pH of water affects noise?

Dichromated Gelatin.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

I was reading a paper on DCG by McGrew

http://www.nli-ltd.com/publications/color_control.php

which includes the following:

"The factors influencing emulsion noise are listed in order of decreasing importance:

1. gelatin hardness
2. processing temperatures
3. pH of baths containing water"

I was wondering if anyone can shed light on #3 (pH of baths containing water). I have seen the effects of 1 and 2 myself, but I don't remember anything in the literature about the pH of baths containing water (contributing to noise levels).
Dinesh

pH of water affects noise?

Post by Dinesh »

In solution the chromate and dichromate ions are in equilibrium. Thus:

2(H[2]CrO[4]) --> ( H )+ (HCrO[4]-)
(HCrO[4]-) -->( H) + (CrO[4]-)

The hardening of the gelatin depends on the HCrO[4]- ion, so you end up hardening the gelatin, which decreases speed and contrast, ie the planes become non-linear because they can't follow the sinusoidal variations rapidly enough. This non-linear response to higher pH may be seen as noise.

Adding alkali produces:

(Cr[2]O[7]--) + (2OH-) --> (2CrO[4]--) + (H[2]O)

That is, it converts the (useable) dichromate into the chromate and so renders the dichromate inert.

Having said this, this is a small effect in relation to all the other effects, because it depends very much on the type and bloom strength of the gelatin. If the gelatin were exceptionally hard or exceptionally soft, the effect may be significant, but not otherwise. One reason McGrew may have seen noise is that silver halide emulsion is considerably harder than "home-made" dcg emulsions. This extra hardness promotes noise because of higher scattering.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

Thank you Dinesh.

I'm using a bovine photo-grade gelatin with a bloom of ~250 (from Photograper's Formulary, supposedly a Kodak product). So I guess this would qualify as hard but not "very" (i.e. 300) hard. For that kind of gelatin, do you think lower-pH, water-containing baths would help reduce noise? I suppose acetic acid could be added to the fixer (haven't checked the pH on this) and to the water (swelling) bath, though I don't know offhand how this would affect the fixer or swelling bath performance.

Finally I'm getting back into DCG (dye-sensitized) and looking into noise sources and how to reduce them.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

After reading more of McGrew's paper (which I should have done before, instead of skipping around) it seems the question of pH affecting noise in DCG relates to gelatin solubility.

McGrew makes the statement:

"5. The solubility of gelatin in water increases as the alkalinity of the water increases."

later on he says:

"Depending on pH, temperature, and hardness, some gelatin will be dissolved at all stages of processing, primarily at the surface and internally in a pattern corresponding to exposure." (By "a pattern corresponding to exposure," I assume he means the unexposed areas will dissolve out more.)

Rallison makes the following comment in "Phase Materials for HOE Applications" (I printed a hard copy of this from the Ralcon site in '98, I don't know if it's still available online):

"When the material (DCG) is used without much hardening it produces hazy holograms that exhibit broad spectral and angular bands but as it is hardened it also narrows and at some point it crosses into the no scatter zone quite suddenly, with no attendant change in other properties. This point is where even unexposed gelatin can no longer be dissolved out with warm water, leaving scattering centers behind.

So the way Rallison describes it, the "dissolving out" of gelatin causes noise (scatter, milkniness, fog, haze, etc. could be other words for it, I guess).
Dinesh

pH of water affects noise?

Post by Dinesh »

Not true, at least, not completely true. It may be true, sort of, in a display hologram.
Joe Farina wrote:"Depending on pH, temperature, and hardness, some gelatin will be dissolved at all stages of processing, primarily at the surface and internally in a pattern corresponding to exposure." (By "a pattern corresponding to exposure," I assume he means the unexposed areas will dissolve out more.)
The gelatin in the "unexposed areas" does not dissolve (how hot were their development liquids anyway. Gelatin usually dissolves around 30 deg C in the free state. Trapped inside a multiple density matrix, it'd be quite a bit higher!)). If it did, then the Bragg structure would collapse. Besides which, there are no "unexposed areas". The popularised books always show the Bragg planes as distinct planes, whereas they're sinusoidal variations in density. Thus, there is a continuity of "unexposed areas" based on the value of the sin function. If you take an arbitrary point on the sin function, say phi, for example phi = (arcsin)(pi/6) (where the value of the sin function is 0.5), then the ratio of the "unexposed areas" to the "exposed areas", R, is

R = (2* <int>{sin(theta)} [lim 0 -> phi])/(<int>{sin(theta)} [lim 0 - > pi)
<int> stands for integral.

In other words, you integrate the sin function from 0 to phi and double (to catch both lobes of the sin) and divide by the same integral of the sin function from 0 to pi. This would then be the ratio of areas of "unexposed areas" to "exposed areas". But, even now, you've arbitrarily determined areas of lower exposure (still not zero) as opposed to areas of higher areas.Actually, you'd have to carry out this analysis for a sin^2 function, since you cannot record the negative areas of the sin function. In a real hologram, the sinusoidal functions of many thousand of rays are recorded in every possible direction from 0 to 2pi. So, you'd have to replace the (rather simple!) sin function in the above integral by the two-dimensional Fourier Transform of the object beam, appropriately truncated to a solid angle subtended by the plate and the object. This means that there will probably be no real "unexposed areas" since what is "unexposed" for one set of sinusoids will be "exposed" for another set. Remember also that the gamma of the material will "flatten out" the sin^2 function, so additional unwanted terms will appear.
Joe Farina wrote:but as it is hardened it also narrows and at some point it crosses into the no scatter zone quite suddenly
It's not "sudden". The reason it may seem "sudden" is that at some point the bandwidth crosses the photopic threshhold and so there is an apparently abrupt change. For example, when you look at the sunset, it appears there is a clear demarcation in the colours of the sky. If, however, you pointed a spectrophotometer at the sunset, you'll not notice any sudden change in the spectrum, the spectrum of the sky will be uniform. If the colours did change suddenly, it'd mean a sudden change of index at the demarcation points, which would give multiple reflections at distances of shorter than lambda/4; you'd effectively see a number of images of the sun next to each other.

What both Rallison and McGrew are noticing is that softer gelatin produces wideband holograms. These wide band holograms give a "soft" muted look to the colours, as opposed to the more narrow band silver halide imagery. In addition, the "flattening" effect of the sinusoid causes additional Fourier terms, which, in turn, cause spurious gratings and so appear to cause noise. However, this flattening of the sinusoid is very much a combination of exposure and development, not the gelatin itself. it also happens, to a lesser extent in silver If you put one of these apparently "noisier" holograms into a spectrophotometer, you'll notice no sudden changes and no spectral peaks off the main image peak. However, I very much doubt that either Rallison or McGrew ever put one in a spectrophotomer.
Joe Farina wrote:at some point it crosses into the no scatter zone quite suddenly
There is no "no scatter zone". Scatter is the spurious reflection of light from random elements in the gelatin matrix. If the elements exist and are larger than lambda/4, they will scatter as per Rayleigh's law (although I think it's possible to make an argument that scatter in silver is a Rayleigh effect and scatter in dcg is a Mie effect).

By the way, apropos of nothing in particular (except perhaps the "expert syndrome effect"!) I was with a very prominent holographer who told me of light bulbs that apparently gave off the same spectrum as the sun. These bulbs were quite expensive because they were "full spectrum" (whatever that means!). At any rate, I told him that this was impossible. He however insisted and backed up the contention with an impressive list of other experts who apparently also believed in the "full spectrum" light bulbs. "Easily settled", I said, "you have a spectrphotometer in you lab. The spectrum of the sun follows the photopic curve pretty closely. Let's see if the spectrum of this bulb also follows the photopic" Well, it didn't, much to the "expert holographer's" amazement!
Tony

pH of water affects noise?

Post by Tony »

For whatever it's worth I tried some quick in dirty pH test on a batch of film.
There were so many variables within variables but I simply tried two different pH rinse waters (same temp and time)
The DI water was like pH= 7 and my tap water was like pH= 6.5.

Exposed a plate, cut it in two and fix time and IPA time was the same.
I could not see any difference so I did not expand the test.

I e-mailed Steve McGrew a long time ago asking him about this. His answer was basically try it. It was not being smug with his answer he was just stating that there are too many variables to make a definitive answer. Within your set up (i.e gel, thickness, hardness etc) it may or may not be a factor.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

Thanks for doing that test. My water is also around pH 7. I have heard that gelatin can become damaged at higher pH levels (above 10 or so), so maybe that was the gist of what McGrew was saying. Dinesh, thanks for your comments.
Dinesh

pH of water affects noise?

Post by Dinesh »

I think I may have been a little confused here. You're asking about the behaviour of the gelatin when you process in the water with high or low pH, as opposed to making the emulsion with gelatin that has a high or low pH?

Well, off the top of my head, it seems that when the gelatin emulsin hits the water, the OH_ or H+ ions, depending on the pH, absorbs into the gelatin, while some of the gelatin dissolves into the processing water. The chemists may have to confirm this, but I believe that alkali solutions dissolve gelatin; I immerse my old plates in a sodium hydroxide solution to strip away the gelatin. If so, then the OH ions entering the gelatin will dissolve small areas inside the gelatin and so create pockets of dissolved gelatin, which may have a different density that the surrounding gelatin. Depending on the size and index difference, these pockets - "micro-bubbles" - will become scattering centres. Now assuming that these scattering centres are uniformly distributed inside the emulsion, it may be possible to determine the S/N due to pH in the water. If there are N such scattering centres/unit volume, then when the hologram is illuminated, there will be NAd scattering centers in the beam profile, where A is the area of illumination and d the emulsion thickness. If each if these centres scatters the incoming radiation by alpha, then if the illuminating beam has an intensity I, the amount of scattered radiation is:
N = (alpha*I*NAd) {alpha will, of course be lambda dependant}

The efficiency of a volume hologram is
eta = exp(-ad/cos(theta))*{sin^2((pi*(delta n)*d)/((lambda)*(cos theta))},
Thus:
S/N ={ (alpha*I*NAd) }/{ exp(-ad/cos(theta))*{sin^2((pi*(delta n)*d)/((lambda)*(cos theta))}

The combination of the exponential function in the denominator and the N in the numerator would seem to indicate that the thicker the emulsion, the lower the noise due to pH. This is just a back-of-an-envelope off-the-top-of-my-head calculation. To be more precise, you'd need the statistical distribution of the scattering centres, their wavelength dependance and the variation of density of these centres.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

Dinesh wrote:You're asking about the behaviour of the gelatin when you process in the water with high or low pH
Yes. At this point, I'm guessing that water-containing baths wouldn't become a problem until pH ~10 or 11. Probably not anything to be concerned about.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

pH of water affects noise?

Post by Joe Farina »

I've been doing a little more digging on the subject of how the pH of water solutions might affect the noise level of some DCG holograms.

An old paper by Sjolinder (Swelling of Dichromated Gelatin Film, Photographic Science and Engineering, Vol. 28, No. 5, 1984) gives a graph of the swelling of an exposed DCG layer, depending on pH. The graph shows the swelling is about the same throughout the range of about 4 to 10. (Lower or higher pH levels causing sharp increases in swelling.)

However, it's been said many times that swelling is at a minimum at the isoelectric point of the gelatin, which is about pH 5 for Type B.

This morning I found the following graph, showing quite a different picture from what Sjolinder indicated:

http://archives.pia.gov.ph/seapavaa/Fil ... elatin.htm

It seems that there are instances (when swelling is not desired in a particular part of a DCG process) that the isoelectric point should be kept in mind, as well as the pH of the given bath or solution. I think uncontrolled swelling at a certain point in the DCG processing is the source of "noise" that I have seen noted occasionally in connection with pH. The interference planes are getting violently pushed around with extreme or non-controlled swelling.

This seems to be confirmed in a Russian paper I have: Unique features of holograms recorded on a layer of hardened bichromated gelatin, developed by buffer solutions (1987). Unfortunately, no online abstract is available for this paper, only the following:

TOPUNOVA, MK, NA VASILEVA, AM KURSAKOVA, TN PARAMONOVA, and LV SHAROVA. "UNIQUE FEATURES OF HOLOGRAMS RECORDED ON A LAYER OF HARDENED BICHROMATED GELATIN, DEVELOPED BY BUFFER SOLUTIONS." SOVIET JOURNAL OF OPTICAL TECHNOLOGY 54, no. 8 (1987): 496-497.
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