submitted to
Leonardo, January, 1991.
A
TOAST TO NICK PHILLIPS
Ed
Wesly
Center
for Photonics Studies
Lake
Forest College
555
North Sheridan Road
Lake
Forest, Illinois, 60045
and
The
School of the Art Institute of Chicago
280
South Columbus Drive
Chicago,
IL 60603
ABSTRACT
The Art of Holography
requires bright holograms with good signal to noise ratio. A gentleman who pioneered techniques in
silver halide holographic material processing to make reflection holograms that
are imminently viewable is profiled.
TEXT
Silver halide based
photographic materials were used to record the first holograms of Gabor,
Denisyuk, Leith and Upatnieks. They will probably remain useful in holography
forever, due to their high sensitivity at all parts of the visible spectrum,
compared to the other choices, which lag an order of magnitude or two
behind. Because of this, they will
continue to be the medium of choice for the self-supporting holographic artists
with their small lasers.
In the beginning,
holographic processing followed basic photographic conventions; develop, stop,
fix, wash, wetting agent, dry. But
there was not much hope for bright holograms in this manner, especially the
reflection type, since the blackened developed silver would absorb much more of
the light than what it diffracted.
Bleaching techniques were suggested by Cathey in 1965[1],
and this resulted in a plethora of papers, many peddling not just the
proverbial snake oil, but ferricyanides and mercuric chlorides.
A paper which remains
a classic to this day is "An Advance in the Processing of
Holograms" by N. Phillips and D.
Porter[2].
They introduce the use of a concentrated photographic developer, Neofin Blue,
as a developer for pulsed holography.
Neofin Blue is not only expensive, but hard to obtain, so its use has
been superceded by special formulations.
More importantly, this paper introduces a relatively benign oxidizing
agent, Ferric Nitrate, in a bleach used after fixing to rehalogenate the
developed silver in the bright fringe areas of the holographic pattern into
silver bromide. This bleach was gentle
to the gelatin, avoiding the formation of noise due to surface relief, plus it
was observed that ferric nitrate also had hypo clearing agent
capabilities. It also incorporated a
desensitizing agent, phenosafranine, to cut out printout, while also inhibiting
grain growth to noisy levels. This
seems to still be the favorite bleach of white light transmission holographers[3].
But Dr. Phillips' next
big paper, "Advances in Holographic Bleaches"[4],
introduced a new concept of bleaching to coincide with the introduction of
Agfa's new line of improved Holotest Emulsions, namely the HD series with the
reduction in grain size from the previous 50 down to a nominal 35 nanometers. By rehalogenating the oxidized silver
directly after development and skipping the fixing step, dramatic increases in
brightness could be achieved. What is
most remarkable about this process, and a tribute to Dr. Phillips' genius, is
that conventional photographic wisdom would dictate that this type of system
should not work!
Holographic plates
start off with a homogenous distribution of silver bromide grains in them, and
if all the developed silver were changed back into its original form, the plate
should then regain its virgin condition, and there would be no modulation of
the incoming reconstructing light since there is nothing to differentiate where
a bright fringe had been as opposed to where the dim ones had been. But this process works really well, and the
theory is that the developed grains migrate into the dim fringe areas as they
are being rehalogenated. The bright fringe areas are represented by pure
gelatin, and the dim fringe areas contain silver bromide, but there is now more
modulable material in those areas so that efficiency is high. Since nothing left the emulsion, things only
having been rearranged in there, the original thickness of the layer is
preserved along with the spacing of the fringes during recording, so that it is
not only possible to replay a reflection hologram with the laser that made it,
but to be able to replace the hologram back onto the object and generate
real-time interferometric fringes with it!
At first I thought
that this migration-diffusion mechanism was unreasonable, but it was proven to
me when I was making some extremely low frequency gratings. They had fringe spacing of about 2 line
pairs per mm; these fringes are visible to the naked eye. I made my first exposure test, developed and
bleached without fixing, and discovered almost no diffraction while wet. It dried while I was interrogating it with
the undiverged laser beam, and I could see in the woodgrain caused by internal
reflection between the two glass surfaces an excellent red Lippmann
mirror. The process made a better
hologram of the back of the glass than it did of the coarse interference
system!
The simple grating had a
fringe spacing of 100 's of microns; the reflection grating's spacing had
fringes about 300 nanometers apart, which is three orders of magnitude
difference. If it were true that the
silver grains were swimming from bright fringe to dim fringe as they
rehalogenated, then this mechanism would be more effective in travelling short
distances rather than longer ones.
Develop ‑ rehalogenating
processes then have a lower limit of useful spatial frequency, and don't really
come into their highest efficiency until about 1000 lines per millimeter, as
papers by Hariharan[5] and Ward[6]
show. Benton had also predicted these
effects when writing about his IEDT processing[7],
which shifts the unexposed silver grains which had been in the dim fringes over
to the developing bright fringe grains.
But gratings with fringes so large that they are visible to the eye are
extreme cases; certainly the process functions well on the size of fringes
formed in the transmission mode by an object placed along the normal to the
plate and a reference at 45 degrees from the normal. The lack of low spatial
frequency response aids in the suppression of intermodulation noise from the
object's light interfering with itself.
The fringes formed by points on the object are very widely spaced for
points immediately next to each other, and are at their minimum for the
interference caused by the extreme ends of the object, but rarely are these
fringes as tiny as the reference ‑ object fringes. The process will tend to ignore these coarse
noise fringes and strengthen the more closely packed holographic ones. Bullseyes caused by dirt on optics will be
less apparent on the developed - rehalogenated holograms as the processing
makes them lower contrast. Ditto for
the dreaded woodgrain. It is strange to think of a holographic material's
modulation transfer function being at zero for the low spatial frequencies,
then climbing to a peak in the 1000's of line pairs per millimeter then falling
off. But silver halide materials
processed in this mode are not alone in this respect, as DuPont's Photopolymers
which work by a diffusion mechanism also exhibit this effect, with products
manufactured specifically for reflection or transmission work.
The trick is that the
rehalogenation must be performed in aqueous solutions for the diffusion of the
silver to take place. For if the developed holographic plate is rehalogenated
by Bromine Vapor a` la Thiry[8]
or Graube[9],
there can be no "swimming" of silver bromide from one area to the
other, so that the plate does return to it original consistency.
This subtle phenomenon can
only work with extremely fine grained holographic or 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
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 experiments
using these new processing techniques, other than those of Dr. Phillips himself[10],[11].
The proof of the processing
is in the holograms, and holograms processed this way helped account for the
success of the Light Fantastic shows in the late seventies, which acquainted
the general public with extremely realistic images thanks to the high
brightness and black shadows due to the low signal to noise ratio. The second PBQ bleach mentioned in reference
4 was adopted by John Kaufmann[12],
the dean of triethanolamine color control, in combination with Kodak D-19, as
his basic process. If not using Dr.
Phillips' recipes verbatim, most workers have adopted the develop-rehalogenation
scheme.
The eradication of the fixing
step also eliminated the characteristic odor of hypo (thiosulfates) which has
been the bane of photographic darkrooms since the days of Daguerre. It is an unpleasant smell for most people,
and some are outright allergic to it. It is also unhealthy for bleached
holograms, since it is a solvent for silver halides, which are the modulation
ingredient in the holographic layer.
Even the airborne particles which account for the smell are capable of
ruining a perfectly good hologram.
But Dr. Phillips has been
criticized by the safety conscious for having introduced an even nastier
smelling chemical, the dreaded PBQ, (p-Benzoquinone), which is what the
developing agent hydroquinone oxidizes into as it gets spent in the process of
reducing silver bromide crystals into elemental silver. It is certainly a
powder from hell when it comes to mixing.
Its very fine dust inflames the sinuses and dries out the eyes. Certainly personal air masks with organic
filters help, and some workers have made spare shower stalls into fume hoods to
control the hazard[13]. It takes a very long time to dissolve, and
the bleach has a covered tray life of a few hours. It can't be left uncovered, as it oxidizes into uselessness even
quicker, but more importantly, even the solution reeks. So he then formulated a
rehalogenation/diffusion bleach based around Ferric EDTA
(EthyleneDiamineTetraAceticacid) as the oxidizing agent, which is the one
commonly used in color photographic processes because it is ecologically benign[14].
Ilford came out with a new
blue-green sensitive silver halide holographic recording material, and he
eliminated their problem of splash marks[15].
The plates were developing splotchily, as the solution did not penetrate the
entire coating evenly and
simultaneously, and the developer darkened some areas more than
others. By using a restrainer in the
developer to hold off developing activity for about a half-minute after
immersion, until the light-sensitive coating was totally full of it allowed
much more even development.
His next processing
publication[16] fine-tuned
the Ferric EDTA formula with a "No Patchy Haze" version of this
bleach, which cuts down local variations in surface scatter. But the paper he delivered at Budapest for
the celebration of the 100th birthday of Dennis Gabor[17],
introduced the greatest improvement in holographic materials' processing since
the invention of PBQ.
After having been impressed
by the high signal to noise ratio of Russian colloidally-developed silver
halide materials in 1979, he tried processing Agfa Holotest materials in
Russian style developers. These
solutions reduce the exposed silver halide crystals into red silver, which get
their color from being small compact grains instead of the long filamentary
strands of the black silver which is the typical product of development. This type of developer works very well with
the Soviet style materials with their extremely tiny grains, which are about a
third of the diameter of those in the Agfa plates. Since scatter is proportional to the fourth power of diameter,
Russian plates have less than 1% of the scatter of Western ones. Because of these small grains, Russian
plates look as clear as glass, since they do not have the large scattering sites
which gives a soft ground glass look to the usual plates. There is less haze in recording and in
replay, contributing to blacker shadows, which adds significantly to the
solidity of the object holographed.
However, since sensitivity is proportional to the third power of
diameter, Russian materials need almost 100 times the exposure of regular
plates!
The Agfa plates were not
responding well to the single step colloidal developers, and since he was
getting such high efficiency with black silver development followed by
rehalogenating bleaches, he decided to change the rehalogenated silver bromide
into colloidal silver, with a highly diluted developer. The plate is exposed to light to the
saturation point but not enough to cause solarization or to the point of
printing out, then immersed in the weak developer formula without any
agitation. The soup breaks down the large highly scattering silver bromide into
little rocks of colloidal silver. There
is quite an appreciable increase in signal to noise, so much so that true color
holograms that the author made with red 633 nm, green 515 nm and blue 476 nm on
a single 8E75HD[18] would not
have worked at all if not for this trick.
The blue image would have been buried in the blue scatter noise of the
Agfa emulsion.
The original colloidal
developing formula, which I had dubbed in the beginning
"Reddeveloper", a pun on red developer and redeveloper, and now call
the "Blood Bath" because of the characteristic dried blood red color
of the finished plate, had six ingredients.
But in a slightly later version of reference 19[19],
he simplified the step to a simple 1% solution of ascorbic acid. This is quite remarkable, as there is no
alkali to provide the proper pH to activate the developing agent, and the bath
runs at a pH of about 3. Again, this
goes against the grain of conventional wisdom.
Not only does this increase
the signal to noise ratio of the Agfa and Ilford plates to a level comparable
to that of soviet style materials, it also prevents the dreaded printout. Colloidal silver is fully oxidized, like the
black silver in a conventional black and white photographic negatives and
prints, and will not change on its own like an unstable silver halide. Holographers looking for that "hologram
as a crystal clear window" effect might be daunted by the red color of the
emulsion, which unfortunately filters out a bit of the blue and green end of
the spectrum, but then again they may take comfort in the fact that the
hologram will be archival, and will not change its color over time.
Currently Dr. Phillips and
Hans Bjelkhagen are researching new formulations of bleaches which create PBQ
in the solution by oxidizing hydroquinone with potassium persulfate. PBQ as an oxidizer does have its advantages
over the others, especially in regards to efficacy, as two grams per liter of
PBQ does the same job in the same amount of time as 30 grams of Ferric EDTA,
ferric nitrate, potassium ferricyanide, mercuric chloride, or copper
sulfate. Plus it also tans or hardens
the gelatin, preventing the shrinkage that often occurs with the other
oxidizers, especially Ferric EDTA.
Their formula, which they have dubbed PBU, (Phillips-Bjelkhagen
Ultimate), does not have the nasty odor of PBQ, yet works identically.
So I propose a toast to the
gentleman who has done the most in inspiring us all in the creation of high
quality reflection holograms, and who has thrown off the bondage of "commercial
ties" to give us the knowledge to get the maximum results of the
materials, so that the medium can progress, cheers to our friend, Nick
Phillips.
Who knows what may
follow? A special monobath, with a
developing agent that changes the silver bromide into black silver but whose
spent by-product oxidizes the developed silver so that it could be
rehalogenated and diffused back to a silver bromide phase hologram but then the
by-product of the oxidizer becomes a weak developing agent to change the silver
bromide into a colloidal silver? That
would be nice. But if that doesn't
come, surely the legacy of Nick Phillips will include the perfection of the
processing of holograms.
THE FORMULAE
(IN THE ORDER OF PRESENTATION)
ORIGINAL FERRIC NITRATE FORMULA
20 g
Glycerol
500 ml Deionized Water
500 ml Isopropyl Alcohol
300 mg Phenosafranine
150 g
Ferric Nitrate
33 g Potassium Bromide
Dilute 1 to 4 with water before use.
Bleaching time: One and a half times the time
it takes to clear.
Temperature: 20C Agitation: Intermittent
For
rehalogenation after fixing. Hans
Bjelkhagen prefers this over the simpler GP 431 formulation for pulsed masters
developed in Neofin Blau diluted 1:1.
This stock
solution lasts indefinitely, working solution about one week.
Source: N. J. Phillips and D. Porter, "An
Advance in the Processing of Holograms,"
Journal of Physics E: Scientific Instruments 9, 631 (1976).
GP 431
150 g
Ferric Nitrate (9-Hydrate)
30 g Potassium Bromide
.3 g Phenosafranine (Optional)
One litre Water
Dilute 1 to 4 with water before use.
Bleaching time: One and a half times the
time it takes to clear.
Temperature: 20C Agitation: Intermittent
The most
enduring of the classical bleaches. The
phenosafranine may need to be dissolved in an alcohol before adding to the
stock solution or in a bit of very hot water.
This stock
solution lasts indefinitely, working solution about one week.
Source: Agfa Gevaert Technical Information Bulletin
21.7271(480).
GP 432
50
g Potassium Bromide
1.5 g
Boric Acid
2
g p-Benzoquinone added just before use.
One litre water
Source: N. J. Phillips, A. A. Ward, R. Cullen, D.
Porter, "Advances in Holographic Bleaches," Photographic Science and
Engineering 24, 120 (1980). Also Agfa
Gevaert Technical Information Bulletin 21.7271(480).
PBQ #3
30 g
Potassium Bromide
15 g
Borax
2
g Potassium Dichromate
2
g p-Benzoquinone added just before use.
One litre water
Bleaching
time: One and a half times the time it takes to clear.
Temperature: 20C
Agitation: Intermittent
This is the
bleach that John Kaufman uses with Kodak D-19 as the developer.
Source: N. J. Phillips, A. A. Ward, R. Cullen, D.
Porter, "Advances in Holographic Bleaches," Photographic Science and
Engineering 24, 120 (1980)
"BENIGN"
Fe EDTA
ORIGINAL REVISED
30 g
Ferric Sulfate 30 g Ferric Sodium-EDTA
30 g
di-Sodium EDTA
30 g
Potassium Bromide 30 g Potassium Bromide 10 ml Sulfuric Acid (Conc.) 30 g Sodium
Bisulfate
One litre
water One
litre water
Bleaching
time: One and a half times the time it takes to clear.
Temperature: 20C
Agitation: Intermittent
Less hazardous
to work with than the dreaded PBQ.
Either version of the recipe gives the same result, the choice depending
on the availibility of the ingredients.
Leaving the solution exposed to air (uncovered tray) will extend the
lifetime of the oxidizer.
Source: Nicholas Phillips, "Benign Bleaching
for Healthy Holography", holosphere, Volume 14, Number 4, p.21, (1986)
NICK'S #5
PART A PART
B
60 g
Sodium Sulfite 20
g Sodium Metaborate
20 g
Catechol 120 g
Sodium Carbonate
10 g
Hydroquinone
One liter water
10 g
Potassium Bromide
One litre water
Mix equal parts
together before use.
Development
time: Four to five minutes
Temperature: 23C +- 1C
Agitation: Constant
Primary
recommendation for transmission holograms on Ilford green-sensitive materials,
followed by a rehalogenating-diffusing bleach. Properly exposed plates will
wait thirty seconds before showing signs of development.
Part A can
last a month in a tightly stoppered bottle; Part B can last indefinitely. The combined solutions can last a day in a
covered tray.
From: Nicholas J. Phillips, "The Silver
Halides - the Workhouse of the Holography Business", Proceedings of the
International Symposium on Display Holography, Volume III, 1988, p.35.
NICK'S #6
PART A PART
B
10 g
Pyrogallol
20 g Sodium Metaborate
10 g
Potassium Bromide 120 g Sodium Carbonate
One liter water
One liter water
Developing
time: Five minutes
Temperature: 23C +- 1C
Agitation: Constant
Primary
recommendation for Ilford green-sensitive materials for same wavelength replay
in the reflection mode when followed by a rehalogenating bleach. SP737T will work in this brew but at a stop
loss in speed. A properly exposed plate
will sit in this brew for a half a minute before any darkening appears.
Tray life of
the combined solutions is about 15 minutes, Part A will last two or three days
in a full stoppered bottle, but Part B will last indefinitely.
From: Nicholas J. Phillips, "The Silver
Halides - the Workhouse of the Holography Business", Proceedings of the
International Symposium on Display Holography, Volume III, 1988, p.35.
"NO
PATCHY HAZE"
12 g Ferric Sulfate
12 g di-Sodium EDTA
30 g Potassium Bromide
50 g Sodium Bisulfate
One litre of water
Bleaching time: To clear plus one
minute. (Usually in excess of six
minutes!)
Temperature: 20C
Agitation: None
A slow,
diluted Fe EDTA bleach which eliminates non-uniform scattering patches
throughout the emulsion. The key to
success is to avoid the urge to agitate, as this one can take up to fifteen
minutes to clear a well-exposed plate.
Source: Nicholas Phillips, "New Recommendations
for the Processing of Ilford Plates", handout at Lake Forest College
Holography Workshop II, July 1989.
REDDEVELOPER
#1
10 g
Sodium Sulfite (Anhydrous)
5
g Hydroquinone
10 g
Ascorbic Acid
23 g
Potassium Phosphate (mono)
30 g
Sodium Carbonate
One litre distilled water.
Dilute 1 part
developer to 40 parts distilled water, otherwise there will be a patchiness to
the final hologram.
Re-expose
plate to UV or visible light and develop for five minutes with the lights on.
Source: Nick Phillips, "Bridging the Gap
Between Soviet and Western Holography", Proceedings SPIE, Conference in
Hungary, in press.
REDDEVELOPER
#2
10 g
Ascorbic Acid
One litre distilled water.
Re-expose
plate to UV or visible light and develop for four minutes with the lights on.
Source: Nick Phillips, "Bridging the Gap
Between Soviet and Western Holography", Handout at Lake Forest College
Holography Workshop II, July, 1990.
ENDNOTES
[1]. W. T. Cathey: "Three-dimensional
wavefront reconstruction using a phase hologram", J. Opt. Soc. Am. 55, 457 (1965)
[2]. N. J. Phillips and D. Porter, "An
Advance in the Processing of Holograms, "Journal of Physics E: Scientific
Instruments 9, 631 (1976).
[3]. R. Berkhout, "Working with Kodak
Plates 120-01, Making White Light Transmission Holograms", Proceedings of
the International Symposium on Display Holography, Volume III, T. Jeong,
Editor, Lake Forest College, (1988)
[4]. N. J. Phillips, A. A. Ward, R.
Cullen, D. Porter, "Advances in Holographic Bleaches," Photographic
Science and Engineering 24, 120 (1980)
[5]. P. Hariharan, C.M. Chidley:
"Rehalogenating bleaches for photographic phase holograms. 2: Spatial frequency effects", Appl.
Opt. 27, 3852-3854 (1988)
[6]. A. A. Ward, L. Solymar:
"Diffraction efficiency limitations of holograms recorded in silver-halide
emulsions", Appl. Opt. 28,
1850-1855 (1989)
[7]. Stephen A. Benton in Handbook of
Optical Holography, H. J. Caulfield editor, Academic Press, 1979, New York.
[8].
H. Thiry: "New technique of bleaching photographic emulsions and
its application to holography", Appl. Opt. 11, 1652-1653 (1972)
[9].
A. Graube: "Advances in bleaching methods for photographically
recorded holograms", Appl. Opt. 13,
2942-2946 (1974)
[10]. N.J. Phillips, H. Heyworth, T. Hare:
"On Lippmann's photography", J. Photogr. Sci. 32, 158-169 (1984)
[11].
N.J. Phillips, R.A.J. van der Werf: "The creation of efficient reflective
Lippmann layers in ultra-fine grain silver halide materials using non-laser
sources", J. Photogr. Sci., 33,
22-28 (1985)
[12]. John Kaufman, "Previsualization
in Pseudo-Color Holography", Proceedings of the International Symposium on
Display Holography, T. Jeong, editor, Volume I, (1982).
[13]. The West Coast holographer Robert
Hess's apartment when he lived in Silicon Valley circa 1985.
[14]. Nicholas Phillips, "Benign
Bleaching for Healthy Holography", holosphere, Volume 14, Number 4, p.21,
(1986)
[15]. Nicholas J. Phillips, "The
Silver Halides - the Workhouse of the Holography Business", Proceedings of
the International Symposium on Display Holography, Volume III, 1988, p.35.
[16].
Nicholas Phillips, "New Recommendations for the Processing of
Ilford Plates", handout at Lake Forest College Holography Workshop II,
July 1989.
[17].
N. Phillips: "Bridging the gap between Soviet and western
holography", Proceedings of the SPIE (1990) in press.
[18]. E. Wesly, T. Jeong, "Progress in
True Color Holography", Proceedings of the SPIE, Vol. 1211, 1990.
[19]. Source: Nick Phillips, "Bridging the Gap
Between Soviet and Western Holography", Handout at Lake Forest College
Holography Workshop II, July, 1990.