Storage_Medium_in_Holography
2.4 Storage Medium in Holography
The storage medium is the medium where, under the influence of light, permanent changes of its physical characteristics occur, i.e. irreversible way of recording. Various energy-sensitive receptors enabling its quantitative registration can be used for recording. Here we can involve photographic emulsions on the basis of sensitized compounds of silver that belong to the oldest but presently most often used storage media of the holographic interferometry.
One of the most important assumptions is the choice of the storage medium whose resolution must make it possible to record the fine hologram structure. The distance of the grating stripes created by the interference of two plane waves depends on the angle between these two waves. Therefore, the holography medium must be adjusted that the used material allow recording of this grating.
The storage media are very important, that is why specific demands are put on their characteristics for their practical utilisation.
Characteristics of a storage medium:
Sensibility characterises the rate of change of the medium physical characteristics induced by the influence of the incident light
Spectral sensitivity indicates for which wavelength the sensitivity of recording material is maximal
Resolution is given by the number of lines per 1 mm that the storage medium is able to distinguish.
Exposure capacity is the storage medium ability to register more than one record of the intensity during their parallel or successive exposure and to allow their independent reconstructions.
Diffraction efficiency is defined as the ratio of the reconstructed and reconstruction waves intensities. It is usually indicated in percentiles, the efficiency for the ideal recording material being 100 %.
Linearity of recording expresses the dependence of the medium physical characteristics on the energy of the light field.
The storage media commonly used in experimental practice can be classified from various aspects according to the mechanism of interaction of light radiation with the material of expected way of utilisation. Considering such criteria as whether the incident wave influences the transparency, refractive index or medium thickness we distinguish amplitude or phase records. This classification dependes on the reconstruction wave that influences either the amplitude or the phase. In practice complex records influencing both characteristics occur (Balaš, Szabó, 1986).
According to the thickness of the storage medium holograms are divided into volume holograms (on the layers whose thickness many times surpasses the distance between the interference maxima) and planar holograms (on thin layers of recording material).
According to the possibility of repeated recording, holograms are divided into: irreversible record – changes of the storage medium physical characteristics are permanent, the original photosensitive characteristics of the material are not possible to restore, and reversible record – with the possibility to delete the former record and register a new one.
To the irreversible recording materials belong photographic emulsions based on sensitized compounds of silver.
The photographic material is the most frequently utilised storage medium in holography. It is created by a sensitive layer applied on a glass plate or film mat. The sensitive photographic layer is a solid substance of fine granules of silver bromide in neutral gelatine. If silver bromide is illuminated by daylight it is decomposed into the components of silver and bromine.
The whole bromide is transformed into metal silver and bromine, either if it is for sufficiently long time illuminated by light or if it is chemically modified after a short illumination. Then chemical development via the developer follows. It continues until the image has sufficient optical density. Then the process is interrupted by water bath in which the developer is removed from the material surface. To remove the residuals of silver bromide the material is put into the fixer whose essential component is the salt cake (disulphate sodium). The final step is washing the material under flowing water. The image of the displayed object has been obtained.
The advantage of this material is its high sensitivity, wide spectral sensitivity and relatively high resolution of several thousand of lines per 1 mm. The disadvantage is the slow processing of the record by the wet method.
Photochemical emulsions also belong to irreversible storage media. This group involves also photopolymers, chrome gelatine, etc. The disadvantage is their low sensitivity, wet processing and short lifetime of the exposed material. The advantage is their great diffractional efficiency.
The reversible storage medium is very practical and prospective. Here belong the photoplastic layers. The fixation of this record is done by fast cool air flow. Under suitable storage conditions it can be kept for several years. The record can be deleted by warming up of its surface layer.
The sensitivity of photochrome material is relatively low and its resolution is only for low wavelengths. Its disadvantage is the undesirable developing process. The material turns back into its original state by radiation with short-wave light.
In holography the photographic sensitive layer with high resolution is most often used as the storage medium (Komar, Serov, 1987). Such photographic materials were originally developed for spectroscopic purposes. At present, there are materials intended especially for holography, for example 649F, AGFA Geavert Scientia, Kodak, Slawich, etc.
