More on the history of color paper
In the last post on this subject, I gave some timelines for Kodak color paper. This post will include more information that I have gathered.
First, it should be mentioned that early color papers used B&W emulsions instead of specially reformulated emulsions. AAMOF, Agfa used basically a Brovira emulsion and Kodak used basically a Kodabromide emulsion.
But, here is an important point. Each coupler reacts differently when silver development forms oxidized color developer. This can be shown by what is called 'dye yield' and is a plot, on film support, of dye density vs mg/unit area of silver. It is done on film support due to the fact that paper support renders the data severely non-linear in the toe and shoulder. The slope can be anything from about 2.0 to higher than 6.0 and infers the equivalency (moles of silver / mole of dye formed) of the reaction. A slope of 2 means that 2 moles of silver are required to form 1 unit of dye.
Therefore, each layer had to be tweaked with addenda such as cadmium, mercury, rhodium and other items to match the curve shape of each dye layer to give a neutral.
This was all before the masked color negative.
When that revolution took place, Kodak had to have graded speeds in the paper to print around the masks, and therefore had to invent new emulsions. Kodak came up with basically 3 new emulsions. These were graded in about 0.2 micron, 0.4 micron and 2.0 micron sizes for the cyan, magenta and yellow respectively and had different halide balances and also different addenda as mentioned above.
In this configuration, the yellow layer was moved to the bottom, the CLS yellow silver layer was removed, and the speeds were further enhanced by other means. With yellow being on the bottom, the yellow dye stability was greatly enhanced. Addition of a UV absorber also improved magenta and yellow image stability.
But, the yellow layer developed very slowly. In fact, Kodak "C" paper took longer to develop at 75 deg F than Gevaert or Agfa papers did at 68 deg F. This was of great concern to Kodak. We could solve it by going to a higher pH developer, and this is where I came in by learning how to design developers and test them. I tried devising new, high pH developers for more rapid processing. It resulted in two patents and my gaining a lot of experience testing developers and developing agents.
BTW, if it were not for the slow yellow development, Kodak "C" paper would have developed properly in about 1/2 the time, and as a result the top cyan layer had to be restrained to prevent it fogging or becoming too high in contrast by the time the yellow layer was ready. A rather ironic twist.
In any event, this higher pH developer was not the direction Kodak took, and the new developing agent (CD-6) was put on hold for a bit. In a parallel effort to remove cadmium and mercury, Kodak had undertaken a big project in color paper to come up with 3 new emulsons, at the same time solving the development rate problems. This new paper resulted in the yellow layer developing in just about the same amount of time as the other two, but with the same photographic speed and no need for Cadmium or Mercury. The big problem was that the curves could not be adjusted to work in the old developer/replenisher combination and so we had to devise a new developer.
At the same time, we had to remove the ferricyanide. So, our group worked on the developer and blix and in 10/66 I formulated the first usable blix that basically is still used by Kodak. We also devised a C41 blix resulting in another patent. This new paper was Kodak Ektacolor 70 paper introduced a year early in 1969.
At that time, Kodak then went on to design a paper based on CD-6 and another based on catalytic imaging (another patent and a paper delievered at the ICPS conference in the 70s). But, when the Ektaprint 3 process was announced, Ansco and several others sued Kodak because they had just released Ektaprint C compatible papers and/or processes. As a result, Kodak management decided not to release the CD-6 paper and chance additional law suits. This was at the same time as the Hunt silver fiasco, and as silver prices fell, then need for CD-6 fell also. CD-6 gave higher dye yield. It also gave better dye stability in some cases.
During that time period, one of the superb researchers in emulsions came up with a rapildy developing large grain yellow layer, and as I left the project for Kodakcolor Gold 400, they were just working on the fundamentals for what became the RA papers of today.
Throughout this all, couplers and addenda were changed, as noted before, to give nearly a 100 fold increase in image stability, a subject that I was associated with in every project.
So, there is a thumbnail sketch of color paper history in more detail in some respects.
that is very informative again.
But I don’t understand why the dye yield is dependant on the emulsion. Or to be more precise, why, concerning the AgX, a new emulsion has to be made. Of course, a certain emulsion and a certain developer give a certain silver density yield. And the higher this would be the higher the dye yield would be.
Or do you mean, that as a coupling developer is introduced the silver density yield would be different in first place. And it is this what makes a special color emulsion necessary?
(I guess I answered the question myself, but to be sure I leave it open…)
Furthermore, I don’t understand:
“…., Kodak had to have graded speeds in the paper to print around the masks,…”
Well, for starters, there are 3 couplers and each reacts at a different rate and with a different yield with oxidized color developer. This changes dye yield and curve shape. Imagine a coupler 50% less active than another, they you can imagine a 50% loss in contrast. This is an extreme, but an example.
Lets now imagine a 2% difference that shows up as mismatched toe and shoulder. Tweaking the emulsion with addenda to change contrast might fix this.
The dmin of the cyan layer is about 0.2, magenta about 0.4 and yellow about 0.8, therefore the speeds relative to each other in the paper are Red = 0, Green = Red + 0.4 log E and Blue = Red + 0.8. This gives a neutral with white light + a piece of fixed out film and no filter pack. Add 50 R and you have Green = Red + 0.4 + 0.5 and Blue = Red + 0.8 + 0.5. The blue layer is as much as 1.3 Log E faster than the red.
In fact, I have posted a wedge spectrogram of Endura paper processed in Dektol here somewhere. The raw blue speed is about ISO 100+ and the grain size is about 2.0 microns. This has the added effect of making the blue sensitivity of the red layer insignificant.
PE, you need to write a book!
Just a compendium of stuff like this.