Holography

Introduction

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 light field, rather than a sound field, is recorded and then subsequently reconstructed when required.

History

The development of holography was initially motivated by the problem of improving the resolution of electron microscopes. In 1947, Hungarian-British physicist Dennis Gabor, while working to improve the resolution of electron microscopes at British Thomson-Houston in Rugby, England, came up with the theory of holography and coined the term. He received the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method".

Principles of Holography

Holography is based on the principle of interference. A hologram captures the interference pattern between two or more beams of coherent light (i.e., laser light). One beam is shone directly on the recording medium and acts as a reference to the light scattered from the illuminated scene.

Wavefront Reconstruction

The process of holography using light involves the use of a laser, lenses, light-sensitive material, and mirrors to record patterns of light. The recorded patterns are a result of the interference of two beams of light: the object beam (light reflected or scattered from the object) and the reference beam (light coming directly from the light source or reflected from a mirror).

Types of Holograms

There are several types of holograms that can be made. These include transmission holograms, reflection holograms, rainbow holograms, computer-generated holograms, and holographic interferometry.

Transmission Holograms

A transmission hologram is one where the object to be imaged is illuminated from the back and the image is viewed from the front, or vice versa. The viewer looks through the hologram, hence the name transmission hologram.

Reflection Holograms

In a reflection hologram, the object and reference beams are incident on the recording medium from the same side. When the hologram is viewed, light is reflected off the hologram, hence the name reflection hologram.

Rainbow Holograms

Rainbow holograms, also known as Benton holograms, are designed to provide color reconstructions under white light illumination. They are produced by a slightly different process than other types of holograms, and can be mass-produced using a printing press.

Computer-Generated Holograms

Computer-generated holography (CGH) is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g. by digitally computing a holographic interference pattern and printing it onto a mask or film for subsequent illumination by suitable coherent light source.

Holographic Interferometry

Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light).

Applications of Holography

Holography has a wide range of applications. These include data storage, security, and holographic optical elements.

Data Storage

Holographic data storage is a potential technology in the area of high-capacity data storage. The ability to store large amounts of information in some kind of medium is of great importance, as many electronic products incorporate storage devices.

Security

Holography can be used to protect products from counterfeiting. Security holograms are labels with a hologram printed onto it for sale security reasons. Holograms are used widely in many currencies, such as the British pound sterling, the Canadian dollar, and the Euro.

Holographic Optical Elements

Holographic optical elements (HOEs) are one of the most important applications of holography. An HOE is a component of an optical device that uses a hologram instead of a traditional (refractive) lens to manipulate light.