A hologram is a
recording in a two- or three-dimensional medium of the interference pattern
formed when a point source of light (the reference beam) of fixed wavelength
encounters light of the same fixed wavelength arriving from an object (the
object beam). When the hologram is illuminated by the reference beam alone,
the diffraction pattern recreates the wave fronts of light from the original
object. Thus, the viewer sees an image indistinguishable from the original
object.
There are many types of
holograms, and there are varying ways of classifying them. For our purpose, we
can divide them into two types: reflection holograms and transmission
holograms.
The reflection hologram,
in which a truly three-dimensional image is seen near its surface, is the most
common type shown in galleries. The hologram is illuminated by a “spot” of
white incandescent light, held at a specific angle and distance and located on
the viewer’s side of the hologram. Thus, the image consists of light reflected
by the hologram. Recently, these holograms have been made and displayed in
color—their images optically indistinguishable from the original objects. If a
mirror is the object, the holographic image of the mirror reflects white
light; if a diamond is the object, the holographic image of the diamond is
seen to “sparkle.”
Although mass-produced
holograms such as the eagle on the VISA card are viewed with reflected light,
they are actually transmission holograms “mirrorized” with a layer of aluminum
on the back.
The typical transmission
hologram is viewed with laser light, usually of the same type used to make the
recording. This light is directed from behind the hologram and the image is
transmitted to the observer’s side. The virtual image can be very sharp and
deep. For example, through a small hologram, a full-size room with people in
it can be seen as if the hologram were a window. If this hologram is broken
into small pieces (to be less wasteful, the hologram can be covered by a piece
of paper with a hole in it), one can still see the entire scene through each
piece. Depending on the location of the piece (hole), a different perspective
is observed. Furthermore, if an undiverged laser beam is directed backward
(relative to the direction of the reference beam) through the hologram, a real
image can be projected onto a screen located at the original position of the
object.
Between the reflection
and transmission types of holograms, many variations can be made.
Embossed holograms:
To mass produce cheap holograms for security application such as the eagle on
VISA cards, a two-dimensional interference pattern is pressed onto thin
plastic foils. The original hologram is usually recorded on a photosensitive
material called photoresist. When developed, the hologram consists of grooves
on the surface. A layer of nickel is deposited on this hologram and then
peeled off, resulting in a metallic “shim.” More secondary shims can be
produced from the first one. The shim is placed on a roller. Under high
temperature and pressure, the shim presses (embosses) the hologram
onto a roll of composite material similar to Mylar.
Integral holograms:
A transmission or reflection hologram can be made from a series of
photographs (usually transparencies) of an object—which can be a live person,
an
outdoor scene, a computer graphic, or an X-ray picture. Usually, the object is
“scanned”
by a camera, thus recording many discrete views. Each view is shown on an LCD
screen
illuminated with laser light and is used as the object beam to record a
hologram on a
narrow vertical strip of holographic plate (holoplate). The next view is
similarly
recorded on an adjacent strip, until all the views are recorded. When viewing
the finished
composite hologram, the left and right eyes see images from different narrow
holograms;
thus, a stereoscopic image is observed. Recently, video cameras have been used
for the
original recording, which allows images to be manipulated through the use of
computer
software.
Holographic
interferometry: Microscopic changes on an object can be quantitatively
measured by making two exposures on a changing object. The two images
interfere with each other and fringes can be seen on the object that reveal
the vector displacement. In real-time holographic interferometry, the virtual
image of the object is compared directly with the real object. Even invisible
objects, such as heat or shock waves, can be rendered visible. There are
countless engineering applications in this field of holometry.
Multichannel
holograms: With changes in the angle of the viewing light on the same
hologram, completely different scenes can be observed. This concept has
enormous potential for massive computer memories.
Computer-generated
holograms: The mathematics of holography is now well understood.
Essentially, there are three basic elements in holography: the light source,
the hologram, and the image. If any two of the elements are predetermined, the
third can be computed. For example, if we know that we have a parallel beam of
light of certain wavelength and we have a “double-slit” system (a simple
“hologram”), we can calculate the diffraction pattern. Also, knowing the
diffraction pattern and the details of the double-slit system, we can
calculate the wavelength of the light. Therefore, we can dream up any pattern
we
want to see. After we decide what wavelength we will use for observation, the
hologram can be designed by a computer. This computer-generated holography (CGH)
has become a sub-branch that is growing rapidly. For example, CGH is used to
make holographic optical elements (HOE) for scanning, splitting, focusing,
and, in general, controlling laser light in many optical devices such as a
common CD player.
