View Full Version : Sensitivity enhancement through doping
02-20-2010, 10:06 PM
I've been reading the emulsion posts here with interest, trying to put together the complex picture in my mind. I ran across a paper by Belloni in 2003:
I'd be very interested in PE's comments as to how the sensitivity enhancements by HCO2- doping fit into the various ways to boost response (2-electron, etc.) or whether this is just another name for something already discussed somewhere in these vast posts. This technique looks like it works both by removing some of the photoholes which scavenge electrons, and creating a formyl radical which enhances the rate of silver ion reduction.
Of course, how practical this is as a technique is completely out of my comfort zone [my day job is space plasma physics].
The "2-electron-sensitization" follows the same principle as the HCO2-doping by scavenging a hole and finally releasing an additional Ag atom (though by different compound and reactions).
Whereas the "2-electron-sensitization" only works at the shell of the crystal and thus is only capable of scavenging those holes that make it that far, the Belloni-Agfa system is situated inside the crystal and thus will be much more effective.
Kodak put their system into production, Agfa did not...
02-21-2010, 08:42 AM
Thanks for the clarification. It does seem like the amplification potential is enormous with this technique, but that of course says nothing about the temperature, etc. stability of the formate doping in practice, nor whether there would be large problems in production line use.
02-21-2010, 10:13 AM
Belloni's method has been discussed here in other threads. The Formate method was quite impractical IIRC. The alternative approach by Munter et. al. at Kodak using Osmate salts was most effective. I believe that Belloni's method was studied at Kodak by Eikenberry as noted in another post. It was found to be quite unstable. Even the Osmate approach renders film less stable and more prone to change during keeping and in my understanding, this is the major design problem faced in the 2 electron sensitization methods.
I believe that Belloni or one of the other workers on the formate method has tried to sue Kodak over the 2 electron sensitization, at least I heard something to that effect. Also, Belloni or some of the workers sold the rights of this method to Agfa just before they went under several years back.
The osmate system is indeed a surface effect, but a very effective one at that surface which yields about one stop in speed from a given emulsion. So, today's 100 speed emulsion, with all that can be done, is actually more like a 12.5 or 25 speed emulsion from years back when one considers Sulfur+Gold, Supersensitizers and 2 Electron Sensitization. This improvement also includes Iridium and/or Rhodium doping for Latent Image Keeping and Reciprocity Failure fixes. So, in spite of all of the "complaints" here, today's films are better for speed, sharpness and grain and actually have better keeping than those of about 25 - 50 years ago.
02-21-2010, 06:30 PM
Thanks PE! I've found the thread from a couple years ago which discusses all of this in detail. Sorry to make you type things twice. In actuality, having your explanation in the second paragraph above is a nice summary of all the improvements applied today to things like Ektar 100. Guess I should go out and shoot some more..