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Technical Research
Holography
Holography is a specialised form of photography that stores a three dimensional image of an object on a photographic plate, as opposed to the two dimensional image achieved with standard photography. The image is a store of the scattered light reflected from a test object, combined with a reference light beam. These two light sources interfere with one another and consequently contain enough information for a virtual recreation of the object.
The image created consists of the recording of an interference pattern from the light scattered by the object and the reference wave from the same source on a high resolution, light-sensitive material. This recorded pattern is called a hologram. To visualise the image, the hologram is illuminated by the reference beam. The light beam incident to the hologram diffracts on the recorded pattern as with a diffraction grating. The observer can see through the hologram as a three dimensional image of the object in the same place as during the recording.
To create a hologram the light source must be coherent and of a single wavelength, therefore the source used is usually a laser. This also provides a very intense beam with very little scattering. The beam from the laser is split into two separate beams, usually by an optical glass prism or similar device; one of these beams forms the reference beam and the other is directed onto the test subject. This light is subsequently scattered from the subject's surface and thus a small fraction of it falls onto the photographic film together with the full reference beam. Lenses are used to spread the beams so they cover the desired areas of photographic film and subject respectfully.
There are also considerable restrictions on the photographic materials which can be
used in holography. The resolution must be very high, typically 1000-4000 lines per
mm, the diffraction efficiency must also be high, and the light energy required to mark it must be low. This requirement is necessary because even though the laser light intensity can be quite high, the duration of exposure is very small.
There are a number of typical types of hologram, these each have a different method of creation. They all contain information about the object and this is stored by the interference of the two aforementioned light beams on the photographic material, but they all have unique set-ups and consequently have separate advantages and consequently applications. There are four basic types, these are:
 | Fresnel Holograms;
These are the most common variety, as described above.
| Fourier Transform Holograms;
The object image is formed with the photographic plate in the back focal
plane, this is usually achieved by the use of converging lenses. This allows the
hologram to be stored on less photographic film than an equivalent Fresnel
hologram. Thus this type is particularly useful for creating high resolution
images.
| Image Holograms;
In this case the object is placed very near to the photographic material when
the hologram is taken. The advantage of the similar length paths for the two
light beams is that the light need not be so coherent, it is even possible to use
white light sources to produce and reproduce the hologram. This type is
particularly useful for display/advertising type holograms.
| Lensless Fourier Transform Hologram;
Here the object is placed in the same plane as the reference light beam. This
method is consequently only practical for two dimensional holograms.
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The Holoviz group is concerned with a laser and camera system that can create any of the first three types of hologram, the lensless Fourier transform hologram is limited to producing only two dimensional images and is therefore inadequate for Rolls-Royce's requirements. It is unlikely that the camera will be required to create image holograms as it is to be used to holograph air flows in a Trent engine (and perhaps the surrounding structure) and thus the camera would not be able to be placed too near the test region as it would be at risk of being damaged. Therefore it is believed that the difference in producing the remaining hologram types will not affect the groups tasks it must tackle. The positioning of the test subject, camera and the use of lenses is entirely dependant upon the Rolls-Royce testing team, and it is appreciated that the group is in no position to recommend test procedures and is therefore unlikely to be required to do so.
In the context of obtaining vibrational information on turbine blades, or in understanding the airflow as it passes through a rotating front fan on a test-bed based gas turbine engine, then holographic techniques give a unique insight into this field. As an example, a wind tunnel is filled with small particles produced by a particle nebuliser spray, and a series of holograms are taken of the particles passing through the blades of a turbine engine.
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