewing window becomes much smaller.

2)Reconstruction of Image

It is based on the principle of Diffraction. In the Reconstruction Process, the hologram is illuminated by laser beam and this beam is called reconstruction beam. This beam is identical to reference beam used in construction of Hologram. The hologram acts as a diffraction grating. This reconstruction beam will be diffracted as it passes through hologram. The reconstruction beam after passing through the hologram produces a real as well as a virtual image of the object. One of the diffracted beams emerging from the holograms appears to diverge from an apparent object when projected back. Thus, a virtual image is formed behind the hologram and real image in front of the hologram. Thus, an observer sees light waves diverging from the virtual image and the image is identical to the object, and also is visible from all perspectives. Therefor the virtual image exhibits all the true 3-dimensional characteristics. The real image can be record on photographic plate. If the rays pass through and it isn’t affected then we will get the zero-order image or maxima.

Applications of Holography

1. Holographic optical elements are used in an aircraft’s “heads up display” to allow the pilots see critical cockpit information while looking straight ahead through the windscreen. Similar systems are being developed by several automobile manufactures.

2. Holography is ideal for the display of fragile artifacts, using holographic display museums.

3. Holography is useful for visualization of biomedical specimen.

4. Geographic intelligence is critical to military strategy. Fully dimensional holographic images are being used for improved reconnaissance. These 3D holographic maps of "battle-spaces" allow soldiers to view three-dimensional terrain, look "around" corners, and train for missions.

5. If you have a credit card, you have a hologram. The small silver rectangle of a Dove on your credit card is a white-light, mirror-backed, transmission hologram. It displays a three-dimensional image which is visible as you move from side to side, and changes colour as you tilt your card up and down.

6. Bank notes are also starting to incorporate secure holograms. In the UK, the newest £5 bank note has an image of Big Ben and uses holography to produce a set of changing colours as you tilt the note. There is also a 3D image of the coronation crown "floating" above the note when tilted.

Holograms in the Future

Information Storage

Holograms are capable of recording huge amounts of pure data. The current prototype systems can store 4.4m individual pages of information on a DVD like disc. They also offer a unique form of long-term security.

Medical

It can be used as a tool for visualizing patient’s data in training students and surgeons. Holograms do not need any viewing devices or glasses. Students and doctors can simply "look", unhindered, at the three-dimensional images. These images can include the incredibly complex organs and systems of the body, like the brain, heart, liver, lungs, nerves, and muscles.

Art

There are artists around the globe using the three-dimensional of holograms to bend and cut space, combine collections of still images or video to produce animated 3D works, and to sculpt pure light.

Recently, an exhibition in Central London presented a show of creative holography. International groups of selected artists contributed work to an exhibition on Governors Island, New York, and artists from Canada, Italy, the US and UK were chosen for an exhibition using holography and the media arts in Santa Fe, New Mexico this summer.

Facts

1. In 2015, the first highly interactive hologram was invented in Japan. It can respond to human touch. These touchable holograms are called “Fairy Lights.”

2. Holograms can be faked. The ghost figures of Tupac and Michael Jackson that appear on stages to surprise many audiences are not actual holograms, but projections on hidden screens.

3. Most driver's licenses include holograms, as well as ID cards and credit cards.

4. Microsoft ( MSFT ) has its HoloLens headset, Apple ( AAPL ) uses ARKit, and Google ( GOOGL ) presents holograms in ARCore (is a software development kit developed by Google that allows for augmented reality applications to be built).

Conclusion

Holographic interferometry is an optical method that makes it possible to visualise transparent objects, helps explain the physical essence of the investigated events, enables to specify and expand the possibilities of visualisation of physical fields while the investigated area is not disturbed by sensors or sensing heads by which, for example the local temperatures or concentrations are detected. Holography technology has endless application as far as the human kind can imagine. Holographic technology isn’t just about art or business communication, they are about safety, security, education planning. Holography technology will become a very integral part of the human societal in the future.

References

1. https://www.scribd.com/document/105403460/ABSTRACT-on-Holography

2. https://www.nasdaq.com/articles/what-can-we-expect-hologram-technology-future-2018-07-17

3. https://en.wikipedia.org/wiki/Holography

4. https://www.explainthatstuff.com/holograms