View Full Version : Changing color sensitivity of film.
11-01-2006, 11:11 AM
Wow, what interesting responses! Thank you all. My reason for originally asking, is that I have enough Kodak aerial dupe film (type 2430) to give me several thousand sheets; but it is blue sensitive. I want to do some experimenting with a very high resolution Wild lens I have and would prefer a panchromatic film but with the resolving power of type 2430.
Which begs my asking another more convoluted question.
Is the change in emulsion sensitivity on a molecular level or is it caused by the physical presence of the dye acting in somewhat the same way as a filter? And if so would the use of an optical filter in the image path perform a similar function?
11-01-2006, 11:34 AM
I think that the dye's function is to capture wavelengths of light to which the silver is not sensitive and donate electrons to the silver in the emulsion
They definetely act on the molecular level.
11-01-2006, 11:43 AM
To be effective, a dye must initially have 2 properties. It must first be colored opposite to the wavelength you wish to sensitize the emulsion to, therefore a green sensitizer will be magenta in color and etc. Second, it must adsorb onto the surface of a silver halide grain. These are the two basic properties.
The third property, acting after the first two, is the ability to pump the energy absorbed by the dye into the grain as if the photon were hitting the grain, so a blue or UV sensitive grain would become green sensitive by adding a magenta colored dye to the grain. The emulsion is still blue sensitive, so you end up with blue-green (ortho) sensitivity by adding a green sensitizing dye.
The two simplest dyes are chlorophyll, a weak red sensitizer, and erythrosine a fair green sensitizer. Using erythrosine will give you an ortho emulsion from a blue sensitive emulsion if added properly. Erythrosine is the active ingredient in some food dyes and also in the old antiseptic Mercurochrome.
11-01-2006, 12:17 PM
That is good for an approximation only. As it says, this is for a chlorobromide emulsion and all film emulsions that I know of are bromoiodide, therefore the spectrum and reactions will be different.
These particular dyes may not even work on a bromoiodide or they may form "J" aggregates. We had tables of dyes with emulsions, pH, pAg and etc at EK that could be used to pick and choose.
I only posted it 'for interest', not even 'for information' never mind 'for production' - but you are right to clarify the practical uselessness of it!
If anyone would like copies of short extracts on the mechanism of spectral sensitization by dyes from Mees (readable) or Tani (a little less readable, more detailed, but still not exactly a DIY handbook) for their personal use - ie not for publication on the web - let me know.
11-01-2006, 01:11 PM
I have posted some of my wedge spectrograms and described my spectral sensitization experiments here on APUG somewhere. If anyone is interested in them, they may look them up.
Spectral sensitization of a raw emulsion either before or after coating is one of the simplest of operations on an emulsion you can do, once you have a known good dye. Remember, that is a raw blue sensitive emulsion, not one already dyed. It also requires a good dye for the emulsion you are using.
11-01-2006, 05:02 PM
Helen, don't worry about the comment you quoted. In the case of 3,3'-diethylthiatricarbocyanine, the spectral sensitivity shifts towards shorter wavelength side and best sensitivity is between 750 and 800nm. This dye works well with bromide emulsions with varying iodide content, as well as various core-shell structures (although there are preferred crystal designs, the preferred form is not too dissimilar to other dyes anyway).
J-aggregate is largely irrelevant in infrared sensitization because very low dye density is chosen. J-aggregate is used when sharp tuning (sharp cutoff) of spectral sensitivity is necessary, such as multilayer color films and modern multigrade papers.
The mechanism of spectral sensitization (and desensitization) has been elucidated quite well in the past decade, and a lot of old theories are now obsolete. It is very nice to be able to get rid of old empirical rules that don't always work and replace them with a few simple physical and chemical principles that explain all known issues that had not been explained before. This is the exciting time to understand sensitivity mechanisms of silver halide media, but like anything, it is necessary to have good background in solid state physics (semiconductor materials) and organic chemistry. Tani's book is excellent on this topic, and many conjectures he stated therein were actually later figured out by himself. But again, as you mentioned, the prerequisite to understand this topic (that is, his book) is not that modest.
When you take spectral sensitivity measured with chloride emulsions and try to infer it to bromide system, there is one issue that needs attention. Some dyes sensitize chloride emulsions but desensitize bromide emulsions. Good dyes that are used in modern practice generally don't have this sort of problems, but historical ones such as erythrosin can sensitize chloride emulsions decently, but it is impractical for bromide system. In this particular case, erythrosin adds a bit of green sensitivity but depresses the intrinsic sensitivity to blue light. Other than this, gross features of the dye rarely change drastically between chloride and bromide emulsions.
11-01-2006, 06:26 PM
An unexpected "J" aggregate can form at any time. For example, a dye that is a green sensitizer for a Bromide emulsion can become a red sensitizer for a Bromo-Iodide emulsion due to strong adsorption. The same can happen for almost any dye used improperly.
BTW, the dye I refer to above is actually used as a red sensitizer in a high iodide Kodak product, so this is no useless or rare phenomenon, but when it hits you, you get unexpected and "wrong" results. For example, expecting green and getting red sensitization.
Therefore, it is possible to imagine a short IR dye becoming a long IR dye if a "J" aggregate forms or a red sensitzer becoming a short IR sensitzer if a "J" aggregate is formed. There is no way to really tell beforehand without testing on the emulsion in question.
Regarding erythrosine, it is usable and gives fine results if you know how to use it. If you don't know how to use it properly it will give poor results. It is also quite economical, as it is about 1/4th the price of many other common sensitizing dyes and is more stable. It is therefore useful for doing a lot of early on experiments without eating up a lot of expensive dye.
Also, the expensive dyes are hard to get, and becoming harder to get all the time.