emulsion thickness

Silverhalide Emulsions / Chemistry.
PRATHEEP
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emulsion thickness

Post by PRATHEEP »

how to find emulsion thickness? please suggest me a good low cost testing any..
PRATHEEP
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Din
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Re: emulsion thickness

Post by Din »

Scratch the emulsion off at some point. Make a mark on the base. Make a mark on the emulsion. Use a traveling microscope to focus on the mark on the base, then raise the microscope to focus on the mark on the emulsion. The traveling microscope has a graduation that shows you by how much the microscope has to rise. The microscope has to have a resolution < 1 u.
BobH
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Re: emulsion thickness

Post by BobH »

Din
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Re: emulsion thickness

Post by Din »

There have been attempts to measure emulsion thickness with interferometric methods, we tried it at POC in the 80's, and, more recently, a group working from the University of Arizona Optics Dept tried it. But, the method is ineffective for this application because the rms is too high.

However, it really doesn't matter, since the only reason to measure the emulsion thickness is to evaluate the efficiency, since the efficiency of a hologram is based on emulsion thickness and other scalar factors (not the polarisation, which is a vector, as R. Rallison has published, and some holographers believe so fervently that they attempt to silence genuine criticism based on the literature of the subject, like some politicians we all know!)

The efficiency of a phase reflective grating is (assuming on-Bragg reconstruction. For off Bragg reconstruction, the efficiency is attenuated by a factor ζ, which is the Bragg dephasing factor)

η = 1/(1 + sh²{√[ν²]}*

ν = i(π)(δn)(t)/{(λ)[√(c(r)*c(s)] : t = emulsion thickness, (δn) = index modulation, [(c(r)*c(s)] are slant factors for the gratings.

Despite the statements published by R. Rallison and believed by some holographers who seem not to know the difference between a vector and a scalar, this is a scalar expression. Anyway, assuming that the reconstruction is by means of coherent radiation, with a wavelength variability of δλ, the uncertainty in η, δη, is given by:

δη/η = √{(δt/t)² + (δn/n)² + (δλ/λ)²}

Now assuming that (δλ/λ) is known via the bandwidth of the coherent radiation, and that (δn/n) is small (roughly ~1%, an assumption of the Kogelnik theory), then it is easy to determine the effect of the rms on δη/η. The variability of the efficiency, as a ratio of the rms of the emulsion thickness is given by:

{(δη/η)/(δt/t)}² = {1 + (δt/t)² + (δλ/λ)²}²/(δt/t)²

But, that being said, most applications on holographic diffractive optics will not be greatly affected for a δη/η < 0.1. The more important factor is δn/n.

"Coupled Wave Theory for Thick Hologram Gratings", H. Kogelnik, The Bell System Technical Journal.
Joe Farina
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Re: emulsion thickness

Post by Joe Farina »

Since this was posted in the AgX section, I'm guessing the emulsion to be measured is rather thin compared to the DCG thickness I normally use. For my DCG (~20 microns), the following was useful:

https://www.harborfreight.com/digital-m ... micrometer

Maybe they have something similar in India. The claimed resolution is 0.0001 inch, or 2.5 microns. I've found it useful for some general and non-precise measurements of ~20 microns DCG. One would need to measure the thickness, scrape away the emulsion, measure again, and repeat a few times in other areas to get an average result.

I would like to try a traveling microscope, but don't have one. If a person has some microscopy skills, perhaps a strip of emulsion/substrate could be cut (say 5 or 10mm wide) and then stood, on end, under a microscope? I wonder how visible the emulsion thickness would be. If visible, one might clamp or glue a "reference material" (some type of plastic film with a known thickness) on top of the emulsion, then the reference material could be compared to the emulsion. Just a thought.
Din
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Re: emulsion thickness

Post by Din »

Joe Farina wrote: Sat Dec 04, 2021 3:27 pm Since this was posted in the AgX section, I'm guessing the emulsion to be measured is rather thin compared to the DCG thickness I normally use. For my DCG (~20 microns), the following was useful:

https://www.harborfreight.com/digital-m ... micrometer
Typically, AgX emulsions are ~ 8 u. The micrometer you link to states a resolution of 0.001 mm, or 1u. Thus, it can measure an 8u thickness, but, with a resolution of 1u, the error is 8 +/- 1 or 1/8, or 12.5%. This is too inaccurate for technical work, but is not a problem for display. Agx has typical efficiencies in the display area of ~25 - 30%, but the brightness is far more important. However, the brightness depends on the bandwidth of the spatial frequencies recorded, and, with only 8u, there is not enough width to support a large number of frequencies. Therefore, AgX is a narrow band material. DCG has a typical emulsion thickness of about 10 - 15u, and so has a greater ability to share different frequencies. Thus DCG has an increased bandwidth and hence is brighter for display work.

Joe Farina wrote: Sat Dec 04, 2021 3:27 pm I would like to try a traveling microscope, but don't have one. If a person has some microscopy skills, perhaps a strip of emulsion/substrate could be cut (say 5 or 10mm wide) and then stood, on end, under a microscope? I wonder how visible the emulsion thickness would be. If visible, one might clamp or glue a "reference material" (some type of plastic film with a known thickness) on top of the emulsion, then the reference material could be compared to the emulsion. Just a thought.
At POC, in 1987, we did what I suggested earlier. We scratched off the emulsion at some corner or edge, till we got down to the glass base. We then used a pen or marker to make a mark on the glass and focused the microscope on the mark. We put a similar mark on the top of the emulsion elsewhere on another edge or corner, placed this edge under the traveling microscope, and raised the microscope to focus on the surface mark. The graduated scale showed how much the microscope had to be raised. Obviously, it's important to keep the plate on the same base of the microscope. My colleague Chris Rich simply drew a line that stretched from the glass base to the emulsion, so the two marks were next to each other. If memory serves, the traveling microscope we used had a resolution of 0.5 u. We were going for emulsion thickness of about 60 u, so 60 +/- 0.5 was an acceptable error.

You may be able to get one at some university 'sales'. The University of California, San Diego (UCSD) has a so-called 'surplus sale', where equipment is bought for some project or study. Once the study is over, the equipment is put up for sale or auction. Here is the UCSD site, and you may be able to have them ship some item: https://surplus.ucsd.edu/. It's possible that other universities do the same. At any rate, we (Triple Take) got our traveling microscope from a UCSD surplus sale.
BobH
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Re: emulsion thickness

Post by BobH »

I'd remove some of the emulsion, clamp another plate of glass to the emulsion surface, and use the spectrometer to measure the air gap over the area removed.
Din
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Re: emulsion thickness

Post by Din »

BobH wrote: Sat Dec 04, 2021 4:33 pm I'd remove some of the emulsion, clamp another plate of glass to the emulsion surface, and use the spectrometer to measure the air gap over the area removed.
This method assumes an rms of zero, which it cannot be.
Martin
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Re: emulsion thickness

Post by Martin »

Din wrote: Sat Dec 04, 2021 4:04 pm
Typically, AgX emulsions are ~ 8 u. The micrometer you link to states a resolution of 0.001 mm, or 1u. Thus, it can measure an 8u thickness, but, with a resolution of 1u, the error is 8 +/- 1 or 1/8, or 12.5%. This is too inaccurate for technical work, but is not a problem for display. Agx has typical efficiencies in the display area of ~25 - 30%, but the brightness is far more important. However, the brightness depends on the bandwidth of the spatial frequencies recorded, and, with only 8u, there is not enough width to support a large number of frequencies. Therefore, AgX is a narrow band material. DCG has a typical emulsion thickness of about 10 - 15u, and so has a greater ability to share different frequencies. Thus DCG has an increased bandwidth and hence is brighter for display work.

[I thought common (holographic) wisdom suggested: the thinner the recording layer the broader the bandwidth of the reconstructed hologram. So if you aim at very narrow bandwidth, you'd need a thick (I think ~ 1 - 2um or so) recording medium. Volume gratings e.g. currently used to run compact ECL laser diodes at single frequency (see Ondax, now Coherent I believe) would come to my mind.

With commercial AgX emulsions (7-8 um) you can get large broadband reconstruction (~200nm) through colloidal processing - let alone SHSG.

As for DCG, FEG, diazo gelatin, monomer sensitized gelatin etc. bandwidth may not only depend on layer thickness but also on the specific processing scheme. My somewhat unqualified guess would be that if you have a large size distribution of scattering particles/voids, you'll get large bandwidths.
BobH
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Re: emulsion thickness

Post by BobH »

Din wrote: Sat Dec 04, 2021 5:57 pm
BobH wrote: Sat Dec 04, 2021 4:33 pm I'd remove some of the emulsion, clamp another plate of glass to the emulsion surface, and use the spectrometer to measure the air gap over the area removed.
This method assumes an rms of zero, which it cannot be.
Regardless, it will work fine as I used the technique for many years while at Digilens.
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