Silver Halide Recording Materials
(excerpted from HMP 6 - Chapter 6)
...More Detailed Information
According to KODAK, "When selecting a silver halide
material, it is important to match peak sensitivity as closely as
possible to the wavelength emission of the laser being used.
Early attempts to produce holograms also demonstrated the need
for emulsions with the lowest possible graininess
characteristics, and highest possible resolution."
There are five atoms which, because of their atomic similarity,
are called the halides. They are chlorine, bromine, iodine,
fluorine and astatine. Silver halide emulsions are made using
either silver chloride, silver bromide, or silver iodide. The
other two halides are not used because silver fluoride is
insoluble in water and astatine is radioactive.
A typical silver halide emulsion is made by adding a solution of
silver nitrate to a solution of potassium bromide and gelatin.
Silver bromide crystals form in the emulsion. The emulsion is
heated for a certain amount of time, which is called the ripening
process.
During the ripening process, the grain size increases and the
speed of the emulsion is increased. Some doping agents may be
added to the emulsion at this time to foster proper crystal
growth. Afterwards, the gelatin is allowed to cool. It is then
shredded, and the soluble potassium nitrate is washed out of the
emulsion.
The emulsion is heated again, with more gelatin added; then it is
cooled and applied to a base. The thickness and hardness of the
emulsion is important in holography because emulsions too thick
tend to deform during development. Emulsions that are too hard
can either retard chemical reactions or create vacuoles in the
emulsion left by migrating atoms. These vacuoles tend to scatter
light.
Lets assume the emulsion is made and we now want to expose
it to light. It sounds surprising, but a perfectly structured
crystal of silver bromide does not react to light in any
appreciable way. A crystal with defects, however, does react with
light. Fortunately, most silver bromide crystals will have
defects which consist of some interstitial (out of order) silver
ions displaced in the crystal structure.
The process of the photochemical reaction is not known in exact
detail, but it is believed that when light strikes a silver
bromide crystal, enough energy is available to remove an electron
from an occasional bromide ion. The electron produced is able to
migrate through the crystal until it comes in contact with an
interstitial silver ion. The silver ion takes the electron and
becomes silver metal. Silver atoms formed by this mechanism
apparently act as a nucleus for the formation of aggregates of 10
to 500 silver atoms, known as latent images because they are too
small to be seen by the naked eye.
After exposure, the emulsion is developed. The developer goes to
the site of any silver bromide crystal with a latent image and
causes all the silver in that particular silver bromide crystal
to be reduced to silver metal and deposited on the
already-existing latent image of silver metal. This causes a
worm-like grain of silver metal to form which is limited in size
by the amount of silver available in the silver bromide crystal.
This growth is considerable, amplifying the size of the latent
image silver metal by a factor on the order of 106.
If the developer is left in contact with the emulsion long enough
it eventually attacks all the silver in the emulsion. The speed
of development is slow enough, though, that you can use a timer
to take the emulsion from the developer just after the latent
image, but not the unexposed silver bromide crystals, have been
developed. At this point the developer has converted silver ions
to silver metal if and only if they belong to a silver bromide
crystal that was exposed to light.
The emulsion is then placed in a fixer solution which attacks all
silver bromide crystals that were not exposed to light. The fixer
makes these silver bromide ions soluble and removes them from the
emulsion. The result is an emulsion with black spots where light
has struck, and clear spots where no light struck.
An ideal silver halide emulsion depends somewhat on its use but
there are three main factors to consider in any emulsion:
thickness of emulsion, grain size of silver halide crystals, and
sensitivity (or density of silver halide crystals) in the
emulsion. We can generally state the following: It is agreed that
emulsions of more than 10mm are neither practical or
theoretically necessary to produce most volume holograms. Thickness above this size causes problems in development.
Grain size becomes an important issue in holography because it
involves recording fringe patterns that are wavelengths apart.
Too large a grain size may create excessive scatter, which may
fog or destroy your hologram, and too small a grain size makes
the emulsion have no usable sensitivity. It is generally agreed
that the most ideal grain size is in the range of .01mm to .035
mm.
The ideal exposure would probably be 100 - 300 mJ/cm2 to give a
useful density (D=2-3). If exposures are much longer than this,
the main attraction of silver halide emulsion, its speed, comes
into question and other emulsions become more attractive.
Additional information regarding
this topic, along with information about other recording
materials suitable for holography is included in Chapter 6 -
Recording Materials
of the Holography MarketPlace 6th Edition.
Total cost for HMP6
Delivered airmail to your door:
$30 (USA) (UPS ground)
$35 (Alaska, Hawaii, Canada, Mexico)
$45 (all other countries).