• Producing a Hologram
• Types of Lasers
• Still Holograms v. Stereograms

Producing a Hologram

The image on a hologram is recorded in an entirely distinct way from the recording of the image on a photograph. With photography, ordinary light reflects off the subject, passes through the camera lens and exposes the film. Bright subjects expose the film a lot, darker subjects expose the film less and the resulting picture we see is composed of varying tones. With holography, the film needs to be exposed to a beam of coherent light - that is, light which is composed of light waves that have identical frequencies and which are vibrating together ‘in phase.' Light from the sun and from light bulbs emit light of varying frequencies with randomly varying phases, and, therefore, cannot be used to make holograms. Lasers emit coherent light and are, therefore, a perfect source for making holograms.

In a basic holographic recording setup, the single beam of light leaving the laser (‘A') is immediately divided into two beams of equal length (‘B'). One of these beams travels directly to the film (‘C'). The other beam reflects off the object and is scattered onto the same film. As the two beams of light converge on the film, they intersect and combine to form a complex pattern. This is called an interference pattern. An interference pattern created in this way and recorded onto the film is called a hologram. The recording of the interference pattern onto the film takes place during an exposure that can last from a fraction of a second to several minutes, depending on the film’s sensitivity and the amount of laser light used.

The film is then developed and processed in order to permanently store the interference pattern on the film. The hologram thus produced, sometimes called the ‘master hologram,' has no recognizable image in ordinary light and only an indecipherable microscopic pattern of closely spaced overlapping lines can be seen. However, when coherent laser light properly illuminates the master hologram, a clearly viewable three-dimensional image appears. The image that appears on the film under the laser light is an exact replica of the image of light waves that bounced off the subject and exposed the film during the recording process.

Essentially, the interference pattern stored on the film interacts with the incoming laser light to reconstruct an image of the subject. What the film ‘saw' during the recording process, the viewer looking at the master hologram can now see. If the film ‘saw' a physical object, the viewer will see a three-dimensional image of the object. Since it is not practical to use laser light to illuminate all master holograms, the image on the master hologram can be transferred to, and recorded on film, so that it can be seen under ordinary light. This transfer process basically involves making a hologram of the master hologram, resulting in a ‘transfer hologram' which can be viewed under ordinary light.

The LitiGraph processes for making master holograms and transfer holograms is extremely sophisticated and based on cutting-edge optical and holographic technology, much of which is proprietary to Liti Holographics, but the underlying optical principles are similar to those in the above discussion.

Want to learn more about holography, and how LitiGraphs are produced? Read more in the other Technology sections we've prepared: Types of Lasers, Still Holograms v. Stereograms.

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