Introduction to Holography How holograms form three-dimensional images

If you carry cash, a driver's license, or a credit card, you are carrying a hologram. The pigeon hologram on the Visa card is probably the most familiar. When you tilt the card, the rainbow-colored bird changes color and appears to move. Unlike birds in traditional photos, holographic birds are three-dimensional images. Holograms are formed by the interference of laser beams.

How lasers make holograms

Holograms are made using lasers because lasers are "coherent." This means that all photons of the laser have exactly the same frequency and phase difference. Splitting a laser beam produces two beams of the same color (monochromatic). In contrast, regular white light is made up of many different frequencies of light. When white light is diffracted, the frequencies split to form a rainbow.

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In traditional photography, light reflected from an object hits a light containing Reactive chemical (i.e. silver bromide) on the film strip. This produces a two-dimensional representation of the object. Holograms form three-dimensional images because light interference patterns are recorded rather than just reflected light. To achieve this, the laser beam is split into two and passed through a lens to expand their beam. One beam (the reference beam) is directed onto high-contrast film. The other beam is aimed at the object (object beam). Light from the object beam is scattered by the body of the hologram. Some scattered light hits the film. The scattered light from the object beam is out of phase with the reference beam, so when the two beams interact, they form an interference pattern.

The interference pattern recorded on film is encoded as a three-dimensional pattern because the distance from any point on the object affects the phase of the scattered light. However, there are limits to the "three-dimensional" presentation of holograms. This is because the object beam only hits its target from one direction. In other words, the hologram only shows the perspective view from the object beam viewpoint. So while the hologram changes depending on the viewing angle, you can't see behind the object.

View a Hologram

A hologram is an interference pattern that looks like random noise unless viewed under the correct lighting. The magic happens when the holographic plate is illuminated by the light of the same laser beam used to record it. If a different laser frequency or other type of light is used, the reconstructed image will not exactly match the original image. However, the most common holograms are visible under white light. These are reflective volume holograms and rainbow holograms. Holograms that can be viewed in ordinary light require special processing. In the case of rainbow holograms, a standard transmission hologram is replicated using horizontal slits. This preserves parallax in one direction (so the perspective can be moved), but creates a color shift in the other direction.

Uses of Holograms

The 1971 Nobel Prize in Physics was awarded to the Hungarian-British scientist Denis Gabor for his "invention and development of the holographic method." Originally, holography was a technique used to improve electron microscopy. It wasn’t until the invention of the laser in 1960 that optical holography took off. Despite the immediate popularity of holograms in art, practical applications of optical holography lagged until the 1980s. Today, holograms are used for interferometry, security and holographic scanning in data storage, optical communications, engineering and microscopy.

Interesting Hologram Facts

If a hologram is cut in half, each piece still contains an image of the entire object. Conversely, if you cut a photo in half, half of the information is lost.

One way to duplicate a hologram is to shine a laser beam onto it and place a new photographic plate so that it receives light from both the hologram and the original beam. Essentially, a hologram is just like the original object.

Another way to replicate a hologram is to stamp it using the original image. This is much the same way a record is made from a recording. The embossing process is used for mass production.