opaque backing for DCG holograms

[Joe Farina]

I've noticed recently how DCG holograms can look better when placed against a dark background. I was thinking about adding a pigmented, opaque, dark color (maybe black) to the backside of the gelatin surface prior to sealing. It's been normal practice over the years to improve contrast with silver halide holograms by simply spraying them with black paint. But I would like to do some experiments with a "dry" coating of pigment, maybe something like a pastel rubbed over the gelatin. Since it will be sealed in glass later, it doesn't need any abrasion resistance. Has anyone tried an opaque backing layer with DCG that's sealed inside?

[BobH]

How about using a piece of black or smoked glass for the cap?

[Joe Farina]

That would be a good solution, but there are a couple problems. First, I would like both halves of the glass "sandwich" to be of the same type of glass. When different types of glass are cemented together, there's a good chance that the thermal expansion rates will be different. The more different they area, the more stress will be put on the adhesive holding them together. Also, I've found it very difficult to find black float glass which is very flat. There is a lot of black glass out there, but it seems to be used mostly for stained glass purposes, and the flatness isn't so good. Also, I want the backing plate to be 3/8 inch thick (with the hologram plate being 1/8 inch) and that thickness would be difficult to find (it may not even be available).

[holorefugee]

I was able to find scrap "architectural glass" that was black float glass made for some high rise installation in SF. I was never able to find the source just the scrap. It was quite thick like 5/16 or 3/8 but was broken and scratched so it was hard to find large pieces. I have some 4x5 pfg-01s I made with it and liked it.

[Tom B.]

Not sure why there would be any more problems with (non-water-based) black spray paint backing for DCGs than for silver halide, but you might try carbon soot from a candle or whatever. The deposited film is black as sin, very delicate but if encapsulated OK. I expect there are ways to produce good dirty black smoke reasonably free from bad impurities and also to reduce the heat stress on the gelatin during deposition. Could try it on blank glass plate before risking a hologram.

An adhesive black backing film, appropriately applied, could work just as well or better if the adhesive refractive index was reasonably close to gelatin (likely). Better still if it was also free from nasty contaminants and also non-destructively removable after years of aging, Hard to test that sort of thing.

[Joe Farina]

Thanks Tom. I'm not sure about the effects of ordinary spray paint on DCG, it would need testing. I'm just cautious about proprietary products, it would be preferable to know what the paint contains. That's an interesting idea about the black candle soot, I thought about it also. I tested a very soft black pastel by rubbing it against the gelatin. This didn't work at all, since the smooth gelatin has absolutely no "tooth" to hold the pastel pigment.

I have some old holograms on Agfa film that I used for tests (I don't have any DCG right now). I added some dry pigment (ivory black) to 90% IPA, and ground this in a mortar and pestle to make a "paint" (no binder however) and then airbrushed this on the gelatin. This works well. However it remains to be seen what happens with DCG, whether or not the alcohol will have any effect on the hologram when this solution is airbrushed on it. I might want to keep the hologram warm or hot while airbrushing the mixture, so that the alcohol evaporates very quickly. A little shellac binder may help (in very small amounts). It needs testing though. One benefit of this technique is that the pigment component is clearly known (desirable from a permanence standpoint). Also, any dark color can be applied, not just black.