Red Sensitive DCG
the latter half of the 1980s I took up the challenge of trying to make DCG holograms with low power HeNe lasers (633nm).
The consequent discovery of the useful
compound TMG and the theoretical side of it is described in ref. (1 ) in the piece below which was mostly written back in
1989. Does it work? It
sure does , but patience is not a virtue-- it is an essential here. However could anyone get a more enthusiastic email than I got in 2002 from a student in South America?
Here it is:-
Date: Wed, 5 Jun 2002 11:18:51
To: Jeff Blyth
I can only say, Oh my goodness! WOW,WOW,WOW!!!!!!
so much, I'll send u a picture of the beautiful DCG that I got yesterday.
while I was in 1989 satisfied to have removed a little of the monopoly power of Agfa-Gevaert as the supplier of red sensitive
holographic emulsions, things have long since moved on a lot as we all know.
over to only working on silver halide based emulsions and because DCG was relatively so photographically slow, I put it on
the back burner. So M's and others and correspondences
about 12 years after I last did any work on it was a surprise.
(What follows was nearly all written in1989 for DIY holographers at the time.)
to make clear and very bright DCG holograms with a HeNe.
(633nm) by Jeff Blyth
private research of mine has uncovered a readily available chemical which enables one to obtain a dazzling bright reflection
hologram of an object of say 1 inch in diameter after an exposure of only 1minute from a 10 mw. HeNe. (i.e. with the beam
spread to about 4 cm.diameter to give even illumination of object.).
This opens up the way for holographers to produce their own holographic plates and get brighter single-beam results
than they could get by purchasing expensive silver halide plates. It also enables them to coat curved surfaces such as wine
glasses. The recipe below really does work and can be great fun.
Unlike conventional DCG recording material,
this one remains sensitive for weeks at room temperature and yet it contains
only a small fraction of the quantity of dichromate normally used. The approach
taken in the instructions below is to enable one to produce remarkably good coatings at minimal cost. There is however one
item that would be very helpful. Control of the pH to a value of about 9 is really important in this system and therefore
a pH meter would be much easier than using pHpapers.
for preparing DCG to record with a HeNe laser
following instructions will produce approximately 100 ml of material. The new discovery centres on 1,1,3,3- tetramethylguanidine ( TMG ).(Aldrich cat. no. 24176-8.). This
is a considerable advance over other similar systems  possibly because this molecule exists as a number of resonance structures
in dynamic equilibrium.[1 ]
TMG is neutralized with acetic acid and added to the gelatin formulation it is a reasonably innocuous substance. However,
the pure chemical is a volatile , smelly and very alkaline liquid which should not be inhaled and eye protection should be worn when using it.
Preparation of stock solution
of 25% v/v TMG ( 1,1,3,3-tetramethylguanidine):
50 cc.Deionized water: add until total volume reaches 200 cc.
Preparation of stock solution of TMG acetate.
stock solution of TMG : 80 cc.
acetic acid 8 cc. (approx). This quantity should be added very slowly while stirring and adjusted if necessary
to give pH of roughly 7. (A bit of fuming occurs here as the two vapours combine.)
Preparation of stock solution of 5% w/v potassium chromate
chromate : 5.0 gmade up to 100 cc. with deionized water.
of stock 0.4% methylene blue solution
blue : 2.0 g.Deionized water: 500 cc.Any undissolved impurity can be filtered out.
of coating solution.
de-ionized water 80 cc.
Gelatin 12 g. (A good quality cow gelatin rather than pig gelatin)(It needs to have the lower
"Iso-electric point" of cow gelatin.) However,
good catering gelatin has given excellent results.
container in a water-bath and heat up contents to between 45 and 50 C. while mechanically stirring slowly. This may require
20minutes or more. Check that no undissolved gelatin particles remain.
the solution through a nylon mesh, eg. a stocking. Maintaining the temperature
between 40 and 45C, continue as follows:
5% potassium chromate solution : 1.0 cc.
solution TMG acetate : 6 cc. **
that this quantity should produce an orange replay color in reflection holograms
made with HeNe laser light, at an ambient relative humidity of 60%. To obtain a green replay color this quantity should be
increased to 8 cc. This will also have the advantage of increasing the light sensitivity but the disadvantage of making the
surface rather tacky because TMG is hygroscopic.
From this point on, before adding the methylene blue, it is VERY IMPORTANT to maintain
the pH between 9.0 and 9.5.
adjustment is carried out by very carefully adding stock 25% TMG solution dropwise while the solution is well stirred. If
the pH rises above 9.5 it should be brought back at once by a drop of acetic acid. It is not possible to give a precise value
of the quantities of either of these reagents at this point because it will depend on the batch of gelatin being used. I found
that it took 1.8 cc. of stock 25% TMG to get pH 9.1. It is a big advantage to use a pH meter rather than pH paper strips.[A useful calibrating solution is 3.80
g. borax crystals in a litre of distilled water. This has a pH value of 9.2 at room temperature.]
Finally, under green or dim yellow safe-light:
6 cc. stock methylene blue solution. Note that this quantity is for a coating thickness between
4 and 5 microns. If you are going to double the thickness of coating by using methods other than the simple one described
below, then the methylene blue quantity might need to be reduced. This is because the dye in the dried coating needs to be
low enough to allow as much light as possible through to the object. However if you do not see any sign of a blue coloration
when your dried, unexposed film is resting on white paper, then that probably means you have let the pH drop too low and it
will have no sensitivity to red light. [
In my earliest formulation I used ammonium dichromate, but found that it caused a wide variation in the pH of the coating
depending on its state of dryness and how much free ammonia had escaped from that salt. Methylene blue forms a useless precipitate
with dichromate ions unless the pH is above about 8.5. Accordingly this problem
was eliminated by using the alkaline salt potassium chromate instead
of ammonium dichromate.]
formulation can remain active for several weeks at room temperature although it is best to store it in a cold place, but not
actually allowed to freeze.
coating method. "Veil Coating" for
a 300mm x 400mm glass plate
plate should be very clean eg. by leaving overnight in neat domestic bleach solution.
Then well rinsed in de-ionized water.
Thankfully a silane pre-subbing is not essential unlike the case of silver halide gelatin work.)
green safelighting (Can be much brighter than would have to be used for silver halide holographic recording).
Warm the solution to between 40 and 45 C. in a beaker with a good spout. Do
Hold a clean and warm glass plate at an angle of about 30 degrees to the vertical with its base resting in a flat and clean tray.
Position the spout of beaker at a point about 1 cm. from the top of the
glass and 1 cm from the edge closest to you .
Start to pour out the solution at a constant rate while moving your arm at a constant rate a centimeter from the top of the
plate. The liquid should fall like an evenly spread veil on that side of the glass.
After the plate is covered, allow it to gel in this position for a few minutes. ( The balance between gravitational and surface tension forces can give a surprisingly even coating.)
After the coating has gelled, the thick layer at the bottom edge of plate
can be either wiped off or removed by dipping the bottom edge in warm water.
Pour the tray contents back into the beaker.
The plate is now best left to dry for at least 24 hours in a darkened room before cutting it up into reasonably sized pieces.
4cm squares. if you only have a 4 mw laser.
make a bright Denisyuk hologram of an object of one inch in diameter may need
about 1 min. exposure to a 10 mw. HeNe laser with beam expanded just sufficiently
to give a reasonably uniform illumination of object. Firstly, a trial-and-error test on exposure levels is essential. A large
bright coin is the standard test object.
initial trials should preferably contain a range of exposure levels on one plate
by masking off the spread laser beam for various times . Please remember that doubling the beam diameter may increase the exposure time needed by a factor of 4 and not 2.
biggest cause of failure in new DCG work is either insufficient exposure time or insufficient settling time to allow micromovements
of the gelatin to cease.
policy was always to set it up and to go away and do something else for an hour or three.! )
Processing. It is very helpful to keep the
following mechanical model in mind. If one hangs weights on to the end of a steel extension spring, then removing those weights
will result in the spring recovering its original un-extended length unless the weight exceeds a critical value. When a weight
is of that critical value, the material of the spring has reached just beyond what is known as the "yield point". When that
weight is removed the spring would normally never completely recover its original un-extended length but will remain permanently
distorted. Had the material of the spring been a little stiffer, then that particular weight would not have stretched the
spring beyond its yield point. If the temperature of the stiffer spring were to be raised however, then that weight may again
take the spring beyond its yieldpoint.
in water behaves similarly to the stretched spring. At a certain temperature, the stretching forces due to swelling in water
may take the gelatin beyond its yield point. Had the water temperature been lower, then the gelatin would not have reached
its yield point.
art of making the most noise-free and yet bright DCG holograms, is to take the gelatin as close as possible to its yield point
without exceeding it. Stretching gelatin beyond its yield point creates noise in the finished hologram. Using the onion layer
analogy of Curran and Shankoff, the holographic fringe layers might resemble dried out onion with crinkled-up layers if the
yield point is exceeded. A sign that you have got the swelling just right is
no milkiness in the exposed area but milkiness in the unexposed area.
A Simple Processing Method
(It is strongly recommended that this simple method is tried before other methods you
may know of.)
meaningful tests should be tried until the coating is at least 24 hours old.
first trial hologram is best processed so that the yield point of the gelatin is just exceeded, and some noise or scatter
is generated deliberately in the finished hologram. This is necessary to be sure that no sign of a hologram after processing,
is not due to insufficient swelling in the first water bath and therefore must be due to either relative movement between
object and holographic plate or insufficient exposure. Once one has established the necessary exposure level then one can
produce holograms devoid of any noise by processing at a level which takes the gelatin just below its yield point.
exposure, the sample should be left for a few minutes before processing.
Thoroughly rinse hologram in COLD running tap water for several
note written in 2002:-
is essential to wash out all traces of that TMG compound to stop it getting into your alcohol baths where it would finish
up attracting in water into the finished holograms and this will fatally affect their long term survival.
The plate is then swollen in water at a temperature of 26 deg.C for about
Shake off excess water droplets and plunge plate into cold iso-propyl alcohol.
Agitate vigorously for at least 1 minute. (The water content of this bath
can be allowed to be anything from zero to 10%.)
Put plate into another iso-propyl alcohol bath but this time insure
that its water content is less than 1%.
for 2 minutes at least. (Insufficiently dry alcohol in this bath will result
in drying marks which replay in the blue.) It must be said that particularly
good results and rapid image-appearance come from having this bath rather warm, but there are fire safety risks involved and
a tepid bath can work very well.
Blow alcohol off the plate with a strong blast of tepid air.
inhalation of the alcohol vapour is harmful and hence the drying process should be carried out with air extraction. The second
alcohol bath should be protected from ambient air as soon as possible as it rapidly absorbs moisture.
image may be visible until all the alcohol has escaped from within the fringes; a process helped by leaving the sample in
a warm, dry place.
the finished result is completely clear and shows neither scatter nor hologram, then the chances are that the fringe voids
or cracks have not formed properly in the alcohol bath by the "Shankoff effect" and the hologram simply needs reprocessing
using water at a higher temperature in the first bath. If the hologram is noisy
or "milky" then that means that the gelatin has had its yield point exceeded and nothing can be done about this noise except
to shoot another plate and process it in water at say 18 deg.C so that the yield point is not exceeded. Plates which are a
month old and have been left at room temperature, may need a water temperature
of about 35 deg. C.
the water temperature just right, can produce the most noiseless and bright single-beam holograms.
holograms should be thoroughly dried for several hours at around 60C. in a dry place space. Once treated like this, they seem
able to withstand ambient relative humidity below 70% indefinitely. (I used to leave my holograms for several days in a laboratory
desiccator containing violet crystals of freshly roasted self indicating silica gel). One should then seal them if they are
going to be carried around, otherwise a cold hologram suddenly brought into a warm room can create surface moisture and cause
the notorious disappearing trick of DCG holograms. (They can be reprocessed in water and alcohol if that happens.) Note that
sprays should not be generally used as sealants since they can penetrate into the fringes.
Viscous UV curing resin and a glass backing sheet seems to work well.
I am very grateful for the help of George Clare who made many independent tests of formulations.
(1) J.Blyth. Methylene Blue sensitized dichromated gelatine
holograms: a new electron donor for their improved photosensitivity Appl. Optics 30,
R. Changkakoti., S.S.C.Babu, and S.V.Pappu,
"Role of External Electron Donor in Methylene Blue Sensitized Dichromated Gelatin Holograms" Appl. Optics 27, 324
(3) R. Curran and
T. Shankoff, "The Mechanism of Hologram Formation in Dichromated Gelatin." App. Opt. 9, p.1651 (1970).