TECHNICAL
The Holographic Film : Silver Halide Emulsion : Silver Halide Films and Plates
The photographic film or plate emulsions are the most convenient and commonly used materials for recording holograms. These are available in a wide range of spectral sensitivities. The emulsion contains silver halide microcrystals, dispersed in gelatin deposited on a glass substrate or on a plastic film. The average size of the grains in the holographic emulsion is about 0.08-0.03 micrometer. To decrease the scattering the crystals must be smaller than the wavelength of light so that Rayleigh theory becomes applicable. The emulsions with smaller grains are `slower' than those with larger grains. The emulsion thickness range from 5 -15 micrometer. They can be used for recording thin or volume holograms of amplitude or phase type. These materials have excellent shelf life.
Latent Image Formation
The mechanism of latent image formation in a silver halide emulsion with regard to the recording of a hologram is well understood. During exposure, the absorption of a photon by a grain of the emulsion can free an electron from a surface localized halide ion. This electron can move through the crystal lattice of the grain to combine with a surface silver ion.
The silver atom has a lifetime of about two seconds and dissociates back to an atom. If, however, another photon is available producing another electron, a larger silver speck is formed. This silver speck does not contain enough atoms to be developable. At least four silver atoms are needed for development.
The silver speck is now stable and developable. This is known as a latent image. The latent image can be converted into a hologram by development and fixing. During development process each exposed grain is entirely reduced to silver by catalytic action. The effect of development about 10 times that of the effect of exposure. The unexposed grains remain without any change. The development process is an amplification process.
If the photon energy is not sufficient to produce developable silver atoms, an initial bias exposure can be given to the emulsion to bring it to the threshold value.
Sensitivity and Speed
Intrinsic sensitivity of silver halide is in the ultraviolet and blue region of the spectrum. Dyes which absorb in the longer wavelength region are adsorbed to the silver halide crystals, so that they can transfer the absorbed energy to the crystals for the latent image formation. Thus the silver halide emulsion can be sensitized to record holograms at different laser wavelengths in blue, green and red region. The spectral sensitization can be extended to near infrared region up to about 1.3 um. But due to their MTF and granularity characteristics, they are unsuitable for recording holograms with 1.06 um laser radiation from a Nd: YAG laser.
The speed of the emulsion can be increased by hypersensitization by bathing. The hypersensitization process removes the excess bromide ions and/or increases the relative concentration of free silver ions in the emulsion. While soaking the photographic plate in a water bath for a few second can increase the speed of the film, bathing the plate in a solution of ammonia yields holographic plates with significant increase in sensitometric speed.
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