Rainbow hologram
Encyclopedia
The rainbow or Benton hologram is a type of hologram invented in 1968 by Dr. Stephen A. Benton
at Polaroid Corporation
(later MIT
). Rainbow holograms are designed to be viewed under white light illumination, rather than laser light which was required before this. The rainbow holography recording process uses a horizontal slit to eliminate vertical parallax
in the output image, greatly reducing spectral blur while preserving three-dimensionality for most observers. A viewer moving up or down in front of a rainbow hologram sees changing spectral colors rather than different vertical perspectives. Stereopsis
and horizontal motion parallax, two relatively powerful cues to depth, are preserved.
The holograms found on credit cards are examples of rainbow holograms.
The developed hologram is illuminated by a beam similar to the original reference beam. A re-constructed image of the original real image can be seen by an observer located to the right of the hologram. However, this image will appear as if it is being viewed through the re-constructed slit to the right of the plate. This means that only a a small horizontal section of the image can be seen from any one location, though if the observer changes his/her viewing position, a different part of the object can be seen. If the hologram is illuminated with a laser beam of a different wavelength, the position of the reconstructed image will change. When the hologram is illuminated with a white light source directed from the left of the hologram plate, each colour re-constructs a different part of the image at a slightly different angle, so that the whole object is now seen, but with the colour varying in the vertical direction.
This hologram is a transmission hologram, where the hologram is illuminated on one side, and viewed from the other. Illumination and viewing can be done from the same side if the hologram is mounted onto a reflective surface. Mass replication of such holograms can be done using an embossing process. These are used in a wide range of security
applications such as credit cards, banknotes and quality merchandise.
Reference sources=
Stephen Benton
Stephen A. Benton was a Massachusetts Institute of Technology professor and the inventor of the rainbow hologram and a pioneer in medical imaging and fine arts holography.- Biography :...
at Polaroid Corporation
Polaroid Corporation
Polaroid Corporation is an American-based international consumer electronics and eyewear company, originally founded in 1937 by Edwin H. Land. It is most famous for its instant film cameras, which reached the market in 1948, and continued to be the company's flagship product line until the February...
(later MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
). Rainbow holograms are designed to be viewed under white light illumination, rather than laser light which was required before this. The rainbow holography recording process uses a horizontal slit to eliminate vertical parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
in the output image, greatly reducing spectral blur while preserving three-dimensionality for most observers. A viewer moving up or down in front of a rainbow hologram sees changing spectral colors rather than different vertical perspectives. Stereopsis
Stereopsis
Stereopsis refers to impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due the the eyes' different positions on the head...
and horizontal motion parallax, two relatively powerful cues to depth, are preserved.
The holograms found on credit cards are examples of rainbow holograms.
How a rainbow hologram works
Figure 2 shows an optical arrangement for making a rainbow hologram. The object is illuminated with laser light (not shown in the diagram), and an image is formed in the plane of the hologram plate used to record the hologram. A narrow horizontal slit is placed between the object and the lens. The hologram plate is also illuminated with a reference beam derived from the same laser (not shown in the diagram), and the interference pattern between object and reference beams is recorded.The developed hologram is illuminated by a beam similar to the original reference beam. A re-constructed image of the original real image can be seen by an observer located to the right of the hologram. However, this image will appear as if it is being viewed through the re-constructed slit to the right of the plate. This means that only a a small horizontal section of the image can be seen from any one location, though if the observer changes his/her viewing position, a different part of the object can be seen. If the hologram is illuminated with a laser beam of a different wavelength, the position of the reconstructed image will change. When the hologram is illuminated with a white light source directed from the left of the hologram plate, each colour re-constructs a different part of the image at a slightly different angle, so that the whole object is now seen, but with the colour varying in the vertical direction.
This hologram is a transmission hologram, where the hologram is illuminated on one side, and viewed from the other. Illumination and viewing can be done from the same side if the hologram is mounted onto a reflective surface. Mass replication of such holograms can be done using an embossing process. These are used in a wide range of security
Security hologram
Security holograms are very difficult to forge because they are replicated from a master hologram which requires expensive, specialized and technologically advanced equipment. They are used widely in several banknotes around the world, in particular those that are of high denominations...
applications such as credit cards, banknotes and quality merchandise.
Reference sources=
- Hariharan P, 2002, Basics of Holography, Cambridge University Press, ISBN 0 521 00200 1