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Murray@uptowngallery
02-24-2008, 09:09 AM
Wow. Interesting. Thank you.

Hologram
02-25-2008, 03:01 AM
That was what I tried to hint at. I assume gelatine gets anistropic at/towards the tiny interface with another medium.
In the context of holography there has been some research on polarization issues related to gelatin. Under certain
circumstances one might get fairly large differences between p and s-polarization.
Having said that, refractive index remains the key parameter regarding internal reflections. The degree of internal reflection may also depend on wavelength...

The reason for this I only can speculate on. It could be mechanical tension. Though I expect this would not be established in case of wet/wet coating of similar gelatine solutions.

Maybe processing (development-fixing) involves significant refractive index changes. Has there ever been research on this subject?

However, even in this case there would be an interface. In the form of a layer of wetting substances.

I guess the wetting substances (being essentially hydrophilic) would get fully absorbed by the gelatin-water layer to be coated on top of it.

Hologram
02-25-2008, 03:04 AM
There is always an interface whether the layers are singly coated or coated in multiples at one time. There are tiny refractive index changes due to the chemicals present in the layers due to the fact that layers differ in content. The difference within the layer itself is substantial.

If that difference is so high, what about the explanation, that gelatin hardening might play an important role?


Think of the lens analogy. You have to coat lenses even though you may use the same glass in every element. It is the interface that counts even if there is no air there.

Far from being particularly knowledgeable about optics I assume that anti-reflection coatings only make sense when put at the air-glass interface.

AgX
02-25-2008, 03:15 AM
I guess the wetting substances (being essentially hydrophilic) would get fully absorbed by the gelatin-water layer to be coated on top of it.


If there are wetting agents in both layers, what I assume, and if they are of that typical kind we know (a hydrophile next to a lipophile part), what I assume too, the hydrophile parts would be incorporated in both layers with the lipophile parts sticking out.
Thus there not only would be an inhomogenity in both layers due the gathering of these wetting agents to the interface, but even even a layer made out of (or at least characterized by) those lipophile elements.

Am I wrong?

Photo Engineer
02-25-2008, 09:23 AM
Just about entirely right.

The wetting agents can be anionic, cationic or neutral. TX-100 is neutral and is a polyoxy compound, TX-200 is basically di-nonyl napthalene sulfonic acid or the like, and the positive charged surfactant (wetting agent) is usually a quaternary ammonium salt. These latter are not often used in photo product manufacture.

They are used at such a tiny concentration during coating that they would be hard to detect, but after drydown, the concentration has the effect of going up. Often, different surfactants are used in adjacent layers.

PE

AgX
02-25-2008, 10:36 AM
Thus

-) there are inhomogenities at the interface between layers

-) they could be explained by those wetting agents

?????

Photo Engineer
02-25-2008, 01:02 PM
There are no homogenieties at the interface.

Problems caused by wetting agents might include changes in refractive index or changes in silver type or grain size or coupler (in color), or combinations.

As I said, you have everything just about right.

In the photos on the left there is a 10 layer color film coated at high speed in one pass. The irregularities in the layers are due to the microtoming and mounting. You can see that there are no intermixed layers in the processed sample on the right. (used with the permission of Bruce Kahn) I have seen others without the 'ripple' but I have none of those. I will look and see if I can find some. The ripple, if real, would mess up the image really badly, so I hope you can understand that this is an artifact.

In the photo on the right, is a 5 layer structure coated one pass at a time. This is a dye bleach (Ilfocolor) prototype I coated and again you can see no interlayer mixing.

PE

Hologram
02-25-2008, 11:18 PM
There are no homogenieties at the interface.
Problems caused by wetting agents might include changes in refractive index or changes in silver type or grain size or coupler (in color), or combinations.

...or increased/decreased silver halide concentrations. The Bruce Kahn photo (Cross Section of EXR 5296, “before processing”) - thanks for posting! - seems to suggest uneven silver distribution within a single layer. The bottom of each layer may have a considerably higher amounts of AgX than the upper regions. I wonder if this is always the case or relates to surfactants, AgX concentrations of the coating solutions.
So if high AgX regions border on low AgX regions, it might explain the refractive index difference and hence the internal reflections.

Photo Engineer
02-26-2008, 09:52 AM
Well, first it should read "Inhomogenieties at the interface". The interface is supposed to be smooth except where distorted by the microtoming process.

I think you have misunderstood the Kahn photomicrograph. This is a 10 layer structure. What you interpret as silver being unevenly distributed is two layers as follows /overcoat/fast yellow/slow yellow/CLS layer/fast magenta/slow magenta/interlayer/fast cyan/slow cyan/AH/support.

The fast layers are coarse grained and the slow layers are finer grained. That is what you see as inhomogeneity and that is what can be seen by the light as changing the nature of the layers.

PE

AgX
02-26-2008, 10:33 AM
Well, with "inhomogenties at the interface" I put it quite badly.

I wanted to say that across a right angle section of a layer I expect a difference in RI between the interfaces and the rest of the layer.
Or to put it differently, I expect an inhomogenity concerning the RI with a change taking place at the interfaces.

Photo Engineer
02-26-2008, 11:39 AM
I meant inhomogenieties as well and mistyped.

However, even though there are RI changes they are rather small but important. And they are not due to intermixing and rough surfaces.

PE

Hologram
02-26-2008, 11:30 PM
I think you have misunderstood the Kahn photomicrograph. This is a 10 layer structure. What you interpret as silver being unevenly distributed is two layers as follows /overcoat/fast yellow/slow yellow/CLS layer/fast magenta/slow magenta/interlayer/fast cyan/slow cyan/AH/support.

Oops, now I can see it - silly me.

The fast layers are coarse grained and the slow layers are finer grained. That is what you see as inhomogeneity and that is what can be seen by the light as changing the nature of the layers.

And so the difference between coarse and fine grained layers, wouldn't it explain for the refractive index difference?

Photo Engineer
02-27-2008, 08:19 AM
To some extent, but the other differences I mentioned count as well. It all adds up. A little here and a little there and pretty soon you are talking about a real big effect!

PE