dave battin wrote:ok, i like the activity over this subject and i want to thank all involved with this thread.
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I have currently switched over to shooting DnS in 475nm, and i am amazed how little dichromate is needed, i took a portion of my original sensitizing solution (for 532nm) and have diluted it by 300% ! The film is a little low in sensitivity, so i probably diluted a bit to much.
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All is good, and I hope for more progress this weekend !
For those who have not seen this before , this spectrum (I made)(that Colin helpfully put in for me) shows just how feeble the absorption is at 532nm. compared to the absorption in the blue wavelengths. I choose to show this as an absorption graph rather than its upside down transmission graph version because after all it is the absorption by the dichromate molecules that has to do the work to crosslink the gelatin film. However it is the transmission aspect that affects how much light gets through the film to illuminate the object and then comes back through the film to form the interference or standing waves of light we refer to as fringes. So to compensate for that tiny absorption at 532nm we have talked about here of packing in as much dichromate as possible without causing crystallization and probably one of the best ways of doing this is to switch to using the incredibly high solubility sodium salt (provided it is reasonably free of traces of metals like copper which cause a rapid dark reaction.). However the signs are that the trouble Filipe was having at the start of this thread was that he had coated them too thickly. Suppose the above graph was given by a 10 micron thickness of film, then if it had been say a 1 mm thick film that would have meant the absorption would have increased by a factor of a hundred and even at 532nm this would have been too much loss of transmitted light to the object. We have then 2 big conflicts of interest here , Factor (1) To get as much light onto and back from the object, and Factor (2) to get as much absorption as possible to increase the rate of photochemical crosslinking of the gelatin by the dichromate molecules.
One can see from the graph that at 475nm there is so much more absorption compared to 532nm that I am surprised you Dave only had to dilute the dichromate to a third of what you use for 532nm. It looks as though it ought to be more dilute than that given that you had been pioneering ways to push AmDi concentration to its limits without crystallization occurring so that you could optimise Factor (2). But you say
the film is a little low in sensitivity, so i probably diluted a bit to much. .
Could it be that factor (1) is the problem?
Your photo’d samples look really good Dave. Were they shot at 475 nm? (I would point out to others that the skull objects Dave used were diffusing ones apparently and that is a real test of diffraction efficiency compared to using metallized objects because you lose so much effective light due to a non-metal object returning light that is randomly polarized unlike the case of metal objects which return pretty much light of the same polarization as the laser’s reference beam. In making initial tests with coins it is helpful to put an aspirin on the coin as a white diffuser. Only when you see the aspirin as a non-black shadow on the bright coin do you know that you really have got good diffraction efficiency.
Another point is that John thinks we are overdoing things with all this packing in so much dichromate for 532nm. Yes if one is using 514nm from an Argon laser like his he is probably quite right. If he is using his 488nm line he is even far more right! You can see from graph a significant increase in the absorption compared to 532nm and that improves the photosensitivity for Factor 2 and one can over do things more easily for Factor 1.
Jeff
