The name of HARMAN technology Ltd may be new, but we have a long history in the field of photography.
The expertise we now apply in other areas is derived primarily from what we have learnt in decades of experience in the photographic field - controlling crystal growth when making photosensitive silver halide emulsions (technically, dispersions), and making multi-layer coatings on film and paper supports.
- The Holographic Process
- Holographic Film
The Holographic Process
We live in a three-dimensional world, but conventional photography reduces it to two dimensions. One exception to this is holography, a specialised photographic process which produces real three-dimensional images.
A hologram cannot be made using an ordinary camera: it requires special equipment, including a laser. The light produced by a laser is said to be “coherent” – that is, all of the light rays are of the same wavelength and “in phase”. As a result, if two beams of laser light meet, they produce an interference pattern of alternating light and dark lines.
When making a hologram, one laser beam is directed at the subject, and is reflected from it onto the holographic film. A second (“reference”) beam is directed at the film, avoiding the subject. The two beams interfere, and what is recorded is the interference pattern in the plane of the film. (The Denisyuk single-beam method for making reflection holograms is the simplest: the light passes through the film and is reflected back onto it by the subject, and the reference beam is light striking the film for the first time.)
After processing, light is shone onto the film from the same direction as the reference beam to reveal the image. A laser is not always required for this - sunlight or a spotlight can work well enough in many cases. The interference pattern recorded in the film modulates the light falling on it so that the viewer sees an impressive three-dimensional image. Depending on how the hologram is made and displayed, the image may be seen to be “hanging in the air” in front of the film, or lying behind it, as though seen through a window.
For more information on holography, see www.holography.co.uk, the website of the Royal Photographic Society special interest Holography Group.
Conventional photography “maps” the scene onto the negative, and in order for the sensitive emulsion to resolve fine details, the sizes of the silver halide grains are typically around 1 micron. Holography has very different requirements. The interference fringes which must be recorded are very close together – depending on the set-up, their separation can be as little as about half the wavelength of the exposing light. In order to record them accurately, the sizes of the silver halide grains need to be a small fraction of the wavelength of light; so holographic emulsions tend to contain grains with a diameter of about 30 nanometres or less.
This picture shows a cross-section through an exposed and developed holographic film. The layer thickness is about 10 microns. The fuzzy bands are the developed images of the interference fringes.
This picture shows an enlarged portion of a cross-section like that shown above: the individual particles of developed silver corresponding to the original grains in the emulsion can be seen clearly.
We have previously manufactured holographic film, under the name ILFORD Holofilm. However, there was insufficient demand for this material in the early 1990’s, and it was discontinued. If enough users were to require a holographic film, a successor to Holofilm could be introduced.
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