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.

Let’s 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).