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25 holography lessons

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20. Preparation of chemical solutions

For every type of photoplates there exist their own optimum way of photochemical treatment. VRP (FPR) photoplates differ to a great extent from the photoplates PFG-03 considered in the lesson 2 both in silver bromide crystal sizes and in gelatine hardness. Big sizes of the silver bromide crystals in the VRP photoplates allows to use a chemical operation of bleaching which sharply increase the hologram brightness. Bleaching is a transformation of the corns of developed metallic silver back into haloid silver (as a rule into silver bromide). Bleached holograms are also called phase holograms since diffraction of light in them takes place due to difference of the refraction coefficients of gelatine and interference layers of haloid silver. In the table the refraction coefficients of gelatine and haloid silver, of silver bromide, silver chloride and silver iodide used by synthesis of photoemulsions are presented:

Gelatine - 1.52
AgBr - 2.25
AgCl - 2.07
AgJ - 2.22

As Kogelnik theory [1] proves thick phase holograms (that is the holograms with thickness of the emulsion layer much greater than the wave length of laser radiation) can have a diffraction efficiency (DE) of 100%! It is quite clear that under real conditions of recording holograms itís impossible to get this maximum value. Although, the holograms on the basis of dichromate gelatine (DCG) give a DE value of 96% and even higher (on mirrors). Phase array in such holograms is formed not on the basis of corns of haloid silver but on differences of the refraction coefficient of hardened and not hardened gelatine that is practically on the molecular level. Unfortunately sensitivity of DCG is very low (about 200 mJ/sq.cm and it is unsuitable for recording pulse holograms. Real DE value of phase holograms for silver halide photoplates reaches the level of 70% what is also quite good. The photoplates PFG-03 have very fine crystals of silver bromide, which on the one hand allows to register the finest interference pattern of the reflecting hologram and on the other hand doesnít have sufficient mass for carrying-out effective bleaching. Bleached (phase) holograms on the PFG-03 plates give a very weak image and therefore itís necessary to carry out amplitude-phase treatment with conservation of the most part of the developed metallic silver. DE value of such holograms doesn't exceed 50%.
It becomes clear even from such brief explanation of peculiarities of phase holograms that photochemical treatment is the most important element in the technological process of making phase holograms. There exist a huge number of methods of photochemical treatment of phase holograms [2] and new methods appear every year. Let us consider two most widespread methods. The first method is based on the developer recommended by the works manufacturing photographic plates and the second method Ė on using well known Western developer SM-6. The first method has an advantage that it's possible to buy the developer at the manufacturing works both in the form of dry batches and in the form of solution. The second method is more simple because it doesnít require fixation operation but the formula of bleacher is more complex and the bleacher is less long-lived by storage. The sequence of carrying out photochemical treatment using the first method is given below.

Development in the developer VRP - 2 min.
Washing - 5 min.
Fixing bath - 5 min.
Washing - 5 min.
Bleaching - up to full clarification
Washing - 10 min.
Drying in ethyl alcohol - during 2 min. in every bath.

The firm developer VRP was improved by the author of these lessons Ė the amount of sulphite in developer was reduced from 140 g/l down to 50 g/l. This allowed decreasing noises holographic image of considerably which evidently were caused by the increased solution rate of silver bromide in large quantities of sodium sulphite in the solution and by its spontaneous precipitation on dislocations of the lattice not being centers of the latent image. Formulas of the improved developer and whitener are given below.

Developer VRP

Bleacher

Metol 6 g.
Sodium sulphite, anhyd. 50 g.
Hydroquinone 20 g.
Potassium methaborate 112 g.
Potassium hydroxide 17.42 g.
Water (distilled) up to 1l
Ferric nitrate 150 g.
Potassium bromide 30 g.
Water (distilled) up to 1 l
Before use it should be diluted in the proportion 1:3

The formulas of fixing bath and of ethyl alcohol solutions for drying the hologram are given in the lesson 9. Advices about correct dilution of chemicals for preparation of the developer and fixer are also given there. If you donít suppose to store the developer during a long time itís possible not to add potassium methaborate. The sequence of carrying out photochemical treatment using the second method is given below.

Development in the developer SM-6 1 min.
Washing - 5 min.
Bleaching - up to total clarification
Washing - 10 min.
Drying in ethyl alcohol - during 2 min. in every bath.

The formula of the developer SM-6 was also improved by the author of these lessons in order to achieve optimum development of the hologram on the photoplates VRP (FPR). The original formula doesnít include sodium sulphite. This fact leads to quick oxidation of the developer. On the other hand a large quantity of sodium sulphite in the developer leads to increased noises in the image for the above indicated reasons. Hence addition of 10 g. of sodium sulphite (anhydrous) to 1 litre of the developer solution appeared to be optimum answer.

Developer SM-6

Bleacher PBU-amidol

Sodium sulphite, unhydrous 10 g.
Ascorbic acid 18 g.
Sodium hydroxide 12 g.
Phenidone 6 g.
Sodium phosphate, dibasic 28.4 g.
Water (distilled) up to 1 l
Amidol 5 g.
Potassium persulfate 10 g.
Citric acid 50 g.
Cupric bromide 1 g.
Potassium bromide 20 g.
Water (distilled) up to 1 l

By preparation of the bleacher itís necessary to observe extreme caution and to work in rubber gloves in order to prevent falling drops of solution on the skin of hands and clothes because amidol and cupric bromide are toxic substances.

References
1. R. J. Collier, K. B. Burckhardt, L. H. Lin, Optical Holography, Academic Press, New York, 1971.
2. H. I. Bjelkhagen, Silver-Halide Recording Materials (for Holography and Their Processing), Springer Series, USA, 1993.
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