Reconstructing and viewing the holographic image
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When the hologram plate is illuminated by a laser beam identical to the reference beam which was used to record the hologram, an exact reconstruction of the the original object wavefront from the object is obtained. An imaging system (an eye or a camera) located in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing the original. If the lens is moved, the image changes in the same way as it would have done when the object was in place. If several objects were present when the hologram was recorded, the reconstructed objects move relative to one another, i.e. exhibit parallax, in the same way as the original objects would have done. It was very common in the early days of holography to use a chess board as the object and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess pieces appeared to change. This effect can be seen in the figure on the right where the orientation of the mouse is significantly different and its position relative to other parts of the scene have changed.
A holographic image can also be obtained using a different laser beam configuration, but the reconstructed image will not match the original exactly. A holographic image can also be obtained using white light in specific circumstances.
Laser beam reconstruction
editThe reference beam is normally a divergent beam incident at an angle onto the hologram plate. Any variation in either divergence, angle of incidence, or wavelength of the illuminating beam will produce some distortion in the reconstruction, changing the relative size, shape, orientation and location of the objects in the re-constructed scene. The main effects of specific changes in the form of the illuminating beam can be summarised as follows:
| Change in illuminating beam | Change in reconstructed image |
|---|---|
| Angle | Angle |
| Divergence | Distance |
| Wavelength | Size |
When a laser is used to reconstruct the hologram, the image is speckled just as the original image will have been.
White light reconstruction
editWhite light consists of light of a wide range of wavelengths. Therefore, if a hologram is illuminated by a white light source, each wavelength can be considered to generate its own holographic reconstruction, and these will vary in size, angle, and distance. These will be superimposed, and the summed image will wipe out any information about the original scene, just as if you superimposed a set of photographs of the same object of different sizes and orientations. Volume holograms and rainbow holograms can be reconstructed with white light and it should be noted that these do not contain speckles.
Volume holograms
editA volume hologram, as explained above, can give a reconstructed beam using white light, as the hologram structure effectively filters out colours other than those equal to or very close to the colour of the laser used to make the hologram so that the reconstructed image will appear to be approximately the same colour as the laser light used to create the holographic recording.
Rainbow holograms
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In this method, parallax in the vertical plane is sacrificed to allow a bright well-defined single colour re-constructed image to be obtained using white light. The rainbow holography recording process uses a horizontal slit to eliminate vertical parallax in the output image. The viewer is then effectively viewing the holographic image through a narrow horizontal slit. Horizontal parallax information is preserved but movement in the vertical direction produces colour rather than different vertical perspectives. Stereopsis and horizontal motion parallax, two relatively powerful cues to depth, are preserved.
The holograms found on credit cards are examples of rainbow holograms. These are technically transmission holograms mounted onto a reflective surface like a metalized polyethylene terephthalate substrate commonly known as PET.