With the coming of high-quality megapixel digital cameras, the creation of digital holograms of real-world objects has become viable today. A hologram is an optical element or an image that can record all information present in a wave front-both amplitude and phase information. In other words, a hologram can reproduce the complete 3D wavefront of a captured image or object . The process of making hologram is known as holography and a sub area of holography is the digital holography. The hologram formed using digital holography is called digital holograms. Digital holograms are used to provide three-dimensional (3D) information of a scene or object. Digital cameras capture images which record the intensity of an optical wavefront. With digital holography, the recording of the intensity and directional information of an optical wavefront is possible, which in turn encode some 3D information about the objects we capture.

Difference Between Digital Holograms and Computer Generated Holograms

The difference between the digital holograms and the CGHs is that in digital holograms, there are a group of digital images synthesized by controlling fractal parameters, which at the same time depict intrinsic relationship and distinction. These holograms reveal the correlation properties between part structure and whole structure, and also between some stages and entire process of scenes.

Process of Making Digital Holograms

Also known as synthetic holography, digital holography, simulates the recording process of optical holography. Depending on the diffraction theory, the wave fronts scattered by objects are synthesized and analyzed by using computers. These objects are numerically specified or by means of mathematical models. During recent years, many interesting results have been achieved in digital holography owing to improvements in computer architecture, computational methods, and electro-optical technology. Digital holograms are a series of computer-generated images in sequential order and these images do not require to record the light distribution of the objects, unlike optical holograms. The research work of digital holograms is mainly inclined towards formation and rendering of images. It is possible to reconstruct the imaginary objects that do not exist physically in digital holography. Digital holograms are initially saved as a file directly and using a computer program they are reconstructed. They are simpler to record and reconstruct, but resolution is much lower than a holographic film.

The content for digital holograms can easily be produced by non-experts, and the printing process is comparatively inexpensive. Usually a three-dimensional graphical scene, a series of digital photographs or a short movie of a real object is enough for producing digital holograms.

An example: A diagrammatic representation of a digital hologram of a car headlight with integrated CAD data.

The above figure shows a digital color white-light reflection hologram of a car headlight. It was formed by taking 360 perspective photographs from different angles. The photographs were multiplexed into different sub-zones. Subsequently, three different partial views (rear, side and front) are observed by moving within the total viewing zone of 110 degree. After calibrating the projector and registering the holographic plane, interactive graphical elements like wire-frame or shaded CAD data can be integrated into the hologram.

Advantages

Compared to a conventional optical holography, digital holograms have many advantages. These are as follows:

• Digital holograms have strong anti-disturb property.
  
  
• They are easy to be modified.
  
  
• They are difficult to be imitated.
  
  
• By using the pixel holograph and embossment, hologram films can be fabricated by transforming digital holograms. This can make digital hologram anti-counterfeiting identifiers.
  
  
• Digital holography uses holographic printers, exposing the photosensitive emulsion with computer generated images. This leads to the creation of conventional holograms with digital content rather than real scenery.
  
  
• 2D/ 3D graphics or digital photographs and movies can be printed which helps in the holographic recording of real outdoor scenes, completely synthetic objects, and objects in motion. This is impossible to achieve with optical holography.
  
  
• Digital holograms can be multiplexed.
  
  
• Another basic advantage is that the content for digital holograms can easily be created by non-experts and the printing process is not very expensive compared to conventional holograms.
  
  
• A series of digital photographs or a short movie of a real object is enough for producing digital holograms.

Disadvantage

Digital holograms lack in the quality in terms of color appearance, resolution, sharpness, etc. of conventional optical holograms.

Applications of Digital Holography

The applications of digital holograms can be seen in the following areas:

• Holographic optical elements (HOEs): Holograms are used as lenses.
• Holographic interferometry: To detect form deformations.
• Optical data storage: In the area of security applications.
• They have a good application prospect in laser holography anti-counterfeiting field.
• 3D displays
• Optical data processing.