by Dinesh » Fri Feb 18, 2011 1:04 pm
Actually, one reason I don't like the term "print out" for dcg is that the opposite problem is more likely to occur. It's not that there's unexposed "stuff" in the emulsion, it's that local environments degrade the emulsion that have little to do with light levels. One of the big problems with dcg is water absorption and/or heat. The weather in San Diego is mild enough that 10 year old unlaminted holograms still look OK. But, holograms that were laminated exactly the same way disappeared within weeks in India, where local temperatures were around 100 or so and humidity was around 80%. Recently, we had a deluge here in Southern California and our lab humidity went up to 80% (it's normally between 35% and 55%). At this level, merely taking the exposed hologram to the laminating station two rooms away was enough to make the hologram disappear. Our experience in India changed our laminating habits so now we can take a hologram to such extreme climates and the holograms will survive (we've tested this for about a year and so far they're good). Generally, for display holograms, this is not always that big of a deal, the image is still visible, if a little green shifted. However, for technical optics, where the customer wants a fairly tight spec - say an efficiency of above 80% at some peak wavelength with a 20nm bandwidth - if the hologram swells just a little my measurements are invalid and the spec is no longer obeyed. Also, we have a set technique for getting these specs - such-and-such an efficiency at such-and-such a peak lambda with such-and-such a bandwidth requires us to shoot it with these parameters at these angles. However, as the film dries in the coating station, it also swells at these high humidities. The film was thicker than our standard thickness and all our strictly derived methods went out the window!
Also, silver holograms backed by some laminating material also went dark, not due to print out, but humidity. My theory, based on an actual experience some years back, is that after time the silver emulsion sticks to the backing. Under high humidity, the backing separates from the glass and takes the emulsion with it. We had one of Edwina's Bog Man holograms on film that's now disappeared. I'm trying to coax it back into existence with a little success. There's got to be something funny in that. A hologram of a 5000 year old mummy that was shot to help preserve the actual mummy is now in need of preservation itself (the hologram was made because the mummy itself was too delicate to be taken to the various universities for study and preservation)!
Actually, one reason I don't like the term "print out" for dcg is that the opposite problem is more likely to occur. It's not that there's unexposed "stuff" in the emulsion, it's that local environments degrade the emulsion that have little to do with light levels. One of the big problems with dcg is water absorption and/or heat. The weather in San Diego is mild enough that 10 year old unlaminted holograms still look OK. But, holograms that were laminated exactly the same way disappeared within weeks in India, where local temperatures were around 100 or so and humidity was around 80%. Recently, we had a deluge here in Southern California and our lab humidity went up to 80% (it's normally between 35% and 55%). At this level, merely taking the exposed hologram to the laminating station two rooms away was enough to make the hologram disappear. Our experience in India changed our laminating habits so now we can take a hologram to such extreme climates and the holograms will survive (we've tested this for about a year and so far they're good). Generally, for display holograms, this is not always that big of a deal, the image is still visible, if a little green shifted. However, for technical optics, where the customer wants a fairly tight spec - say an efficiency of above 80% at some peak wavelength with a 20nm bandwidth - if the hologram swells just a little my measurements are invalid and the spec is no longer obeyed. Also, we have a set technique for getting these specs - such-and-such an efficiency at such-and-such a peak lambda with such-and-such a bandwidth requires us to shoot it with these parameters at these angles. However, as the film dries in the coating station, it also swells at these high humidities. The film was thicker than our standard thickness and all our strictly derived methods went out the window!
Also, silver holograms backed by some laminating material also went dark, not due to print out, but humidity. My theory, based on an actual experience some years back, is that after time the silver emulsion sticks to the backing. Under high humidity, the backing separates from the glass and takes the emulsion with it. We had one of Edwina's Bog Man holograms on film that's now disappeared. I'm trying to coax it back into existence with a little success. There's got to be something funny in that. A hologram of a 5000 year old mummy that was shot to help preserve the actual mummy is now in need of preservation itself (the hologram was made because the mummy itself was too delicate to be taken to the various universities for study and preservation)!