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  1. #1
    richard ide's Avatar
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    Changing color sensitivity of film.

    One can hypersensitize film to increase effective film speed before or after exposure. My question is: Can the color sensitivity be changed in somewhat the same manner; or is that only possible when coating the film? I realize that it would have to be done with a dye rather than a vapor bath. My reason for asking is that ortho emulsions do not excel at cloud photography.

    Thanks

    Richard

  2. #2
    Photo Engineer's Avatar
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    You can change the color sensitivity by the use of spectral sensitzing dyes. They are typically added just before coating the emulsion, but old workers used to bathe the film in the dye solution.

    You can dye in the blue, green, red, short IR and long IR regions of the spectrum. You should not try to dye a predyed film. So, if you have a pan film, you probably cannot undo the pan sensitivity and make it ortho, nor could you easily add an IR dye and make an IR film, although adding is possible under some circumstances.

    PE

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    Quote Originally Posted by richard ide View Post
    My reason for asking is that ortho emulsions do not excel at cloud photography.
    The two reasons, that I can think of, that people use ortho films are that they like the films color response and/or they want to develop by inspection. If you add a red sensitizing dye to an ortho film you will lose both reasons to use an ortho film. If you have difficulty in rendering clouds with an ortho film why not just use an panchromatic one instead?

  4. #4

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    You can dye factory-manufactured film to modify spectral sensitivity. Some dyes require lower adsorption density to be effective than others, and these are the ones you want to use if you want to apply a dye on pre-manufactured materials. Many infrared dyes can be used in this fashion but there aren't many dyes for visible light that can be used effectively for this application. If what you want is panchromatic, it's easiest and probably cheapest to buy the finished film. Good cyanin dyes aren't cheap.

    But there are people who soak pan film in infrared dye bath to make infrared films. Big PITA but that is an option for nice fine grain IR film after discontinuation of Sakura (Konica) IR 750.

  5. #5

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    Quote Originally Posted by Ryuji View Post
    You can dye factory-manufactured film to modify spectral sensitivity. Some dyes require lower adsorption density to be effective than others, and these are the ones you want to use if you want to apply a dye on pre-manufactured materials. Many infrared dyes can be used in this fashion but there aren't many dyes for visible light that can be used effectively for this application. If what you want is panchromatic, it's easiest and probably cheapest to buy the finished film. Good cyanin dyes aren't cheap.

    But there are people who soak pan film in infrared dye bath to make infrared films. Big PITA but that is an option for nice fine grain IR film after discontinuation of Sakura (Konica) IR 750.
    Can you post a link to more details on this?

  6. #6
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    Doing this is not a *good* idea. You have to do it in such a way as you displace the dye already on the film, and sometimes it does not work or does not work the way that you want.

    Sometimes a "J" aggregate forms causing a shift to longer wavelengths and giving an unwanted spectral sensitization.

    Even worse, sometimes the dye sticks to the emulsion and leaves a severe stain in the negative.

    BTDT. Sorry.

    PE

  7. #7

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    Quote Originally Posted by wirehead View Post
    Can you post a link to more details on this?
    I don't have a web page describing this technique, but it's a common trick among infrared community (as there are a couple of very good infrared dyes for this technique). A common method is to make 0.05% dye solution in 91% 2-propanol or methanol, immerse film for a predetermined period (say 2 min) and dry at once. This level of dye is sufficient for some of post-1930 infrared dyes but vastly inadequate for many dyes, so you have to select the dye very carefully. Again, I don't recommend doing it if what you want is panchromatic emulsion. Just buy it in that case.

  8. #8
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    If you do what Ryuji suggests, work in total darkness for the entire operation of re-dying, drying, loading into the film holder (or whatever) and then the dark steps of processing. Be aware that you may encounter drying spots on the film and that the dying process may wash the acutance and AH dyes out of the coating depending on the type of dye already present and the type of solvent you use.

    Don't expect stellar results. It may not work.

    Being one of the few here that has done spectral sensitization routinely, I am familiar with the pitfalls of this. In fact, I doubt if anyone has done this with really good results.

    PE

  9. #9
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    This is well outside my knowledge and experience, so all I'm doing is passing on this link which has contributions from Ryuji as a starting point for a bit of web surfing for the sake of interest (I hope that Ryuji doesn't mind me linking to that thread).

    You could also google 'diethylthiatricarbocyanine sensitization', as I did to find the above, or 'diethylthiatricarbocyanine' or similar and follow links. Tadaaki Tani calls it ‘Dye 8’ in ‘Photographic Sensitivity’. It is dye 69 in Mees, who also gives examples of the spectral sensitivities of a chlorobromide emulsion sensitized with a number of infrared sensitizers.

    Best,
    Helen
    Last edited by Helen B; 10-31-2006 at 05:53 PM. Click to view previous post history. Reason: Mees reference added.

  10. #10

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    That's right, and good job on finding it! 3,3'-diethylthiatricarbocyanine iodide is a good dye, and if used on undyed emulsion, it can be used with dim green safelight because of deep spectral notch. (But if you begin with panchro emulsion, work in total darkness.)

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