Hardening Holograms to Fix the Color
The color of a hologram can change as the humidity changes. Efforts have been made to epoxy glass plates to the back, glue Oracal to the back and coat the back with cyanoacrylate glues. Jeff Blyth offers a chemical method:
One way to stop something breathing is to strap it up tightly. The easiest way to do this is to strap it up with chromium oxygen bonds. Inevitably strapping up leads to contraction. A bonus is that using dichromate can lead to a more hydrophobic gelatin. To get a longer replay wavelength than the 532nm pulse I suppose this is a bit awkward. Shooting the plates in a very dry state helps. The processing chemistry can also help but I appreciate that this must not reduce brightness.
The answer I think is to thoroughly post harden the gelatin for ever. So put a test hologram in a solution of ~5% ammonium dichromate, squeegee off excess drips, blow dry it ,and leave it out in sunshine all day. If you leave it like this without rinsing it will be extremely resistant to any future print out but it might be prudent to rinse out any soluble salts. This post hardening procedure will cause some final but permanent contraction so you would just need to do tests to compensate by adjusting the swelling state for your initial laser exposure of course. My tests on this method allowed the pictures to be viewed in or out of water. So your customers could then view the holograms underwater in their swimming pools !
Yes I am glad to report a successful experiment last night that left sunlight completely out of it.
I took 2 bright bleached Denisyuk holograms, one on a BB640 plate and the other on a Slavich PFG-03M The plates were first dipped vertically into a solution of 7% ammonium dichromate in DI water so that they were only partially covered . After about 2 minutes they were wiped and the dipping solution had 1 % glycerol added to the 7% ammonium dichromate so that other sections of the plates were then treated with this solution. Finally a fresh 7% ammonium dichromate solution was prepared with 2% conc. sulphuric acid.
So I finished up with 4 striped sections on a plate.
1) Untreated 2) 7% Ammonium dichromate in DI 3) 7% Ammonium dichromate in DI + 1% glycerol 4) Ammonium dichromate in DI + 2% conc sulfuric acid.
Plates were blown in cool air till touch dry and then placed in a preheated oven at 110 C for 12 hours.
Then plates were both cooled and washed in cold running tap water for 2 minutes and then dried.
Results: Dry plates Stripes 1,2, and 3 showed bright images with no indication of loss of brightness except perhaps stripe (3) may have lost a little. Stripe (4) was completely ruined and out of the running.
Plates soaked for 10 minutes in tap water at ~20C. Stripes 2 and 3 showed good clear images still. The BB640 plate had changed color from yellow green to light orange. The PFG-03M had changed hardly at all!... from light yellow to golden yellow. (In view of the notorious softness of this brand it was a surprise to find it had hardened up even better than the BB640 )
So this hardening method does effectively strap up the gelatin film so tightly that it cannot “breathe” and change its color with ambient humidity.
Note: This was done on bleached finished holograms. It cannot be used on unbleached reflection ones, the colloidal silver would have to be converted to AgBr first. Jeff