What's the relationship between latent image sites and rate of reduction from a developer?

srb383 · Aug 22, 2023

I read somewhere that developers distinguish between exposed and unexposed silver halide crystals by a particular threshold determined by the amount of reduced silver ions in the crystal lattice structure. They particularly observed a threshold of around 3 - 5 reduced silver ions to silver atoms in a single latent image site, not for the entire crystal but rather 3 - 5 to be concentrated at a single site. Indicating the total number of reduced silver ions could be much higher for the entire crystal.

What I'm trying to better understand is the relationship between such a threshold and the rate of reduction from a developer. From what I understand, if a developer were to perfectly distinguish between exposed and unexposed silver halide crystals, it would be a binary system, however given that high densities can be achieved by leaving unexposed film in a developer for long enough, this would indicate there's a probability, a rate of reduction for all silver halide crystals regardless of exposure.

Firstly, if this "threshold" is met, does this indicate reduction of the entire crystal would take place given regular development conditions? A kind of "clipping point" for singular crystals. Where for crystals below this threshold, it would be a kind of broader spectrum of reduction, meaning they may only be partially reduced given their size and silver atoms present.

Kino · Aug 22, 2023

I am far from the person to intelligently respond to this inquiry, but have been around photographic endeavors long enough to hopefully be able to point you toward a book that may help. Apologies if you are familiar with the reference...

"The Theory of the Photographic Process" by C.E. Kenneth Mees

The Theory of the Photographic Process : C.E. Kenneth Mees : Free Download, Borrow, and Streaming : Internet Archive

1st edition

archive.org

Alan Johnson · Aug 22, 2023

https://www.researchgate.net/publication/285538827_FORMATION_NATURE_AND_ACTION_OF_SENSITIVITY_CENTERS_AND_LATENT_IMAGE_SPECKS

There does appear to be a certain grain size or "clipping point" as you call it below which grains do not grow.
However , I never got the full paper or followed up on later results.
See , eg, citation by Belloni, 2003.
Search "evolution of the concepts of photographic sensitivity"

ic-racer · Aug 22, 2023

Each individual grain is either black or colorless; the fraction of grains that can be developed, however, depends on the statistics of the incidence and absorption of random photons of light and the randomness of the efficiencies of individual grain for forming a latent image speck.

From The photographic latent image, L. M. Slifkin 1972

srb383 · Aug 23, 2023

Heaps of great resources shared here. Thank you.

Kino · Aug 23, 2023

Another good reference that is terribly hard to find is: "Modern photographic processing". Vols. 1 & 2 / By Grant Haist. TX 170-576 (1979) & TX 200-881 (1979).

You should try an InterLibrary Loan search at a major university or state library, otherwise the set is practically impossible to purchase at a reasonable price.

In a stroke of unbelievable luck, I stumbled onto a copy of the two volumes at an estate sale. It is a work that should be freely available to photographers, but the copyright (1988) is still well enforceable and maintained by a large publishing house, so scanning it and putting it online would no doubt result in legal action.

If you should run across an estate sale of an older photographer, go to the books FIRST and then look at the cameras! It's a lot harder to find some of these books than a good example of just about any camera...

Alan Johnson · Aug 23, 2023

Up till 1995, Tadaaki Tani: https://en.wikipedia.org/wiki/Latent_image

Photographic Sensitivity: Theory And Mechanisms [PDF] [4r9dt1kp5h60]

Photographic Sensitivity: Theory And Mechanisms [PDF] [4r9dt1kp5h60]. Notwithstanding the current excitement surrounding cutting-edge digital imaging techniques, photographic film still prov...

vdoc.pub

Mr Bill · Aug 23, 2023

I would say that Tadaaki Tani's book is probably about the best semi-current info (1995?) if you can follow it. (I don't, at least more than halfway at best.) (Fwiw hard for me to believe that the uploader really had permission. But whatever...)

I've heard him speak a couple of times at IS&T symposiums where he has been well-regarded as an authority. (Imo there is not much higher recognition than this.)

Mr Bill · Aug 23, 2023

srb383 said:
I read somewhere that developers distinguish between exposed and unexposed silver halide crystals by a particular threshold determined by the amount of reduced silver ions in the crystal lattice structure. They particularly observed a threshold of around 3 - 5 reduced silver ions to silver atoms in a single latent image site...

Hi, I would not normally want to weigh in on a thread like this as I don't really have a good understanding. But since I already sorta started... keep in mind that I'm not trained as a chemist, so I have a lot of holes in my understanding of this and there is no solid authority behind it - so probably better seen as perhaps "possible" explanations.

Regarding number of silver ions clumped in a crystal, there is a minimum number required to be "stable," meaning that this latent image won't be lost. (Now, it may not be "developable," only stable, meaning that a later "latensification" or random accident might render it developable.) The minimum number depends on how the crystal was sensitized (I think Tani gives some numbers comparing something like sulfur vs sulfur plus gold.) Again, don't trust my words, but rather only use them as possible ideas to research further.

I would GUESS that if there were a large crystal (AgX, silver halide) with multiple dislocations/kink sites these could get multiple silver ion "clumps" due to light exposure, and these might increase the statistical probability of development being initiated. Then once initiated the development tends to go until the entire crystal is fully converted. But... I would further guess that a certain proportion of the crystals are not fully reduced (to metallic silver) because thr process was interrupted or there possibly was not enough developing agent present at the crystal to complete (the developing agent, as I understand it, acts as an intermediary to pump electrons into the crystal, being supplied by other developer molecules). But in literature I think that the individual crystals are seen as either fully reduced or not.

So given as either developable vs not, crystals, I think your interests would be more along the line of what they call kinetics of development. There are certain classes of developers with so-called induction periods before significant development begins. My questionable understanding of this is that certain developing agents have a hard time approaching the individual crystal, perhaps being repelled by the charges of attracted bromide ions, or whatever (the so-called bromide ion shield, etc.). So there may be some measurable time period before enough developing agent can make its way through.

Now, some developers are known to have "high fog," loosely meaning that they are not as selective about which crystals they will develop. So this is another aspect to consider. In other cases the silver clumps may have formed beneath the surface of the grain and, as is, are not amenable to development; if sulfite ion is present in the developer this can help erode away the edges of the crystal and make development possible.

As a note earlier in this thread I did a search for kendall-pelz rules for developing agents, to see if anything useful showed up, and the first hit was within photrio, by Gerald Koch. I've not seen him post here for a long time but I believe he had an advanced degree, so you might look for his posts. Also posts by a guy named Ryuji (?) who did substantial research into the mechanisms of development. And of course the well-known PE (photo engineer) who is sadly no longer with us. Best of luck.

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Alan Johnson · Aug 24, 2023

As regards the rate of reduction by a developer, Tani p205 Fig 7.2 notes that in the cases studied this was found to correlate well with the potential for the one electron oxidation of the developing agents.

However, on p219 he reports that some workers have found that "photographic sensitivity did not depend upon the kind of developers when each was used under its optimum conditions"
At first sight this seems to contradict conventional belief, but it might just mean, eg, his optimal conditions involve adding an amount of sulfite to uncover latent image specks and are not the same as the optimal conditions for the practical use of the developing agent.

The different EIs given by different developers for the same film do not appear to be explained by the theories Tani discusses.

srb383 · Aug 25, 2023

Mr Bill said:
Hi, I would not normally want to weigh in on a thread like this as I don't really have a good understanding. But since I already sorta started... keep in mind that I'm not trained as a chemist, so I have a lot of holes in my understanding of this and there is no solid authority behind it - so probably better seen as perhaps "possible" explanations.

Regarding number of silver ions clumped in a crystal, there is a minimum number required to be "stable," meaning that this latent image won't be lost. (Now, it may not be "developable," only stable, meaning that a later "latensification" or random accident might render it developable.) The minimum number depends on how the crystal was sensitized (I think Tani gives some numbers comparing something like sulfur vs sulfur plus gold.) Again, don't trust my words, but rather only use them as possible ideas to research further.

I would GUESS that if there were a large crystal (AgX, silver halide) with multiple dislocations/kink sites these could get multiple silver ion "clumps" due to light exposure, and these might increase the statistical probability of development being initiated. Then once initiated the development tends to go until the entire crystal is fully converted. But... I would further guess that a certain proportion of the crystals are not fully reduced (to metallic silver) because thr process was interrupted or there possibly was not enough developing agent present at the crystal to complete (the developing agent, as I understand it, acts as an intermediary to pump electrons into the crystal, being supplied by other developer molecules). But in literature I think that the individual crystals are seen as either fully reduced or not.

So given as either developable vs not, crystals, I think your interests would be more along the line of what they call kinetics of development. There are certain classes of developers with so-called induction periods before significant development begins. My questionable understanding of this is that certain developing agents have a hard time approaching the individual crystal, perhaps being repelled by the charges of attracted bromide ions, or whatever (the so-called bromide ion shield, etc.). So there may be some measurable time period before enough developing agent can make its way through.

Now, some developers are known to have "high fog," loosely meaning that they are not as selective about which crystals they will develop. So this is another aspect to consider. In other cases the silver clumps may have formed beneath the surface of the grain and, as is, are not amenable to development; if sulfite ion is present in the developer this can help erode away the edges of the crystal and make development possible.

As a note earlier in this thread I did a search for kendall-pelz rules for developing agents, to see if anything useful showed up, and the first hit was within photrio, by Gerald Koch. I've not seen him post here for a long time but I believe he had an advanced degree, so you might look for his posts. Also posts by a guy named Ryuji (?) who did substantial research into the mechanisms of development. And of course the well-known PE (photo engineer) who is sadly no longer with us. Best of luck.

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Your assumptions seem quite sound. I had a quick skim through "Modern Photographic Processing Vol.1" which was recommended above. They presented quite high resolution photomicrographs of the reduction process of individual crystals. The reduced silver ions form more of a filament structure, even in the case of full reduction it never seems to perfectly fill the volume of the crystal. I would send images through but given the copyright of this book that probably wouldn't be a good idea.

Rudeofus · Aug 25, 2023

Alan Johnson said:
As regards the rate of reduction by a developer, Tani p205 Fig 7.2 notes that in the cases studied this was found to correlate well with the potential for the one electron oxidation of the developing agents.

However, on p219 he reports that some workers have found that "photographic sensitivity did not depend upon the kind of developers when each was used under its optimum conditions"

Previous posts in this thread have already established, that a certain number of metallic silver atoms need to be together to form a latent image center. Dependent on number of atoms a certain level of energy is needed to cover the whole process from "reduction of silver ion to silver" all the way to "new silver atom sticks to existing latent image center.

So in theory there should be a sharp cutoff level of energy, which determines whether a silver grain can be developed or not. Larger image specks would require less energy to begin with, so a weaker developer could still develop silver grains if they saw more light. In the same fashion each development agent could be activated to whatever level is needed to reach maximum speed (typically by raising pH).

However, in reality things are rarely binary: that silver speck can sit on different locations on the silver halide crystal, requiring different energy levels to develop silver. We also don't operate at zero Kelvin temperature, which means that not every developer molecule will have exactly the same level of energy. That's, where unsharpness of energy levels comes from, that's, why eventually even unexposed silver halide will be developed.

Alan Johnson · Aug 25, 2023

Yes, but so far [up to 1995] one thing has not been explained, dependence of effective film speed on developer,
eg, with a few dissenters, Xtol gives higher film speed than Rodinal.
So there are limits to what theory can explain and in all technical text and articles [I exclude those behind paywalls] the authors refrain from commenting on the dependence of effective film speed on developer.
This was largely left to careful practical observers like Crawley, widely quoted in The Film Developing Cookbook.

srb383 · Aug 25, 2023

This section seems to line up with what I've read from other studies:

"A photographic emulsion layer may be exposed and developed, then all but a single layer of grains next to the base washed off. By studying the remaining grains with a technique called residual grain analysis, described by G.C. Farnell and J.B. we can relate the proportion of grains developed for each size class of grains to the number of quanta absorbed. An analysis of this information reveals that no grains are made developable by the absorption of only two quanta of light. Some grains may possibly be sensitive to three quanta. But large numbers of grains are developable after absorbing four or five quanta. Most grains would be developable after absorbing ten quanta. Other grains may require many tens of quanta before being developable. Some grains do not appear to be developable at all. The need to absorb three or four quanta would appear to make latent image formation occur in a number of repetitive steps."

relistan · Aug 25, 2023

Alan Johnson said:
Yes, but so far [up to 1995] one thing has not been explained, dependence of effective film speed on developer,
eg, with a few dissenters, Xtol gives higher film speed than Rodinal.
So there are limits to what theory can explain and in all technical text and articles [I exclude those behind paywalls] the authors refrain from commenting on the dependence of effective film speed on developer.
This was largely left to careful practical observers like Crawley, widely quoted in The Film Developing Cookbook.

Agreed, and this is why no one was made the "perfect" developer that develops only and ALL of the exposed silver and nothing else.

Kino · Aug 25, 2023

Well funded studies of photographic science pretty much died a few decades ago.

Kinda doubt if anyone now will ever throw enough money and time at these mysteries to substantially solve them.

But maybe that's the attraction to this medium; the mystery and uncharted territories....

Rudeofus · Aug 26, 2023

Kino said:
Well funded studies of photographic science pretty much died a few decades ago.

Kinda doubt if anyone now will ever throw enough money and time at these mysteries to substantially solve them.

But maybe that's the attraction to this medium; the mystery and uncharted territories....

However, research in this topic 40 years ago weren't a bunch of freaky alchemists mixing frog guts while saying praises to pagan deities. This was top notch research back then, and the most absurd compounds were properly investigated for suitability. This research was not done to give Aunt Tilly and Uncle Jack good holiday pics: these materials were of great military relevance and a lot of effort was poured into research. In the end they chose Phenidone derivative plus suitable secondary developer (ascorbates or HQMS) or PPD derivatives plus color and DIR couplers to obtain optimal results. If there is a molecule better than these two combos, then it's likely complex enough to be unfeasible.

We're not going to find a new prime number below 1000, and we're not going to find a new developer combo, which does visibly better than the three combos mentioned above. They are available to us in the form of XTol and E-6 FD for regular B&W film, and C-41 CD for chromogenic film, so we don't even have to mix our own chemistry to get that kind of performance.

Kino · Aug 26, 2023

Rudeofus said:
We're not going to find a new prime number below 1000, and we're not going to find a new developer combo, which does visibly better than the three combos mentioned above. They are available to us in the form of XTol and E-6 FD for regular B&W film, and C-41 CD for chromogenic film, so we don't even have to mix our own chemistry to get that kind of performance.

"And it is a foolish photographer who believes that the creative possibilities of photography are entirely independent of the processing chemistry that produces the silver image. Yet, writers discourage non-professionals from modifying the chemical composition of photographic solutions. These journalists state that if the research laboratories of the large photographic manufacturers have not introduced a better product, then there is little hope for the individual to succeed. This fallacious reasoning presupposes that human ingenuity or serendipity occur only within the confines of the research laboratory. In truth, invention is the result of individual genius and can come from those who never enter a research building. Industrial laboratories excel in perfecting ideas that often have been originated and nurtured by non-professionals." Grant Haist, "Monobath Manual" (1966), Morgan and Morgan Publishers, Ch 3, pages 44 and 45.

It's a quote, so the pejoratives cannot be attributed to me, but even Haist obviously did not agree!

Rudeofus · Aug 26, 2023

Well, it's a difference, whether you want to change a developer to fill a very specific niche, or whether you want improve a developer into a direction, where countless amateurs and professionals have banged their head against for decades. Kodak came up with this performance triangle "speed grain sharpness", and focused most of their professional efforts on speed and sharpness, treating granularity as a distant third.

Therefore I can fully accept, that amateurs can find improvements on the grain front (sacrificing only a small amount of the other two), but would be very surprised to see them beaten in the speed+sharpness domain (without exploding grain), especially through introduction of a novel developer compound. But well, nobody will stop you to try it anyway.

What's the relationship between latent image sites and rate of reduction from a developer?

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