What Does Spectral Sensitivity Actually Involve?

[thisneumann]

Can someone explain how a film color layer responds to photons in terms of spectral sensitivity/quantum efficiency?

Is the physical issue a matter of how much a photon of a particular wavelength will effect the silver halide crystal, or is it how likely the photon is to be absorbed (instead of passed through or reflected)? Or a combination of the two? Something else?

Thanks.

 

[AgX]

To make it even more complicated: there are two players involved in at least two layers.
The sensitizing dye and the halide crystal.

 

[thisneumann]

Indeed... This may have been too technical of a thing to ask, but I figured apug was the place to try.


Anyway, I've been referred to a book. Here's hoping I'll get my explanation there.

 

[Bill Burk]

Good luck.

I had a feeling the explanation might come from statistics... the probability of a grain of silver halide being provoked to develop-ability... is affected more or less by all the filters in the emulsion, and in front of the camera lens, light quantity for each part of the photograph, dyes in the emulsion...

But from a chemical viewpoint I'd be unable to help.

 

[Mr Bill]

Anyway, I've been referred to a book. Here's hoping I'll get my explanation there.

If the book is by Tadaaki Tani, It will most likely be to your satisfaction.

I don't understand, fundamentally, how it works so have no hope of explaining, but can give you several concepts to research.

First, the silver halide grains, the light-sensitive part of film, are inherently only "sensitive" to bluish light. As AgX mentions, sensitivity to other colors is added by the use of the "sensitizing dyes."

In both cases, the quantum efficiency tends to be fairly low - I'm thinking in the general range of 10 or 20% but don't rely on these numbers. Anyway, if you look at a piece of unprocessed film in the light, it generally appear to have a fairly light tan color. So you know right away that a lot of light is being reflected back, and is not absorbed by the film. And we know, historically, that a certain amount of light passes through the emulsion, because of something known as "halation." Some earlier films would get "halos" around bright spots of light. It was possible to coat the film, beneath the emulsion, with dark materials, and this would eliminate the halos. So it seems to show that the halos were caused by light, having passed through the emulsion, being reflected back into the emulsion by the film base.

There is another part to this. Even after light is absorbed, it is not necessarily adequate to form a developable image. It is generally considered necessary for a minimum of 4 (or possibly 3) silver atoms to come together in a single grain to form a stable, developable image. Anything less may disappear on its own, so even if a photon was initially absorbed, it might not be useful with respect to forming an image. There is an older book that can be found online, here, that may be useful. https://archive.org/details/PhotographicSensitivity

In fairly recent times, the past dozen (?) years or so, a couple new technologies were developed to improve sensitivity. One is with "fragmentable electron donors," aka two-electron sensitisation, where it is possible to form a stable latent image from only two photons, instead of four. The other uses something called "antenna dyes," which are essentially dyes attached to the existing sensitizing dyes; the quantum efficiency may be roughly doubled by these. There have been technical papers with A. Muenter of Kodak as one of the authors, but probably have to be paid for.

Hope this will give you a good start.

 

[Photo Engineer]

There are also supersensitizing dyes which are added to the emulsion to increase sensitivity.

Basically, all of the above is correct. It generally takes 3 electrons to cause a developable center to form without regressing. Adding metal dopants (Muenter) will change that to about 2 electrons. Workers in France make the same claim but by a different route using Formalin or other reducing agents.

A fully sensitized emulsion is gray or nearly black indicating absorption of all colors. The brown color results from trimmer dyes that are there to either adjust speed or adjust color balance / sensitivity.

An emulsion is blue sensitive and UV sensitive. A UV overcoat is placed on films to limit the UV sensitivity and the sensitizing dyes are added to the emulsion just before coating. They are usually cyanine, merocyanine or carbocyanine dyes (to pick a few types). The procedure is simple. Just dump and stir for about 15'. Then the emulsion is UV/Blue/whatever... The whatever is the color of the dye you have chosen, and the emulsion takes on that color + yellow (due to the blue sensitivity). As the "whatever" sensitivity goes up, the blue sensitivity goes down because sensitizing dyes are also desnsitizers for the blue (native) region of sensitivity. Thus, the level of dye to be added is critical. And thus, supersensitizers become important in this as well as 2 electron sensitivity.

With all of these tools in hand, an emulsion that gave us ISO 25 in the '90s, can give us 100 - 200 today with much lower grain! That is part of why EKTAR 100 is so good. It uses all of the most modern tools.

PE

 

[Truzi]

...
With all of these tools in hand, an emulsion that gave us ISO 25 in the '90s, can give us 100 - 200 today with much lower grain! That is part of why EKTAR 100 is so good. It uses all of the most modern tools.

PE

Ah, now I see. I know film had gotten better, but didn't quite understand why.

 

[Bill Burk]

And we know, historically, that a certain amount of light passes through the emulsion, because of something known as "halation." Some earlier films would get "halos" around bright spots of light. It was possible to coat the film, beneath the emulsion, with dark materials, and this would eliminate the halos. So it seems to show that the halos were caused by light, having passed through the emulsion, being reflected back into the emulsion by the film base..

Polaroid was able to take advantage of this to get speeds of 3000 by putting effectively a mirror behind the emulsion... since for Polaroid purposes you were looking through the front of the emulsion anyway - you didn't have to have a transparent film when you were done... And since you were looking at the final (not enlarged) image, the slight degradation didn't matter.

 

[AgX]

The advantage that Polaroid took was in the characteristic of the diffusion transfer reversal process itself.
Not in some mirror.

 

[Bill Burk]

I'd caught that thought from a paragraph in "Instant" p.56 describing where Meroe Morse told Al Bellows that the emulsion of Type 47 film was very thin and the backing was reflective so that light bounced back and exposed the film twice...

 

[AgX]

The diffusion process itself already introduced a loss in resolution. One would not want to reduce resolution even more.
And calculate yourself: how much gain in speed could one achieve by that?

 

[Photo Engineer]

Any emulsion on a reflective support gains at least a stop in speed due to the technical details of back reflection. So, a 400 is an 800 if coated on paper support. Now, to take it a step further, if you lower the Dmax, which one of the results of diffusion transfer, you gain more speed, so that 800 now becomes a 1600 if Dmax goes from say a norm of 2.2 down to about 1.8.

So, sometime just measure the Dmax of a Polaroid (or other instant print material) and see what it is. I'll bet you will be surprised. They were all kind of low to get good speed.

Of course, Land introduced special nucleating chemicals to make the image look blacker, and he did use very high speed emulsions to get the best of all possible worlds from his products. There is no denying that they fiddled with a lot of things to improve quality.

But, here I have given you two common methods of getting speed from instant. I've coated an ISO 25 emulsion on highly reflective RC support and gotten ISO 100 from it using these methods. This is a 2 stop increase just by going from film to paper (RC) support.

PE

 

[AgX]

Though we must not mix up the base of the negative with the base of the positive.

 

[Bill Burk]

Though we must not mix up the base of the negative with the base of the positive.

Yeah, I got that mixed up... the donor had the reflective back not the print, and Polaroid could get away with because of the diffusion transfer processing...

But, gaining that stop of speed... would it be possible to incorporate a reflective layer in an emulsion intended for enlarging? Maybe a semi-reflective "pellicle" type layer that only gains a third stop could be used in a film, and it could stay in after processing, since it does not completely block the light going through for enlarging later...

 

[Bill Burk]

I've coated an ISO 25 emulsion on highly reflective RC support and gotten ISO 100 from it using these methods. This is a 2 stop increase just by going from film to paper (RC) support.

PE

Haaa, right... Paper. Then scan or rephotograph the developed image.

 

[thisneumann]

An explosion of things to study; I appreciate it, guys.

 

[AgX]

That is a complex matter. Better handled in private tuition than on a forum.

 

[kb3lms]

Add "Grotthuss–Draper law" to your research list. Basically, light has to be absorbed by the system to make a photochemical change. So however you can get more light absorbed, the more sensitive you can make the material. I think that is one part of your question.

The other part may have to do with sensitizing dyes themselves. As PE mentions above, an AgBrI emulsion (Silver Bromo Iodide) is sensitive to Blue and UV light. This is the reason you can use a red safelight in the darkroom. In the mid 1800's, a fellow named Vogel discovered that dyes could be used to extend color sensitivity into the green. Later researchers learned how to extend to yellow, then red and infrared using cyanine type organic dyes. There are other dyes that can be used but the cyanines and related are the best.

Now in your basic unsensitized emulsion, a photon is absorbed by a silver halide crystal and hits a bromine atom. The energy of this photon kicks an electron into a higher energy state and into what is called the conduction band. Electrons can move between atoms in this conduction band. So the electron moves about in the conduction band until it finds a random silver ion, Ag+, that is looking for an electron. The electron discharges it's extra energy into attaching itself to the Ag ion, thereby reducing it. The Ag atom now has a full set of electrons and becomes an atom of metallic silver. When you get 4 of them in a group you have enough to catalyze reduction of the entire crystal during development.

In the case of a dye sensitized emulsion, as PE says we add a cyanine dye that absorbs, say, red light. Now in addition to absorbing red light, the dye must also adsorb (or stick to) the AgX crystal. This is one big reason not just any dye can be used because not just anything will stick to the crystal. In dye sensitization, the dye molecule takes the place of the bromine atom. So our photon hits a dye molecule and kicks one of those electrons into the conduction band. Because the dye has tightly adhered to the crystal, the electron can move into the AgX crystal and find an Ag ion to reduce. If the dye doesn't absorb the photon it is free to continue on into the AgX itself.

On of the big problems in sensitivity is that when this electron that reduces the silver leaves a bromine atom, it leaves a "hole" in the electron shell. Therefore the oxidized bromine atom also now wants a new electron to fill that hole. So our electron that has been raised into the conduction band can just as well stumble into a bromine ion (including the one it came from) in a process called recombination, which as you can see, accomplishes nothing and the energy of the photon was wasted. Various schemes to increase sensitivity focus on preventing recombination.

That's over simplified and I probably got something not quite right but it's my understanding in a nutshell.

Another interesting thing to look up on this subject is dye-sensitized solar cells, where a TiO2 molecule takes the place of the AgX, but otherwise works on roughly the same principle. There are a number of simple dyes that can be used for solar cells, such as blackberry juice, which I would like to try on AgX some day. Of course, like film the cyanines still work the best and many of the dyes are actually the same or very similar.

Jason