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25 holography lessons

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21. Chemical processing of holograms

Development of transparent master-holograms differs from development of reflection holograms on the plates PFG-03m and more resembles development of ordinary photomaterials. In principle this is a classical “chemical” development, when the exposed microcrystals of silver bromide are reduced by the developing substance of the developer (metol and phenidone) up to metallic silver (details are described in the lesson 9). In this case sizes of the developed metallic grains are greater than by physical development but this fact doesn’t diminish diffraction efficiency of the hologram since frequency of the interference pattern for transparent holograms is considerably less than for reflect holograms (about 2000-3000 lines/mm). Sequence of operations by chemical treatment of transparent holograms is given in the lesson 20 hence we shall dwell only on peculiarities of some operations.

1. Development.
Emulsion layer of photoplates VRP (FPR) is strongly hardened and weakly swells in water hence temperature of all solutions can be set on the level 20-220C. Prepare phototrays and turn on nonactinic light. For VRP photographic plates the light is red. The sensitivity of VRP photoplates is sufficiently high, hence if you use the photolamp for the first time it’s necessary to check it up whether exposure is absent. To do this cut off a small piece of not exposed photoplate and put it for 5-6 min on the place where trays will stand. If this piece didn’t become dark it means that there is no actinic component in the photolamp spectrum. But if the piece became dark it means that the light filter lets pass a part of green light and it’s necessary either move the photolamp farther from trays or replace the light filter by another one with the following obligatory check-up. The same verification should be carried out also for nonactinic illumination of the shooting cabin in order that the photoplates weren’t exposed by photolamp when the hologram are recording. Chemical developers are active developers. The time of development in them doesn’t exceed 1-2 min. hence the main attention should be paid to quick submersion and withdrawal of the photoplate out of the developer. Take the exposed photoplate out of the box, check the emulsion side and quickly and accurately put it into the bath with developer so that the emulsion side looks upwards. Immediately after this swing the bath in order that the developer covers all surface of the photoplate and then switch on a stopwatch. At once the photoplate begins to darken. Continuously swing the bath in order that development proceeds actively. After 30 seconds lift the edge of the photoplate and look at it through the photolamp. Density of development should be perceptible and uniform over the entire surface of the photoplate. If darkening is weak or it’s nonuniform it means that the hologram was recorded unsuccessfully (the reference beam was displaced, pulse energy was insufficient and so on) or for some reasons the developer has lost its activity, for example grew old. After 50 seconds of development draw the photoplate out of the bath and let developer flow down from the surface into the bath. At the same time the development process still continues. Moreover it becomes more active because by contact with air temperature of the thin surface layer of the developer rises rapidly! During the time when the developer flows down it’s necessary to evaluate once more darkening density of the emulsion layer. Brightness of the photolamp should be considerably weakened when you look at it through the photographic plate. If nevertheless density is insufficient you can raise it with the help of additional development. We don't recommend carrying out development longer than 2 min. since noises can appear in the image which has been caused by dispersion of light on the ravel of fine filament of the developed metallic silver and formation of such ravel sharply increases with prolongation of the development period.

2. Bleaching.
Bleaching is a specific operation for getting phase holograms with high brightness of an image. During the process of bleaching the grains of metallic silver in the emulsion layer again transform into microcrystals of silver halide and the photographic plate gradually becomes transparent. The bath with a bleacher should be continuously swung. Time of bleaching is evaluated only by the appearance of the photoplate when there remain no traces of metallic silver on it and it becomes totally transparent. Approximately for the fresh bleacher and optimum density of darkening (D=1.5) the process takes 2-4 min. If bleaching time prolongs up to 6-8 min it means that the solution became exhausted and it should be replaced by a fresh one. Don’t forget about toxicity of the whitener solution and work only in rubber gloves! Drying the hologram is carried out in the spirits as it is described in the lesson 11. The ready hologram is similar to the not exposed photographic plate, see fig. It doesn’t give image in white light, at best you can see a fuzzy multicolored spot. This fact is connected with properties of transparent holograms [1] which don’t exhibit spectral-selective properties by their illumination by white light. If the reflect holograms themselves “cut out” a narrow band from the wide spectrum of white light and so restore a monochrome image (due to Bragg's properties of the three-dimensional diffraction grating), in the case of transparent hologram diffraction takes place over the whole spectrum of white light. As a result separate monochrome components of the image are superimposed on each other and dispersion (difference between angular directions of the restored beams for different colors) mixes them thoroughly. Therefore a transparent hologram can be restored only in the monochrome laser light. If you illuminate the hologram under necessary angle you can see a bright exceptionally sharp developed over the whole depth three-dimensional image seen through the hologram as through a window. Historically such holograms were received as the first. You can imagine delight of the discoverers – American scientists Leith and Upatnieks [2] when they saw such an image! But necessity of the laser source for restoration strongly restricted use of such holograms especially under domestic conditions. And only due to discovery of the Russian scientist Y. N. Denisyuk, who had developed principles of getting reflect holograms, holography has transformed from exclusively scientific domain into application area and has conquered numerous applications. (The problem of restoring the image from the transparent hologram was solved by Benton. To do this he had to introduce a narrow slot on the stage of recording a transparent hologram. So a transmitting “rainbow” hologram visible in a white light appeared. In order to transform a transparent hologram into reflecting one visible in a white light it’s necessary to copy it using a special scheme. The next part of our lessons is devoted to description of this transformation.

1. R. J. Collier, K. B. Burckhardt, L. H. Lin, Optical Holography, Academic Press, New York, 1971.
2. Leith E. N., Upatnieks J., Journ. Opt. Soc. Amer., 52, 1123 (1962).