A hologram is basically a flat structure that bears or projects 3D images by diffracting light under special illumination. The word ‘hologram’ can be used as a term for both the encoded material and the generated 3D image.
A holographic image can be made visible through an illuminated holographic print or by directing a laser beam through a hologram and projecting the image on a screen.
There are two main types of holograms: a reflection hologram, which is seen when lit from the front; and the transmission hologram, which is viewed when the light is directed through it from the backside.
A transmission hologram can be viewed from the front when it is backed with a mirror-like substance in a setup known as an embossed hologram. A hologram can be made to show objects in motion, or stereograms, which can run for 3 to 20 seconds.
Setup of a Transmission Hologram
Like a photograph, a hologram is a virtual representation of an object, but it differs a great deal from photographs. While each portion of the image contained in a photograph represents the corresponding section of the object, each portion of a holograph contains an image of the entire object, so that the images seen the most from a certain vantage point depends on the portion’s position in the hologram.
Hence, a hologram can still produce a 3D image of the complete object even if the transparent plate containing the hologram gets broken, although the image will be seen from a different point of view.
Since holographs are much more difficult to replicate than many other forms of imaging, they’re widely used to minimize the possibility of counterfeiting in documents such as credit cards, driver’s licenses, etc. In these applications, holograms are imprinted onto a foil.
Also, since holograms can be created through other mediums such as sound waves, x-ray, and ultraviolet, holographs can be used to analyze radio waves in deep space and to record images of objects covered by solid objects like walls or a woman’s womb.
How Holographs Work
A holograph is produced when the interference patterns of intensity peaks and elimination in electromagnetic fields between two or more beams from a laser light is captured on film through a process dubbed ‘window with memory’.
The holograph setup entails directing a beam, which serves as the reference, on the recording medium. The reference beam is then scattered across the illuminated scene to produce the holograph. The film is made of photo-sensitive, fine grain materials such as silver-halide emulsions, dichromate gelatins, and photopolymers.
With a coherent laser light and a stable geometry, a stationary interference pattern can be captured on the hologram’s photosensitive emulsion. Then through chemical processes, the density of the emulsion can be modulated to confine the interference patterns into ‘fringes’. These fringes are frozen distributed recordings of the phase, amplitude and the direction of the visible electromagnetic radiation spectrum.
Upon re-illumination, the hologram diffracts light through these fringes. And by diffracting light that has the same curvature and direction of the reference beam into the shapes of other wavefronts, the hologram reconstructs the recorded image.
With these processes, holographic imaging facilitates the analyses of optical information in various ways. Holograms can be produced in different ways, depending on the aesthetics and image quality required. However, all holograms are based on the same underlying mechanism: the encoding and recording of interference patterns between multiple beams of coherent light. With this mechanism, a hologram can be used to record and reconstruct any spatially-dependent image.
The holographic image is generated through an optical material interface, rather than sensors and programs used in cameras. The processes of recording and reconstructing images in a hologram enfolds information within the flat surface of the hologram, rather than applying the information onto the surface. When the hologram reconstructs the image, it reshapes the image optically in a manner that renders the material surface optically irrelevant.
The light flowing through the hologram appears to carry its own holding space and presence. This apparent space depends on the angle from where space is observed. Hence, the hologram can generate a dynamic spatiotemporal scene where the observer can move around the 3D image without losing sight of the imaged object in its entirety.
Holograms are increasingly becoming a highly sought-after component for consumer products, advertising materials, and sensitive documents. Holograms are also being used in special applications, including space exploration, analyses of artifacts, etc., to generate 3D images that provide optical information on distant and fragile objects.
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