Rainbow holograms are a very popular type of holograms which can be viewed in white light. The rainbow holograms are very practical holograms displaying three-dimensional images because the images are reconstructed by white light. It is also known as Benton hologram, named after its inventor, Dr. Stephen A. Benton of MIT. The Benton hologram or the rainbow hologram is a transfer transmission holograms. It is essentially a hologram of a hologram. Here the first-generation hologram , that is the "master" hologram is masked with a narrow slit. The rainbow hologram distributes wavelengths of white light in different directions. This leads the viewer to see the image by light of only one wavelength and the actual wavelength is determined by the viewpoint. Depending on the viewing angle, the color of the hologram changes and hence the term "rainbow". A popular example of the use of rainbow hologram is in visa cards.

Property of Rainbow Hologram

An important property of rainbow hologram is that perspective information is lost in one axis. That means you may not be able to see a change in perspective when looking from above or below.

Recording Process of Rainbow Holograms

Rainbow holograms are made by a double holographic process. Here an ordinary hologram or a transmission hologram is used as the object. The second hologram is made through a slit. This is a horizontal slit which limits the vertical perspective of the first image and there is no vertical parallax in the resultant rainbow hologram. During image reconstruction, the horizontal slit operates. It removes or eliminates the coherence requirement on the viewing light. This makes it easy to attain the three-dimensional character of an image while the viewer moves horizontally. The brightness of the image is also maintained. If the viewers eye is moved vertically, no parallax is seen and the image color moves through the rainbow spectrum from blue to red. It is seen that when a rainbow hologram is viewed at an average height, the image appears yellow-green. On a little higher height , the color changes from orange or red and if we dip, it becomes blue or violet. Thus, it is referred as rainbow.

Technological Steps of Producing Embossed Rainbow Hologram

The procedure of producing embossed hologram in mass production is explained below in the following steps:

• First step: Origination. The result is a three-dimensional model or several slides for 2D/3D holograms
  
  
• Second step: Recording a normal transmission hologram. In this step, respective image is seen in coherent laser light.
  
  
• Third step: Recording phase-relief master hologram viewable by sunlight. This is done by using reconstructed image of the transmission hologram as a subject.
  
  
• Final step: Producing metallic copies of rainbow hologram by electroplating. Mass production of rainbow hologram copies by embossing as a final or last stage.

Advantages of Rainbow Hologram

Rainbow holograms are popular commercial items because of the following reasons:

• Easy viewing
• High brightness
• Vivid dimensionality
• Ease of replication

Applications of Rainbow Holography

Rainbow hologram is widely used in a variety of applications such as follows:

• Decorative and product packaging
• Promotional labels
• Protection of financial and other documents
• Protection of branded goods
• Gift wraps
• Greetings cards
• Used in advertising, magazines and other high volume projects
• Stickers
• Displays
• Image processing