What's the modus operandi of holograms?

JAKARTA (JP): Holography is a special photographic technique using two laser beams that are in interference with each other on a photographic plate of film (photographic emulsion). The first beam is directed immediately towards the film, while the second beam is directed at the object, which reflects the rays onto the film. The interference of the rays is captured on the film.

To reiterate, in greater detail: the laser directs rays towards a beam splitter. Beam A continues straight ahead, while Beam B is diverted. Reference Beam A is reflected by a mirror and enlarged with lenses before continuing onto the film. Object Beam B is diverted towards a mirror and enlarged by lenses, projecting rays onto the object which is to be recorded. The rays are reflected onto the film as well. Before reaching the film, the two beams create an interference pattern, and this pattern is what is captured by the film.

All light waves reflected from the object interact with the reference beam and are recorded on the film. Once the film is developed, the interference pattern is turned into a hologram, which is a three-dimensional record of the light waves reflected by the object.

There are two main kinds of holographic images. The transmission hologram is created by projecting laser generated reference beams towards the film. The beam projected towards the film and on to viewers diffracts when it hits the film. The diffracted beam projects different wavelengths of light. Hence, viewers see the object formed by different configurations of these light rays. The complex combination of light waves creates both a horizontal and a vertical parallax. As you change your viewing position you perceive the object from different angles of the rays, and the object appears in three dimensions.

While you can only view the laser transmission hologram using laser light, a rainbow hologram can be mass produced. Usually, the vertical element of the three dimensional effect is reduced by masking the hologram. A horizontal slit eliminates the vertical parallax from the hologram. Through the horizontal slit, the laser constructs a second hologram, called a rainbow hologram. This hologram only has horizontal parallax, and the three dimensional effect can only be seen from side to side. A multicolored spectrum of light is instead projected vertically. So, as you move your viewing position up and down, you see the holographic image in different color tones.

A special emulsion ("photoresist") is used to record the hologram, which renders the interference pattern of the hologram as ultrafine ridges. Through a process of electrolysis using nickel, a mold is created. By means of the mold, the pattern is transferred onto a plastic sheet. It is then coated with aluminum. The mirror-like surface of the sheet reflects white- light waves, creating a holographic image of the object.

As holograms can not yet be duplicated, the same process needs to be repeated for every single reproduction.

-- Amir Sidharta