RElationship of Sensitising Dyes, Film Speed and Expiry of film.

[cmacd123]

Don't know if this forum is the right one to ask this question, but is sorta does involve chemistry...

I have a clear mental model that the Sensitising Dyes in Film adsorb light of various colours and somehow pass that energy on to the silver halide crystals. If they did not exist we would all be using a blue sensitive film with ASA 3 resembling Eastman Fine Grain Release Positive 5302. They are also horribly expensive because they are so special, with Ron the Photo Engineer quoting one that is 300 Dollar US a GRAM in another thread.

Do the dyes also have a major effect on the available film speed?

The reson I am asking is that I obtained a few rolls of various B&W films that expired back in the 1970 era. The ones I have exposed and developed seem to follow my mental model of film life in that the 125 ASA films like FP4 still seem to have fairly good speed, even though the image has a cast from what I assume is the grain of the backing paper, and small flecks of the emulsion came off on that paper. While a roll of Agfa Isopan Ultra. (ISU) originaly ASA400 seemed to have fallen to about ASA12.

I am wondering if the Dyes used may have been different to get the higher speed film, and those may have faded more than the ones needed to make the ISO 125 product. Naturally the experiment would have worked better if the e-bay seller who found these in the basement of a former camera Store had found some HP3 or HP4 that had also remained in the same condition, which would have avaoided many variables.

So in sumamry, do the Dyes increase what would otherwise be the speed of the film, and second does part of the speed loss experienced in out of date film have to do with the breakdown of the dyes?

 

[Photo Engineer]

Charles, there is no easy answer.

Dyes do increase speed if you consider the green and red regions to be part of the speed point, but then if you meter overall speed it really stays the same or goes down. This is because spectral sensitizers decrease blue speed at the same time they impart green and red speed.

Dyes are different for higher speed films, but need not be different. Better grains can do the same and then you add the dyes on top.

I suspect that what you are seeing is the normal aging of those particular emulsions apart from any dye effects. But again, you never can tell.

I've been teaching workshops at the EM101 level , but you are asking questions at the PhD level!

PE

 

[cmacd123]

Charles, there is no easy answer.
I've been teaching workshops at the EM101 level , but you are asking questions at the PhD level!
PE

And It does not help that my understanding of the Dyes is at the general concept level also. Thanks. I know that it is really hard to answer a question when it would take a week just to explain the conditions that are behind the answer.

I just seems "funny" that overaged to the extreme High speed film seems to lose speed at a higher rate than slower film, to the point where the shower film ends up faster. But You are right that my hapenstance experiment has way way to many variables.

 

[Photo Engineer]

Faster films are more IR and heat (same thing) sensitive. They go bad faster than slow speed films and finally they are less sensitive in some cases depending on chemicals used.

This is kinda hard to explain. I'm working on means of doing so via a graphics tablet.

PE

 

[Rudeofus]

As Ron describes it, and from what I also read in literature, there are two separate forms of sensitization: spectral sensitization (i.e. make film sensitive to wavelengths it would normally not respond to), and sensitization for extra speed (make film more sensitive at all wavelengths it is sensitive to). While spectral sensitization is generally done with very complicated and expensive dye molecules, sensitization for speed was usually done with Sulfur and Gold until supersensitizers were discovered, which are again some very complicated dye like molecules.

If faster films' speed relied more on these (then novel) supersensitizers, and if these supersensitizers from that era decay faster than regular speed sensitization, then we might have an explanation for Charles' observation. Charles: would you care to test/check, whether this decayed 400 film is still equally sensitive to red, green and blue?