B&W Film chemistry question!

Discussion in 'B&W: Film, Paper, Chemistry' started by Pixie.Lou85, May 9, 2011.

  1. Pixie.Lou85

    Pixie.Lou85 Member

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    Hi there,

    I am a student at college studying photography and I have a test tomorrow for my monochrome processing class.

    We have to know the make up of film and i pretty much have it covered, but there are a couple of things I have tried researching for weeks and have came up blank!

    Can anyone explain what binders and their qualities are? I think binders is the chemistry that bonds the emulsion to the film base? Also, dye sensitising?

    If anyone could explain these even just a little i would be so greatfull!

    Thank you.
     
  2. Vlad Soare

    Vlad Soare Member

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    I may be wrong about this, but I think that "binder" designates anything that holds a light sensitive mixture together, like gelatin in the case of films and photo papers, or collodion in the case of wet plates.

    Short answer: certain dyes are added to the emulsion to increase the sensitivity (particularly spectral sensitivity) of the silver halides. Without them the emulsion would only be sensitive from blue upwards.

    The idea is that the effect of the photons upon the silver halide grains is proportional to the extent to which they are absorbed into the grains. By coating the grains with a dye of a certain color, the absorbtion of light of the complementary color into the grains is facilitated. For instance, you can make an emulsion sensitive to green by adding a magenta (more or less) dye. The theory is long and complicated. There's more to these dyes than their color, and only a few of them have all the properties necessary to work as sensitizers.
     
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  3. michaelbsc

    michaelbsc Member

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    Tremendously simplistic answer coming.

    As stated, ordinary silver halides are only sensitive to blue light and upward naturally. Keep this in mind.

    The way a silver halide crystal gets "exposed" is by having a few photons kick off a few electrons in the crystal. But plain Jane silver halides don't react with green, yellow, or red light. So those colors don't excite any electrons for them to get kicked off.

    Dyes, by their nature, must interact with light of different colors so they can reflect different colors. Basically a dye absorbs light of many colors and reflects light of it's native color. So if you have a yellow shirt, and you're standing in a white light, the red and blue light get absorbed by the shirt, and the yellow light gets reflected by the shirt.

    OK, now we have a thing that's absorbing energy at let say red light wavelengths. Dudes like PE worked to find ways to couple this "red light absorbing" dye chemical thingy to the silver halides so that when the "red light absorbing" dye gets a photon it can transfer the energy over and kick an electron off of the silver halide crystal.

    Voila, a silver halide crystal that has been dye sensitized to red light!

    Note, this simplistic explanation isn't going to get you a job at Kodak, Ilford, or Fuji, Maco, EFKE, FOMA, or even the local coffee shop. But I hope it helps you pass the test.

    MB
     
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  4. holmburgers

    holmburgers Member

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    Out of curiousity, what school is this?
     
  5. RalphLambrecht

    RalphLambrecht Subscriber

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    One with very tight deadlines.
     
  6. michaelbsc

    michaelbsc Member

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    Or a student who waits until the last minute? :whistling:

    He posted yesterday that he has a test today.
     
  7. RalphLambrecht

    RalphLambrecht Subscriber

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    That's OK. We might be his last resort.
     
  8. Sirius Glass

    Sirius Glass Subscriber

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    Hence the desperate cry!
     
  9. michaelbsc

    michaelbsc Member

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    I can confidently say that when I was a young student I certainly had no clue how film was manufactured. I was also never in a photography class where it was expected. So the only things I knew were things that piqued my interest enough to track down on my own.

    I did, however, find a book in the school library that explained the silver halide mechanism, but not the manufacture of pan film. It took me a few more years to get curious enough to track that down.
     
  10. Vlad Soare

    Vlad Soare Member

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    Don't be so mean, guys. I don't know about you, but I (and 99% of my colleagues, for that matter) used to wait until the day before the exam to start learning. :smile:
    Besides, I'm happy when someone is interested in film and its inner workings, regardless of the reason.
     
  11. michaelbsc

    michaelbsc Member

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    Point taken. You are right.