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  1. #1

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    Emulsion layers "Thick as a Brick..."

    Anyone know how thick an X% gelatin layer dries down to at RT...

    How does one calculate/guesstimate it, allowing for various gelatin concentrations, and assuming the gelatin retains say 10% moisture?

  2. #2
    Photo Engineer's Avatar
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    I used to know these, but remember that the figures for dry gelatin at 100 mg/ft square will be different for gelatin with silver halide in it or whatever, to bulk it up. That is the problem. Usually, the only sure solution was to cross section the film or paper and then take a photo of it and measure it.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    I used to know these, but remember that the figures for dry gelatin at 100 mg/ft square will be different for gelatin with silver halide in it or whatever, to bulk it up. That is the problem. Usually, the only sure solution was to cross section the film or paper and then take a photo of it and measure it.

    PE
    Any idea what the magnification for such estimates would have to be?

    Actually I was just looking to get in the ballpark... do you think the variation from the gel only figure would be more than 10%?

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    Firstoff, magnification can be anything at which you can see and measure the layer. I used 2500X very often.

    Second, if you coat 10% gelatin at 0.005" (5 mil) this will cover about 1 square foot. On drydown it will lose about 90% of its bulk. You take it from there. That is the best estimate I can give you.

    I have posted sample cross sections here on APUG in this forum that might help.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    That is the best estimate I can give you.
    PE
    OK.

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    An easy way to measure thickness of an emulsion layer is to coat it on a piece of glass and then reflect light off it from a distant light bulb. If you then look at this reflection through a cheap toy spectrometer you will see a pattern of dark lines across the spectrum. If you count the number of dark lines between two wavelengths you can calculate the thickness of your layer. I regularly measure the thickness of my emulsions this way and once you have done it a few times it takes no more than a few seconds to do it. The coating has to be an emulsion with AgBr or the likes in it. Otherwise the index of refraction of the emulsion will be too similar to that of glass. If you want to do this with a pure gelatin coating, you have to coat it on a first surface mirror.

    Here is a link to a forum where I posted the formula to calculate thickness and also some foto's of what the spectrum will look like: http://holographyforum.org/phpBB2/vi...ickness#p41567

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    Great to hear from you again Hans!

    Thanks for the link.

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    Hans,
    Any ideas as to where to find a cheap specroscope? I cannot find anything under $800. If you have a brand name, that might be helpfull.
    Bill

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    Look for the Project Star spectrometer. They have one that is cheap and quite good. It is also very useful for checking the quality of your safe lights.

    Here is a link for example: http://sciencekit.com/project-star-s...r/p/IG0023913/

  10. #10

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    Thanks Hans.
    I will buy one of these soon.
    Bill



 

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