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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 it’s 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 doesn’t have sufficient
mass for carrying-out effective bleaching. Bleached
(phase) holograms on the PFG-03 plates give
a very weak image and therefore it’s 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 – 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
doesn’t 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 – 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 don’t suppose to store the developer
during a long time it’s 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 doesn’t 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 it’s 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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