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Thread: Collodion is only sensitive to visible light, not UV

  1. #1
    Tim Meisburger's Avatar
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    Collodion is only sensitive to visible light, not UV

    Yesterday we shot a lot of plates under constantly changing skies, from direct sun to heavily obscured by clouds. It was a pain in the butt, so this morning I have been looking at UV meters to try to determine if it would be useful to get one for wet plate. My (hour of) internet research has enabled to be definitively state that UV is not relevant for wet plate, as wetplate is only sensitive to visible light. Here is why I can can confidently state this with a level of certainty only the internet can provide:

    • There are three types of UV: A, B, and C.
    • UV C cannot penetrate the atmosphere.
    • UV B is 290 – 320 nanometer, but does not penetrate glass so cannot get through your lens to the collodion.
    • UV A is 320 – 400 nanometer, and can get through glass (unless it has a UV coating). 95% of the UV that reaches earth is UV A
    • According to Lund Photographic the spectral sensitivity of collodion is 403 to 521 nanometers. According to the internet violet (color) begins at 380 nanometers and extends to 450. This means that there is a disagreement between the description of UV A extending to 400 nanometers and the description of violet beginning at 380 nanometers, but both would agree that by the time you get to 403 nanometers you are in the visible spectrum.
    • So, collodion is sensitive to almost all of violet, all of blue, and half of green, so a lightmeter that measured only this section of the visible light spectrum could be accurately used to predict wet plate exposure.

    I rest my case!

  2. #2

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    Re: Collodion is only sensitive to visible light, not UV

    Good analysis Tim, you're probably right. I remember when I started, a lot of people figured using blacklight bulbs would be the Next Big Thing to allow short indoor exposures. But I noticed it never caught on, and those that tried it didn't get the fast exposures they were expecting. Probably most of the exposure was from the visible violet, like you say. But you should confirm the sensitivity of Collodion. Just because Lund is saying that's the range, they may have just pulled those numbers out of the air.

  3. #3
    Drew Bedo's Avatar
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    Re: Collodion is only sensitive to visible light, not UV

    A UV filter on the lens would eliminate any exposure from UV-A. This would allow you to use a regular light meter and simplify the process.

    Am I missing something?
    Drew Bedo
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    There are only three types of mounting flanges; too big, too small and wrong thread!

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    Re: Collodion is only sensitive to visible light, not UV

    You are right, that would make it where you don't have to factor for UV. But you don't really use a meter anyway. Wetplate collodion changes in sensitivity daily, going from possibly ISO 5 when fresh, to ISO 1 or slower as it ages over weeks and months. The aging isn't predictably either, it's based on temperature, light, and the particular iodizing salts you are using. Using a light meter would only get you close, which most people can do with an educated eye, and no meter. I've taught wetplate to people, and they can reliably calculate the exposure after a few shots, even in diminishing light.

    Tim may be onto somthing, but I do suspect collodion is somewhat sensitive to intensities we cannot see. And our eyes cannot detect when one day has more "blue" light and the next day less, due to atmospheric conditions. But your exposures will change, based on time of day (It seems just as bright at 4PM as 11AM, but it's not) and the particular day, even at the same hour!

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    Tim Meisburger's Avatar
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    Re: Collodion is only sensitive to visible light, not UV

    Yes, I agree with you Garrett that a meter will never account for the changing nature of collodion, but it might have helped us in the variably cloudy conditions over four hours (or maybe not). But, it was pointless to make tests, as by the time you developed and were ready to shoot again the light had changed. One of the people who joined us kept trying to use her meter on and for the last shot (a group picture, in both subject and execution) recommended f/16 and six seconds, with predictable results. I poured another plate, looked at the sky, spit int he wind, and stated--oracle-like--32 seconds at f/8. Spot on!

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    Re: Collodion is only sensitive to visible light, not UV

    That cloudy overcast got me one time too, shooting at a Paris photo fair with a friend, Quinn Jacobson. Every shot was radically different, 8 seconds overexposed. Then 4 seconds extremely dark. Then back to 8, dark again. Then to 15, good exposure, then the next one overexposed at that. We finally figured that there was a low ceiling, but different thicker clouds blowing over every few minutes. You couldn't tell below, it just looked uniformly fairly bright grey. We finally move away from several green trees that were heavily in leaf, and faced the sitter north. That helped. Stay out of the trees/jungle, remember I told you trees can do weird things too.

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    www.alexgard.com AlexGard's Avatar
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    Re: Collodion is only sensitive to visible light, not UV

    This has happened to me a couple of times, most notably probably late summer during days when overcast light is constantly changing, between patches of sunny sky.

    I found for me the best way to overcome it was to practice developing by eye, which I'm still trying to get myself (hard under a red light)

    I made a test plate in the overcast light and it gave me as expected 1 second exp bang on with 16 secs of development.

    The proper plate I made about 15 minutes later under development showed to be massively underexposed, mids and shadows were very slow coming in so I took the risk and pushed development into the late 20 seconds until it looked good under the red light, plate came out perfect. I thought it may have had something to do with the light but I wasn't sure (light was changing) if it was that or my poor technique

    This is an interesting thread thankyou for the information.... I'll keep an eye on this great to know.

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    Re: Collodion is only sensitive to visible light, not UV

    I am skeptical. Nice hypothesis, and in a perfect world theory (or hypothesis) would agree perfectly with experiments. But, frequently theory and practice disagree.

    There has to be some upper limit of the energy at which photons interact with collodion suspended silver halides. For example, most XRay photons would pass right through it. However, so much effort has been expended figuring out how increase both film speed and spectral range the last 150 years or so, and the energy differences are so slight (I think we'll with an order of magnitude between UV-A and infra-red) that I remain skeptical that there's a super sharp cut off excluding UV with collodion/Ag Halides. One Getty PDF looking at conservation properties lists a UV transmission of cellulose nitrate polymer (no silver halides) at 313 nm.

    A Schott glass (no pun intended, it's a real Company) tech paper PDF suggests that for an inch of glass depending on glass the UV cutoff could be as low as 250 nm, but I think high 300's is more common. UV transmission of glass is dependent on the kind of glass and its thickness. Many lenses have less than an inch of glass so ymmv.

    Honestly, I'm inclined to believe you are right, that UV may not matter for wet plate. But, the scientist in me would like to see some data.

    I suspect that exposure guidelines like in old 35mm film packaging i.e. If it's open shade do this... Combined with experience and good notes (lens, conditions, age and kind of emulsion, how you timed the exposure, etc) will be more reliable than a meter.

    I admire y'all wet plate folks. I'm not quite ready to play with such highly energetic materials nor tote a darkroom everywhere. I think if I'm going to make "film" it's going to be gelatin emulsions. I would be curious to know if anyone's made Ortho chromatic or even pan film versions of wet plate. I strongly suspect both could be done since it has been done with gelatin emulsions even by do it yourself-ers. I greatly prefer the look of at least Ortho films.

  9. #9
    Tim Meisburger's Avatar
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    Re: Collodion is only sensitive to visible light, not UV

    I simply don't know, as the information I have been able to access so far is inconclusive or contradictory. I would be surprised if no one has ever looked at this in a serious way, so I'm still hoping to track down more conclusive info.

    It would be interesting to look at uv transmission of various lens designs. Should it be possible to set a few examples on a sheet of cyanotype and expose through them with a UV light source (maybe the sun?) and compare the density of the various lens circles with the surrounding paper.

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    Re: Collodion is only sensitive to visible light, not UV

    I also believe at least some of the invisible UV spectrum is sensitive. One thing that we wetplaters today discuss, and they did in the old days, is the "actinic light." We use Northern light, because it has more of it. I think some UV gets through.

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