DEVELOPER,
BLEACH, AND REDDEVELOPER ROUNDUP:
SUMMER 1990
The processing recipes and suggestions
have accumulated once again and so it is time to find out the truth about what
is really the best chemistry. I started
with single beam reflection holograms, with my standard test object, the silver
painted waffle iron.
The first test was to find the better
reddeveloper formula. The reddeveloper,
or third step, is the greatest thing in holographic processing to come along
since PBQ. Nick Phillips had been using
it on his holograms since 1979, after having been impressed with the very high
signal to noise ratio of Soviet style processed holograms. Their single processing step, developing the
silver halide grains into compact colloidal red silver particles instead of
black filamentary threads in their emulsions reduces scatter quite
dramatically, while at the same time introducing enough modulation in the
coating to give high diffraction efficiency.
Using the Russian developers on Agfa materials works, but not as well as
it does on the materials with the extremely tiny grains. Developed and bleached holograms on Agfa
materials are brighter than those simply colloidally developed, but of course
much noisier.
The trick then is to turn the bleached
hologram into a colloidal one. It was
known that colloidal silver can be produced in an emulsion by developing it in
a highly diluted solution under intense light, (Carey Lea filters), and that is
what Phillips has done for the Agfa materials.
After the hologram is processed in the usual way, it is re-exposed to an
intense light, either UV or visible, capturing enough photons to bring it
almost to the point of printing out, and then immersing it in the diluted
developer with the lights on. The
hologram takes on the blood-red color typical of the Eastern bloc colloidal
processing, and exhibits a dramatic increase in S/N ratio as compared to the
untreated one. I have christened this
step the "REDDEVELOPER", a pun on re-developer and red developer.
My first experiment was to determine
the better of the two reddeveloper formulae that Nick Phillips had published.*,** The first one which he proposed has five
ingredients, while his newer concoction has only ten grams per litre of
Ascorbic Acid in its stock solution. I
made an exposure series on three Agfa Holotest 8E75HD plates of 50, 100, 200,
and 400 microJoules per square centimeter of Single Beam Reflection flux*, developed them all for two minutes in
the classical CWC2 developer, bleached in CWPBQ2, and washed well.
The first one was left in the bleached
state and Photo-Flo-ed, the second one was reddeveloped in the first formula,
the third in the second reddeveloper formula, followed by a thorough wash and
wetting agent. The last two were
identical, so now I standardize on the simple reddeveloper formula, 10 g/l
Ascorbic Acid, diluted 1 part developer to 40 parts distilled water, since it
is so cheap. One of the amazing things
about this bath is that it is acidic, running at a pH of about 3. Standard photographic wisdom says that
developers work in an alkali environment.
(Except for Amidol, which is the exception that proves the rule.) Once again, holographic practice runs
counter to established principles, making discoveries like this all the more
remarkable, as most researchers would not even think of experimenting
with a bath like this since they would be prejudiced by prior knowledge into
believing that it could never work. But
seeing is believing, and the results are definitely revolutionary, as either of
the two reddeveloped plates are the same brightness as the simply bleached
plate but the object is so much solider looking, since the shadows are really
black without that milky opalescence of the Rayleigh scatter off the
rehalogenated silver bromide grains.
Such is the genius of Nick Phillips, as his discovery of PBQ also runs
counter to conventional wisdom.
Although he has been chastised for
introducing the nasty smelling and toxic chemical p-Benzoquinone into the
holography lab, the beauty of the bath is not in its composition but in its
action. The bleach rehalogenates the
developed silver grains back into their original form, silver bromide. One would think then that the exposed and
developed plate has been converted back into its original, out of the box
condition, with absolutely no modulation present so why even bother trying
it. However, this processing scheme of
development followed by rehalogenation provides the brightest of holograms!
What is really going on is that the
exposed and developed silver grains of the bright fringes migrate over to the
undeveloped silver bromide grains in the dim fringes as bleaching takes
effect. This subtle phenomenon can only
work with extremely fine grained Lippmann-type emulsions. To illustrate how un-obvious Phillips'
method is, not one of the researchers in Lippmann photography at the turn of
the century, including some of the greatest minds in photographic research
ever, like the Lumiere brothers who co-authored the paper introducing PBQ, even
attempted this simple experiment. For
if they had, they could have solved one of the basic processing problems of
Lippmann photography, that of retaining emulsion thickness to preserve color
veracity. Most Lippmann photographers
either developed to colloidal silver, or developed, fixed, and bleached in
Mercuric Chloride and used some plumping agent in the emulsion to bring it back
to its original thickness. But with
Phillips' scheme, all the modulable material has been rearranged in the layer,
with none leaving, so there is no shrinkage of the fringes to shorter
wavelengths. It will be interesting to
see if there will be any renewal of Lippmann experiemnts using these new
techniques.
The next batch of experiments was to
sort out the bleach recipes. The PBQ
formula which I had been using, CWPBQ2 adds 2g/l PBQ just before use. This is the step that really gives the brew
a nasty reputation. A respirator should
be used while doing this, and then keep the bleach tray covered with another
one the same size for a good seal. A
fume hood would be even better. But it
would be nice to find a cure for PBQ. I
tried four other formulas to see if there was anything that worked just as
well.
Ten plates were exposed to 200
microJoules/cm squared on the SBR test object, developed for two minutes in
CWC2, and then two each were bleached in the following formulae: CWPBQ2, PBU, Jeff Blythe's Copper Sulfate
Bleach, "No Patchy Haze" Fe EDTA, and TJ Bleach. One of each of those pairs were
reddeveloped, the other not.
The conclusion was that all these
bleaches worked just as well as the other with respect to brightness, however
the TJ bleach was a bit noisier than the others. There was a noticeable color shift to shorter wavelengths with
the Fe EDTA, but it could still be replaced on the object and generate real
time fringes under laser light. When
placed under UV light, the non-reddeveloped Copper Sulfate bleach plate printed
out most rapidly, the others all at about the same rate. The reddeveloped plates remained at a
constant density (about 1.0) under UV and sunlight. This suggests that reddeveloping the plates should be a cure for
printout, as the silver in the emulsion is not kept in the unstable halide form
but in a happy, fully oxidized state, like a black and white photograph, except
that its form is not black filamentary silver but small clumps of red colloidal
silver. Long term exposures to sunlight
are now being undertaken to ascertain how printout proof this technique really
is. The drawback is that the hologram
is red with a density of 1.0, so that those looking for that crystal clear
window transmission hologram aesthetic may not be too happy, but if it stays at
that density for the life of the hologram, who cares. Preliminary experiments with reddeveloped white light
transmission holograms (one step rainbows) show that the S/N ratio is improved
with negligible, if any, loss of brightness.
This will soon be quantified.
A batch of developers were tested; the
standard CWC2, LN-7, Ilford SP678C, Pyrochrome, and an old favorite, Kodak
D-8. Except for the Pyro developer, all
were used with the CWPBQ2 bleach since it was on hand. All test plates had quadrants which were exposed
to 50, 100, 200 and 400 microJoules per square centimeter. Development times of 1, 2 and 4 minutes were
given to the CWC2 and Pyro developers.
Two, four and six were used on SP678C and LN-7, while 1/2, 1 and 2
minutes were the times for the three D-8 testers. The results? Basically,
if a density of about 2.5 were raised by whatever combination of exposure and
development, you got a good hologram.
Kodak D-8 was a bit noisier than the others, but only if you compared
them side by side. If you never saw any
of the others, you would be quite satisfied with the D-8 results.
RESULTS
REDDEVELOPER TESTS: Reddeveloper #1 and #2 produced identical
results. Conclusion: Use #2 because it's cheaper. Reddeveloper time: Four minutes seems to be fine.
BLEACH TESTS: All bleaches gave same brightness; the
dichromate-based bleach was slightly noisier.
Copper Sulfate, CWPBQ2, and PBU gave little or no shrinkage; "No
Patchy Haze" Fe EDTA gave slight shrinkage; dichromate based bleach gave
some expansion.
DEVELOPER TESTS: All developers gave pretty much the same
results, if the final developed density were around 2.5.
* I
define Single Beam Reflection flux as twice the energy in the reference beam,
since we hope the object is returning all its light back to the holographic
plate. With the plate resting on the
shallow (about 8 mm) waffle iron which has been painted with Krylon #1401
Bright Silver paint (full of tiny aluminum particles) this definition is
practical for the purpose of this paper.