Page 454 - Introduction to Information Optics

8.3. Optical Storage Media 439
modulation [7]. In order to produce phase modulation, it is necessary to
obtain the final image in the form of grains of a transparent dielectric
compound (such as silver halide) instead of the opaque metallic silver. This can
be achieved by bleaching, following the development process, to remove the
silver image from the emulsion. The remaining silver halide is then desensitized,
The desensitized silver halide modulates the phase of light passing through it.

8.3.2. DICHROMATED GELATIN

Gelatin is traditionally made from the parts of cows that people do not eat
[8]. If one takes all the gristle, hooves, and bone and boils them for a long
time, they end up as glue. If this glue is further refined, the final product is
gelatin. Gelatin can absorb a very large amount of water and still remain rigid;
in other words, it swells in water. Gelatin film is not intrinsically sensitive to
light. Nevertheless, through chemical sensitization, usually by adding ammo-
nium dichromate ([NH 4)] 2Cr 2O 7), it is possible to induce changes in the
gelatin that make it light sensitive.
The mechanism of the photochemical process which occurs when the
dichromated gelatin is exposed to light is not well understood [7,8,9],
6+
However, it is generally accepted that absorption of light is by the Cr ion,
3 +
and that reduction occurs to Cr ions. As a result of reaction with the gelatin,
the gelatin molecular chains in the regions exposed to more light have more
cross-linking. These regions swell less when immersed in water, and, if rapidly
dehydrated by exposure to alcohol, differential strains are produced between
regions of maximum and minimum swelling. These strains modify the local
index of refraction.
Gelatin film can be prepared by dip-coating and doctor-blading on a
substrate. Since a photographic plate contains gelatin in the emulsion, an
alternative approach is to remove the silver halide from the emulsion of a
commercial photographic plate; e.g., Kodak 649F. As mentioned in the
previous subsection, the silver halide can be removed by washing the unex-
posed photographic plate in fixer and then water.

8.3.3. PHOTOPOLYMERS

Polymerization is a process in which two or more molecules of the same
substance unite to give a molecule (polymer) with the same composition as the
original substance (monomer), but with a molecular weight which is an integral
multiple of the original. Of the many methods available for polymerizing a
substance, one important method involves the use of light; hence the term
photopolymer [7,10]. In a photopolymer, the required state of polymerization