What's the modus operandi of holograms?

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