by Dinesh » Thu Jul 05, 2012 11:11 am
holomaker wrote:Not to cause confusion to this subject, but during this past week @ Isdh, i had a long conversation with Auggie about this very same subject and what i got out of it was in practiaity thickness dose not play a role in narrowbanding only fringe spacing, this was my original observation and now reconfermed by August Muth ...
Actually it is fairly confusing because there's no single beast in dcg as there is in silver. Dcg has such a vast dynamic range and so many parameters that you can pretty well place whatever characteristic on it, regardless of any other characteristic! You have the relative concentration of the dichromate/gelatin, you have the type of gelatin, you have the drying time, the drying humidity (what was the humidity while the plates were drying?), the dark reaction rate (dependant on, amongst other things, the temperature), the pH etc etc etc. This is before you've actually shot anything. Once the film has been exposed the variables mount: the exposure, the wavelength, the delta n/delta rho factor (rho being the density), the temperature and humidity of the lab while the exposure is happenning etc etc. Then you process: now there's the amount of pre-hardening, the actual pre-hardening agent, the temps and humidity of the processing room etc etc. So, the actual thickness of the film is one amongst many different parameters, depending on what you intend to do. In our case, for example, our emulsions are pretty standard at about 12 microns. However, for display we tune them pretty broad band and they turn out a bright orange (the broader the bandwidth, the brighter the hologram for obvious reasons) but we've tuned them to an off-white colour. However, for tech work, I've got them as narrow as 5nm and as broad as 30nm all across the spectrum - all with the same thickness. In other words, we've got a range of about 10nm bandwidth to a 30 nm bandwidth at every wavelength range from 780,to 450. We've even got multiple reflection gratings at various parts of the spectrum at various bandwidths; say the customer wants the following: three bandwidths of 10nm centered at 630, 20nm centered at 530 and 25 nm centered at 480, all reflecting at different, but given, angles. We can do this on the same 12nm thick emulsion (While these wavelengths and bandwidths are not what we actually did - can't give specifics of a customers spec for obvious reasons - these are pretty close to what we already did). The point here is that, as I mentioed, the thickness is but one of several parameters and, for dcg, you may need to manipulate all those parameters.
Augie only does mainly broadband display stuff. Insofar as that is concerned, he's right, thickness doesn't matter.
Joe Farina wrote:Don't know why the noise would be reduced however.
Because noise is spurious light at random directions. As the noise propagates through the emulsion, it impinges on the various Bragg planes and, since noise is usually off-Bragg, it scatters within the emulsion. What does not scatter within the emulsion exits the emulsion and is seen as noise. If the emulsion is thick enough, none of the random, spurious light makes it out of the emulsion. The Bragg planes act as a sort of low frequency filter.
[quote="holomaker"]Not to cause confusion to this subject, but during this past week @ Isdh, i had a long conversation with Auggie about this very same subject and what i got out of it was in practiaity thickness dose not play a role in narrowbanding only fringe spacing, this was my original observation and now reconfermed by August Muth ...[/quote]
Actually it is fairly confusing because there's no single beast in dcg as there is in silver. Dcg has such a vast dynamic range and so many parameters that you can pretty well place whatever characteristic on it, regardless of any other characteristic! You have the relative concentration of the dichromate/gelatin, you have the type of gelatin, you have the drying time, the drying humidity (what was the humidity while the plates were drying?), the dark reaction rate (dependant on, amongst other things, the temperature), the pH etc etc etc. This is before you've actually shot anything. Once the film has been exposed the variables mount: the exposure, the wavelength, the delta n/delta rho factor (rho being the density), the temperature and humidity of the lab while the exposure is happenning etc etc. Then you process: now there's the amount of pre-hardening, the actual pre-hardening agent, the temps and humidity of the processing room etc etc. So, the actual thickness of the film is one amongst many different parameters, depending on what you intend to do. In our case, for example, our emulsions are pretty standard at about 12 microns. However, for display we tune them pretty broad band and they turn out a bright orange (the broader the bandwidth, the brighter the hologram for obvious reasons) but we've tuned them to an off-white colour. However, for tech work, I've got them as narrow as 5nm and as broad as 30nm all across the spectrum - all with the same thickness. In other words, we've got a range of about 10nm bandwidth to a 30 nm bandwidth at every wavelength range from 780,to 450. We've even got multiple reflection gratings at various parts of the spectrum at various bandwidths; say the customer wants the following: three bandwidths of 10nm centered at 630, 20nm centered at 530 and 25 nm centered at 480, all reflecting at different, but given, angles. We can do this on the same 12nm thick emulsion (While these wavelengths and bandwidths are not what we actually did - can't give specifics of a customers spec for obvious reasons - these are pretty close to what we already did). The point here is that, as I mentioed, the thickness is but one of several parameters and, for dcg, you may need to manipulate all those parameters.
Augie only does mainly broadband display stuff. Insofar as that is concerned, he's right, thickness doesn't matter.
[quote="Joe Farina"]Don't know why the noise would be reduced however.[/quote]
Because noise is spurious light at random directions. As the noise propagates through the emulsion, it impinges on the various Bragg planes and, since noise is usually off-Bragg, it scatters within the emulsion. What does not scatter within the emulsion exits the emulsion and is seen as noise. If the emulsion is thick enough, none of the random, spurious light makes it out of the emulsion. The Bragg planes act as a sort of low frequency filter.