Page 351 - Electrical Properties of Materials

Applications 333

Hologram
B

Eye
A
Object

Lens Fig. 12.28
Schematic representation of viewing a
Laser hologram.

on the amplitude of the scattered light, storing this information on a photo-
graphic plate, we have our ‘picture’, which bears no resemblance to the object
at all. However, when the hologram is illuminated by a laser (Fig. 12.28) the
original object will dutifully spring into life. The reconstructed waveforms ap-
pear to diverge from an image of the object. Moving the eye from A to B means
viewing the object from a different angle, and it looks different indeed just as
in reality. So the picture we obtain is as good as the object itself, if not better.
For examining small biological specimens, for example, the picture may be
better than the original because the original will not sit motionless under the
microscope. A hologram can be investigated at leisure without losing any of
the details, and one can actually focus the microscope to various depths in the
three-dimensional picture.
A variation on the same theme offering some advantages is volume holo-
graphy, to which we shall return in more detail in Section 13.5. It uses a certain
volume in a photosensitive medium, in which a three-dimensional interference
pattern is recorded in the form of refractive index variation. One of the ad-
vantages of volume holograms is that the holographic reproduction is strongly
wavelength dependent—it is the Bragg effect again, contributions must be
added in the correct phase. Hence, the image may be viewed in white light,
with the hologram selecting the wavelength it can respond to from the broad
spectrum available. The Bragg effect is of course strongly dependent on incid-
ent angle as well. The wavelength and angular dependence together make it
possible to record multiple holograms in the same material.
What about holographic movies? Could one arrange conferences at which
only the holographic images of the participants talk and walk in the conference
room? Not impossible. Under some restricted conditions, successful experi-
ments have been conducted while some participants were strutting and fretting
on the stage.
The entertainment provided by holography has so far been confined to the
laboratory with the exception of a few exhibitions and a few art shops. To
produce a holographic image is expensive, and the rich have decided to spend
their money on other objects of luxury. So are there any commercial applica-
tions? Probably, only two. There are, as everyone knows, holograms on credit