Page 453 - Introduction to Information Optics

438 8. Information Storage with Optics

where d is the thickness of the volume hologram. By combining Eqs. (8.5) and
(8.7), the storage capacity of a volume hologram can be calculated as follows:

2 2
a 1 A Volume ,. )
VH 3
d/A /.
Referring to Eqs. (8.8) and (8.4), the upper limit of the storage density for a
3
3-D medium is I/A regardless of whether we record directly the bit pattern
plane by plane in the 3-D storage or we record and multiplex each individual
volume hologram. This principle was first formulated by van Heerden in 1963
[5-6].

8.3. OPTICAL STORAGE MEDIA

Prior to discussing the architecture of optical storage, this section overviews
commonly used materials for optical storage. Only the basic storage mechan-
ism of each material is discussed in this section.

8.3.1. PHOTOGRAPHIC FILM

Photographic, or silver halide film is the most popular medium for storing
an image. A photographic film or plate is generally composed of a base made
of transparent acetate film or a glass plate, and a layer of photographic
emulsion. The emulsion consists of a large number of tiny photosensitive silver
halide grains which are suspended more or less uniformly in a supporting
gelatin. When the photographic emulsion is exposed to light, some of the silver
halide grains absorb optical energy and undergo a complex physical change;
that is, grains that absorb sufficient light energy are immediately reduced,
forming silver atoms. The aggregate of silver atoms in the silver halide grain is
called a development center. The reduction to silver is completed by the
chemical process of development. Any silver halide grain containing an
aggregate of at least four silver atoms is entirely reduced to silver during
development. The grains that have not been exposed or that have not absorbed
sufficient optical energy will remain unchanged. If the developed film is then
subjected to a chemical fixing process, the unexposed silver halide grains are
removed, leaving only the metallic silver particles in the gelatin. These
remaining metallic grains are largely opaque at optical frequencies, so the
transmittance of the developed film depends on their density.
Silver halide emulsions can be used to produce phase as well as amplitude