View Full Version : Transparency of Silver Emulsions - Multi-Layer Emulsions
12-06-2011, 02:55 PM
Specifically I'm thinking about bi-packs & tri-packs, but monopacks certainly apply as well.
The azo emulsion that we made at GEH is fairly opaque; creamy & off-white, like a thin layer of gesso or something. So I'm wondering, if I wanted to layer a blue-sensitive emulsion over a green or red sensitive emulsion, how would I ensure that light passes through the blue unscathed?
Now, reading an F.E. Ives patent on a tripack he mentions that the front blue emulsion should be of the Lippmann variety (iodide-bromide??), which has to be quite transparent. They are nearly glass-like, judging from some holography plates I have.
12-06-2011, 03:04 PM
The Azo emulsion is white because it absorbs very little visible light. A true blue sensitive emulsion is yellow. A green sensitive emulsion is red and etc.
There are internal reflections between layers and with the support, and there are internal reflections. These are compensated for by acutance dyes that reduce internal reflections. The AH layer works for the support.
Ives information is seriously out of date. The CLS layer, the 3rd in a color tripack, is the only truly deep yellow layer and it is there to prevent the passage of blue light to the underlying layers.
I've said it before and I'll say it again.... Watch out for old works that are much before 1945 - 1970. and go to the original if you can because of serious errors in derivative works. For this see the handout that I gave at class Chris.
12-06-2011, 03:22 PM
Well, Ives was the only example I had on hand, and out of date or not it doesn't change the fact that you need a transparent emulsion.
I thought that the CLS (yellow filter) layer would be #2, right behind the blue emulsion. What am I missing?
12-06-2011, 04:34 PM
/overcoat/blue sensitive/CLS/etc..... That is the layer order making the CLS the 3rd layer (nominally). If the blue sensitive layer is multicoated then the CLS is buried even deeper.
As for the "old" stuff, you must remember that in Ives day they may have needed a 1 micron grain to get the same speed that we can get today from a 0.1 micron grain. in fact, back at that time, Chlorides were not really much more than UV sensitive curiosities, but today they can get ISo 100 on RC support with no UV..
This affects turbidity and other properties related to grain size vs speed.
At least my emulsions (BrI) were somewhat "transparent" in the sense that they might work in a multilayer setup with limited sharpness and limited speed... I mean, of course they are "milky" or diffuse, but if you'd place a newspaper in a close contact with the emulsion, you might be able to read the text through the emulsion. So when the distance gets shorter, "translucent" changes to "transparent".
Of course, the wavelength which the grains are sensitive to is absorbed by the grains, but what happens to other wavelengths? They do not go through the grain which would be optimum, right? Instead, they are reflected by the grains and get scattered. So, to make the top (blue) layer transparent enough, it needs to be "inefficient" so that it passes part of the light through it by having fewer grains than would be optimum for single-layer? Then, the blue wavelengths of the "pass-through light" is absorbed by the filter layer. I guess there are more optimum ways because what I propose wastes light quite a bit?
12-06-2011, 06:12 PM
Well, actually, red light will pass through a green or blue sensitive emulsion to some extent. However, if you have t-grains this is optimum because the grains are flat and parallel. With octahedra, the angled edges cause more reflection. Cubes are in the middle. Each size and type (AgBr/I ratio) and sensitizing dye) will govern turbidity and transmission.
12-06-2011, 10:09 PM
This is great stuff fellas... keep it coming!
Was this big speed increase that occurred due to the discovery of sulfur sensitization? Or was it a host of things?
So basically there is no magic formula to make a emulsion completely transparent, but rather a game of finding optimum compromises. I can imagine how the use of acutance dyes is key in all of this. How about silver excess and things like that?
Basically, I just need a copy of Ron's book STAT... ;)
- hope all are having a nice evening
12-06-2011, 10:13 PM
Sulfur was always with us, but we did not know what was going on!
The sequence over the years was Sulfur, S + Au, S + Au + 2 electron sensitization to simplify things. And, I didn't really cover this stuff in the book as it is supposed to be a "starter book". I guess I have to stop and re-write or do a 2nd edition!
But, there really is no one method to come up with one best solution. There are computer models to help predict what to do though.